Crop Science

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Potassium studies on an Avalon medium sandy loam.
(1970) Farina, Martin Patrick Warner.; Hulme, Sampie A.; Sumner, Malcolm E.
Abstract available in pdf file.
The use of climatic data for maize grain yield predictions.
(1972) Mallett, John Bertram.; De Jager, James M.
The development and testing of a mathematical model for maize grain yield predictions is described. The model is based upon daily considerations of soil moisture, atmospheric evaporative demand and stage of crop development. Final yield predictions depend upon a knowledge of yield decrement due to moisture stress and the number of occasions that stress is recorded. This information was determined in the following manner:- (i) Stress imposed in lysimeters before and after anthesis was found to reduce grain yields by 3,2% and 4,2% per stress day respectively. (ii) A stress day was identified with the aid of mass-measuring lysimeters and a U.S. Weather Bureau Class A evaporation pan for measuring atmospheric evaporative demand. A nomogram constructed in terms of evaporative demand and available soil moisture, which discriminates between stress and non-stress days, was obtained for the Doveton soil used in the lysimeters. The model was applied to Cedara rainfall and evaporation data and yield probability patterns for three planting dates were obtained. It was found that highest yields (8,5 Mg ha(-1)) and least seasonal yield variation, may be expected from the earliest planting data 15/10. The Cedara : Doveton yield prediction model was also applied to climatic records for two other Natal stations (Estcourt and Newcastle) and six stations outside Natal (Bethlehem, Potchefstroom, Hoopstad, Standerton, Ermelo and Krugersdorp). Interesting comparison of the suitability of their respective climates for maize production was obtained. A method which uses the predicted number of stress days and the resultant yield decrement to determine the most effective and economic irrigation scheduling is developed and described. The effect of moisture holding characteristics of various soils upon the shape of the discriminating curve is discussed, and a method of obtaining discriminating curves for other soils by modifying the Doveton curve is described.
Nutritional studies with a spanish-type groundnut on an avalon medium sandy loam soil.
(1972) Snyman, Jacobus Wilhelmus.; Nathanson, Karl.
Calcium requirements of a Spanish-type groundnut cultivar on an Avalon medium sandy loam soil. The effect of different levels of application of agricultural lime, dolomitic lime and gypsum on yield and quality of the Nelson Spanish groundnut cultivar was studied in a field experiment. The effect of the applied treatments was studied by means of detailed soil and plant analyses. The results reported were obtained in a season with below normal rainfall. The so-called "typical drought damage" symptoms were found to be largely due to nutritional deficiency conditions. A linear increase in kernel yield was obtained as a result of increased levels of exchangeable calcium in the soil. Calcium uptake by the fruit proceeded more efficiently where gypsum was used as calcium carrier. The calcium content of the fruit was increased as a result of increases in the level of exchangeable calcium in the soil. The gypsum treatments resulted in a marked increase in shelling percentage, percentage ovarian cavities filled and increased kernel yield. The mechanism of action of the gypsum treatments was not solely improved calcium nutritional conditions. A poor correlation between calcium content of the soil and the groundnut fruit, and shelling percentage and percentage ovarian cavities filled was obtained in the case of agricultural lime and dolomitic lime treatments. The level of exchangeable soil calcium and soil pH on the majority of groundnut fields investigated was found to be low enough to expect considerable increases in yield following on applications of agricultural lime. Such an increase in pH would result in an improved nitrogen status of the plants as a result of more efficient nitrogen fixation. Applications of agricultural lime should be supplemented with a supply of calcium in a more soluble form, such as gypsum. The more soluble calcium carrier would provide a supply of readily available calcium for uptake over periods of moisture stress. The relationship between chemical composition of the soil and plant, and yield and quality factors were studied. The occurrence, cause and control of hollow heart and black plumule damage in groundnut kernels Hollow heart and black plumule are symptoms of nutritional abnormalities in groundnut kernels. Although the occurrence of these symptoms are associated with the occurrence of drought conditions, the calcium and boron status of the Avalon medium sandy loam is low enough to result in the occurrence of these symptoms on a small scale under normal rainfall conditions. The supply of available boron in the soil is decreased as a result of fixation into an unavailable form during dry periods. Decreased boron concentrations in the plant under these conditions results in the occurrence of boron deficiency symptoms in the kernels (hollow heart damage). At the same time, it is suggested, that due to the nutritional association between calcium and boron in the plant, the decreased levels of boron in the plant results in a decreased metabolic activity of calcium indicated by the occurrence of black plumule damage (a calcium deficiency symptom). The critical level of boron in the kernel as far as both hollow heart and black plumule damage is concerned, appears to be between 10,2 and 13,9 ppm. Applications of gypsum resulted in a marked decrease in hollow heart and black plumule damage. This treatment resulted not only in increased levels of calcium in the kernel, but at the same time appears to prevent the fixation of boron thus allowing normal uptake of boron by the plant. Applications of agricultural and dolomitic lime had no effect on the occurrence of either form of damage. These treatments were associated with a marked increase in calcium content and a decrease in boron content of the kernels. The ratio Ca content of the kernel (ppm)/K content x Mg content of the kernel was found to be fairly closely correlated with the occurrence of black plumule damage. The calcium, potassium and magnesium status of the soil was of little value for the purpose of predicting hollow heart damage. The intensity of hollow heart and black plumule damage was increased by applications of urea. Cultivars differed in their susceptibility to hollow heart and black plumule damage. An application of 26 kg/ha borax virtually eliminated both forms of damage.
Some effects of environment, age and growth regulating compounds on the growth, yield and quality of sugarcane in southern Africa.
(1974) Rostron, Harold.; Nathanson, Karl.; Thompson, G. D.
An assessment has been made of the potential for increasing yields of well grown irrigated sugarcane crops beyond their present maxima. The effects of age at harvest and artificial growth regulators on yield have also been investigated. Measurement of the apparent maximum yield of cane fresh weight (tc/ha) and total dry matter (t.dm/ha) of three varieties grown on a specially prepared site, indicated that actual cane yields were 63-70% of the estimated potential maximum. Differences in morphology between two extreme varieties, NCo 376 and CB 36/14, were insufficient to affect growth and yield under good growing conditions. The average productivity over a period of one year of plant and first ratoon crops of three varieties was 65 t.dm/ha and 150 tc/ha. This is a crop growth rate of 17,0 g/m(2)/dy, representing an average conversion into plant dry matter of 1,9% of total incident radiation (ri.) or 4,3% of visible ri. This level of productivity is equivalent to rates reported for sugarcane growing in more favourable parts of the world and for other highly productive crops. In an experiment in which a series of crops were ratooned at different times of the year and harvested at ages ranging from 32 to 72 weeks, sugar yield and all measures of cane quality were closely correlated with cane yield for crops of all ages. Sugar yield varied with age at harvest, according to the cycle of weather conditions experienced by the crop. Crops ratooned in January and February produced the highest yields of 23 tons estimated recoverable sugar per hectare ( ters/ha) at 64-72 weeks of age. The average rate of sugar production at this time ranged from 1,3-1,5 ters/ha/month. At 12 months of age crops ratooned in July and September had the highest yields of 17,2 ters/ha, but they then made little further growth. High rates of sugar production of 1,4-1,6 ters/ha/month were obtained from crops ratooned between June and September and harvested between May and. August at 40-56 weeks of age. Considering data for all crops, cane yield was correlated with weather conditions only up to 40 weeks of age. After this, increments of cane yield were correlated better with the amount of growth already made than with either crop age or the average weather conditions experienced. Artificial chemical ripening was successful on young immature crops harvested in May at the beginning of the milling season, but there was only a small response under less favourable growing conditions and when the crop was older and more mature. Ethrel and Polaris were the most successful of several ripeners tested, Ethrel being more active than Polaris. Cane quality was improved and sucrose storage was increased despite reductions in rates of photosynthesis and sheath and lamina size. The ripening response varied with variety, condition of the crop at the time of spraying, rate of chemical application and the time interval between spraying and harvesting. From this work it is concluded that it will probably be easier to raise the sugar yield of existing high yielding varieties by altering the proportioning of photosynthate in favour of sucrose storage, rather than by increasing cane yield.
Soil amelioration and boron nutrition effects on the growth of sunflowers (Helianthus annuus L.) on an Avalon medium sandy loam.
(1975) Blamey, Frederick Paxton Cardell.; Nathanson, Karl.
Studies with sunflowers (Helianthus annuus) on an Avalon medium sandy loam, both in pots and in the field, indicated that the poor growth of this crop on this soil was largely due to soil acidity. Marked improvements in plant growth were brought about by liming as shown by measurements of emergence, seedling mass, population, plant heights, leaf areas and yield. Based largely on soil and plant analyses, it was concluded that improved growth with soil amelioration resulted from reduced aluminium toxicity and, since lime was more efficient than gypsum in neutralizing toxic Al, lime was markedly superior to the latter in improving plant growth. An annual application of 2 400 kg agricultural lime/ha increased seed yields >5 fold in the first and >10 fold in the second season in which the field experiment was carried out. In the pot experiment, no benefit of liming above the level required to neutralize toxic Al (-pH (N KCl) 4.5) was recorded and, on the contrary, yields tended to be depressed above this level. Highly significant linear relationships between yield and exch. Al (meq100g) were recorded in the pot and field experiements and, averaged over two seasons, seed yields in the field were increased 12% for each 0,1 meq/100g reduction in exch. Al. Another factor which decreased sunflower seed yields on this soil was boron deficiency and symptoms of B deficiency in the field were identified and described. At the levels of boron in the unfertilized soil, this deficiency affected the reproductive, rather than the vegetative stage of growth and correction of B deficiency by applying 10 kg borax/ha per annum increased seed yields by 38% and 18% in the two seasons, respectively. Soil amelioration had only a slight effect on the boron nutrition of sunflowers in this study. The chemical composition of the plant tissue was found to be a suitable means of quantifying B deficiency in sunflowers, the B concentration being a slightly superior method in most cases to the Ca:B ratio in plant tissue. Using field data, critical B concentrations in (i) month-old seedlings, (ii) the topmost, fully-mature leaf at flowering and (iii) in the seed were determined. The relationships were established between the amount of borax applied to the soil and the B concentration in plant tissues and these relationships could be used as a basis for recommending corrective B fertilization.
Maize crop growth and development related to solar radiation.
(1976) Kaiser, Heinz Werner.; De Jager, James M.
The interception and reflection of solar radiant energy by maize crops was measured at Cedara using tube solarimeters. From the measured data the amount of PHAR that was intercepted was related to the L value of the crop. K(v), the extinction coefficient of PHAR thus obtained was found to vary with row spacing, plant population density and cultivar, whereas growth stage, time of day and weather conditions did not appear to effect the K(v) value markedly. On clear days the albedo (R(v)) of a ma1ze crop was of the order of 0,05 to 0,08 during the middle of the day. However, at low sun angles in the early and late hours of the day, R(v) increased to values of the order of 0,2. On overcast days, the diurnal variation was considerably lower. The rate of photosynthesis in a ma1ze crop was measured using the plastic enclosure technique. The rate of CO2 uptake was related to the amount of PHAR absorbed by the crop. A rectangular hyperbola (Eqn. 2. I) was found to describe this relationship well. The photosynthetic response of a crop to population density, plant spacing and temperature at different developmental stages is discussed. The seasonal distribution of dry matter in the maize crop was studied using normal growth analysis techniques. From these data functions describing the partitioning of DM to the various plant organs (e.g. roots, leaves, grain) at different growth stages were developed. The functions describing the absorption of light by the crop canopy, its photosynthetic rate and the partitioning of photosynthate were incorporated in a computer simulation model, which was used to estimate the growth of a maize crop from meteorological radiant flux density and temperature data. The functioning of the model and its possible use are discussed.
An evaluation of source-sink relationships in three dry bean (Phaseolus vulgaris L.) cultivars.
(1989) Liebenberg, Andries Johannes.; Lea, J. D.
The effect of intensities and times of source or sink related stresses on the growth and development of a determinant (Teebus) and two indeterminant (NEP 2, Bonus) dry bean (Phaseolus vulgaris L.) cultivars was measured in a series of field experiments at Potchefstroom Research Station. Variation in stress levels was attained by defoliation, thinning, light intensity manipulation (shades and reflectors) and removal of reproductive organs. No permanent detrimental effect on vegetative or reproductive organs was observed when source was reduced or increased during the vegetative period (V1-V6f) . Flower initiation (V6f-R1) was identified as the period most sensitive to defoliation as expressed in vegetative growth and economic yield. The negative effect of shading on vegetative development was reduced by an extended growing period . Thinning during flower initiation increased the vegetative and reproductive sink. A source stress (defoliation and shading) during the flowering period (R1-R5) restricted partitioning to the reproductive organs reducing seed yield and harvest index values. Reduced interplant competition during flowering favoured partitioning to the reproductive organs. Source size had a direct relationship with economic yield during flowering. This was confirmed by the absence of a yield response to partial depodding. A lack of response to defoliation (NEP 2) and shading (Bonus) may indicate a limited sink size in these two cultivars. During seed filling (R5-R9) Bonus was very sensitive to defoliation while NEP 2 was insensitive. Bonus was less sensitive to shading than Teebus. Thus in certain cultivars the level of current photosynthesis had a significant effect on seed yield throughout seed filling. The lack of a yield response to thinning in all cultivars during this period indicated that the potential sink size was set before R5. The results provided strong evidence supporting the concepts of yield component compensation in dry beans. Pod number was most seriously affected by defoliation during flower initiation and flowering. The potential sink size was determined mainly through the number of pods per plant which was in balance with the source unless some stress factor was present. The number of seeds per pod responded to current photosynthesis as well as the previously set pod number. Seed size was the least responsive yield component and it had a consistent negative relationship with the number of pods per plant.
Relationships between climatic indices and soil properties that reflect leaching and weathering in the Natal Midlands, and their use in the assessment of afforestation potential.
(1991) Donkin, Michael John.; Fey, Martin Venn.
In the summer rainfall regions of Southern Africa, knowledge of the mean annual effective rainfall (MAER) pertaining to a site is critical for the assessment of that site's afforestation potential. The ability to estimate MAER at a site from soil properties was sought for the forestry regions of the Natal midlands. The S-value (sum of basic cations) expressed per unit mass of clay (a diagnostic property in the South African soil classification system) is currently used for this purpose, but until now this usage has not been validated. Jenny's conceptual model of state factors of soil formation was employed to demonstrate, within a climosequence, the empirical relationships occurring between soil properties and climatic indices. Weather stations from around the Natal midlands were selected on the basis of the reliability of their rainfall and temperature records and 33 of these sites were subsequently incorporated into a conceptual climosequence. At each site, representative soil profiles were excavated, described and intensively sampled. Soil samples were analysed for a variety of properties indicative of the degree of leaching and/or weathering. At each site, indices of climate (mean annual precipitation, drainage from various depths and sub-horizons in the soil profile, and MAER) were calculated using an agrohydrological water-budgeting model (ACRU). Selected soil properties were systematically related to these climatic indices. The results showed that within the climosequence concerned, of all the soil properties considered, the effective cation exchange (ECEC, sum of basic and acidic cations) of the B1 horizon, expressed per unit mass of soil, was best related to the climatic indices.
Fluctuation of non-structural carbohydrates in the stem and ears of maize (Zea mays (L.)) during grain fill as influenced by water stress.
(1991) Shanahan, Paul Edward.; Greenfield, Peter L.
Stems of maize plants may serve as reservoirs for photosynthate produced in the leaves which may then be utilized for cell growth and maintenance requirements of the plants, and in particular for grain requirements during grain fill. Experiments were designed to ascertain the extent to which non-structural carbohydrates accumulate and are depleted in the stem and ears of locally cultivated maize hybrids during grain fill under conditions of water stress. Maize plants were grown: (i) under field conditions; (ii) under a rain-out shelter; and (iii) in pots placed inside a growth tunnel during grain fill. In the latter experiment whole maize plants were exposed to (14)C0(2) at selected intervals during grain fill. In the field trial large differences in the accumulation and depletion of total non-structural carbohydrates (TNC) were found between the six hybrids tested. The water stress conditions that prevailed from mid-grain fill (MGF) to physiological maturity (PM) resulted in TNC content levels being lower at PM than at anthesis in all hybrids except for SR 52. Total non-structural carbohydrate content in the whole stem of PNR 6427, CG 4602 and PNR 473 declined from anthesis to PM. In contrast TNC content in the whole stem of SA 60 and HL 1 declined from anthesis to MGF and then increased substantially in SA 60 and marginally in HL 1 from MGF to PM. In the rain-out shelter trial, water stress resulted in a 38 % reduction in final grain yield in SA 6 compared to 25 % in K78Y x I137TN. The greater tolerance to water stress of the more modern hybrid K78Y x I137TN compared to the obsolete hybrid SA 6 may be attributed to a number of factors, namely: (i) K78Y x I137TN recorded a higher leaf area index throughout grain fill under stress and non-stress conditions compared to SA 6; (ii) it did not partition as much non-structural carbohydrate to the stem during the first three weeks of grain fill as did SA 6 and did not markedly deplete stem non-structural carbohydrate pools to the same extent as did SA 6 under stress and non-stress conditions; and (iii) in the last week of grain fill as the leaf water potential of K78Y x I137TN increased sharply under stress conditions, it exhibited an ability to deplete stem non-structural carbohydrates to supplement the supply of current photosynthate to the grain. In the 14(C)-labelling pot trial, the maize single cross hybrid B254W X M162W generally depleted TNC in vegetative organs in the latter half of grain fill under stress conditions, while under non-stress conditions TNC continued to accumulate in vegetative organs until PM. Both stressed and non-stressed plants assimilated less 14(C) on consecutive labelling occasions during grain fill. The amount of 14(C) assimilated at six weeks after anthesis was only 12,1 and 16,3 % of that assimilated at anthesis in stressed and non-stressed plants, respectively. Stressed and non-stressed plants labelled at anthesis translocated a smaller proportion of assimilated 14(C) to the grain during grain fill than plants labelled later. Consequently, stressed and non-stressed plants labelled at anthesis recorded the highest proportion of whole plant 14(C) recovered in the whole shoot at PM compared to plants labelled on any of the other occasions. At anthesis the primary ear was not yet established as the major sink for photosynthate and much of the 14(C) assimilated at anthesis was utilized for final structural growth of the whole shoot including the cob and husks of the primary ear. Stressed and non-stressed plants assimilated similar amounts of 14(C) at anthesis and two weeks after anthesis, however, stressed plants assimilated less 14(C) than non-stressed plants at four and six weeks after anthesis. Forty-eight hours after each labelling occasion, the stressed plants had partitioned a higher proportion of assimilated 14(C) to the grain than the non-stressed plants. However, by PM the non-stressed plants had partitioned an equal or greater proportion of whole plant 14(C) recovered at PM to the grain compared to the stressed plants. Radioactivity associated with component non-structural carbohydrates, was determined using ion-exchange column chromatography and thin-layer chromatography. These procedures provided detailed data of the partitioning of 14(C) among glucose, fructose, sucrose and starch.
Modelling deficit irrigation of wheat in Zimbabwe.
(1993) MacRobert, John Findlay.; Savage, Michael John.
Wheat is grown in Zimbabwe during the relatively dry, cool winter with irrigation. On most large-scale farms, land resources exceed irrigation water resources. Consequently, the efficient use of water is of prime concern. This has led to the development and adoption of deficit irrigation techniques, with the aim of maximizing net financial returns per unit of applied water rather than per unit land area. This often requires that less water be applied than that required for maximum yields, which implies that water deficits are allowed to develop in the crop. Although the basic principles of deficit irrigation are known, there exists no systematic procedure for advising farmers on whether or not to, or how to, employ such a management option in Zimbabwe. This research was therefore undertaken to develop an interactive computer programme that would assist farmers in determining optimum irrigation strategies for wheat. The CERES-Wheat version 2.10 crop simulation model (WHV21) was chosen as the basis for this programme. In order to validate and modify, where necessary, WHV21, a series of field experiments were conducted at a number of wide-ranging locations in Zimbabwe during the period 1986 to 1992. These included sowing date x cultivar, sowing date x seeding rate, growth analysis and irrigation experiments. In all, 122 data sets were collected, of which 47 were used for model validation and 75 used for calibration and modification of WHV21. The initial validation of WHV21 showed that the model gave biased and imprecise predictions of phenological development, particularly under deficit-irrigated conditions. The simulation of tillering was poor and the model tended to over-predict dry matter accumulation and under-predict leaf area indices. The yield component and grain yield predictions were also generally imprecise. On the other hand, for most data sets, the simulated soil water contents were similar to measured soil water contents. These inconsistencies prompted a revision of the phenological and growth subroutines of the model. In the phenological subroutine, new thermal time durations and base temperatures (Tb ) for all growth phases were determined from regressions of the rate of phasic development on mean air temperatures. For growth phases one, two and three, a Tb of 4°C was established, whereas for growth phases four and five, a Tb of 3°C was used. The revised model included the prediction of leaf emergence (as apposed to leaf appearance) and first node appearance (Zadoks growth stage 31). In order to hasten plant development under conditions of soil water deficit stress, daily thermal time was made to increase whenever the actual root water uptake declined below 1.5 times the potential plant evaporation. These changes improved the prediction of crop phasic development: for example, the Index of Agreement for the prediction of physiological maturity was improved from 0.643 with WHV21 to 0.909 with the revised version. Many changes were made to the growth subroutine, inter alia: 1. the extinction coefficient in the exponential photosynthetically active radiation (PAR) interception equation was reduced from 0.85 to 0.45; 2. an allowance was made for the interception of PAR by the wheat ears during growth phases four and five, with the statement IPAR=1-EXP(-O.004*TPSM), where IPAR is the proportion of PAR intercepted and TPSM is the number of tillers m-2; 3. the area to mass ratio of leaves was increased from 115 cm2 g -l to 125 cm2 g -l during growth phase two and this was allowed to decrease under conditions of water deficit stress; 4. tiller production during growth phase one was made a function of daily thermal time, total daily solar radiant density and plant density, moderated by high air temperatures and a new soil water deficit factor that takes the dryness of the surface soil layer into account; 5. a cold temperature routine was added to reduce kernel numbers whenever the exposed minimum air temperature decreased below 0°C during the period ear emergence to the start of the linear kernel growth phase (cold temperatures during anthesis occasionally cause reductions to kernel numbers in Zimbabwe); and 6. the kernel growth rate was gradually increased during growth phase four, and the rate of kernel growth was increased under conditions of water deficit stress during growth phase five. The modifications made to the growth subroutine of WHV21 improved predictions of tillering, ear density, yield components and yield on the independent validation data set. The modified model (WHVZIM22) was used to evaluate wheat sowing date and irrigation strategies on ten-year sets of weather data from representative locations in Zimbabwe. The results indicated that the highest yields were obtained with sowings during the latter half of April and the early part of May at all tested locations. Yields were greater for each sowing date and irrigation regime at the high altitude (1480 m) location than at warmer, lower altitude locations. The response of wheat yield to irrigation application was typically curvilinear, particularly on the soil with a high water holding capacity. Maximum yields were attained with the application of 400 to 500 mm (net) water. Soils with low water holding capacities produced lower mean yields than soils with a high water holding capacity. Maximum financial returns tended to occur with the application of less water than that required for maximum yields, particularly on the soil with a high water holding capacity. However, the variance of financial returns increased with reductions in the amount of water applied. These simulation results corroborated field observations and, taken together with the improved predictive ability of WHVZIM22 over WHV21, provided sufficient justification to use the revised model as a basis for the development of a pre-season irrigation optimization computer programme. This programme seeks the intraseasonal irrigation regime that maximizes the total gross margin for a particular soil, cultural and weather scenario, within the constraints of land and water availability. The programme is written in Microsoft QuickBASIC 4.00 and can generate an optimized irrigation regime within 4 to 5 minutes when executed on an IBM AT-compatible 80486 computer running at 25 MHz. It is envisaged that the programme would be used as a pre-season management tool, but the literal application of the results in the field is not recommended in view of the fact that the WHVZIM22 model has a number of inherent limitations and is therefore not a perfect predictor of crop growth and yield. The optimum irrigation solution generated by the programme simply provides a basis from which a farmer can plan irrigation management strategies. The actual intraseasonal irrigation schedule would necessarily depend on the real-time crop, soil and weather conditions.
Effects of different levels of competition by Cyperus esculentus L. on the growth and surcrose yield of irrigated sugarcane (Saccharum officinarum L.) in Northern Swaziland.
(1995) Manana, Shesi Vusumuzi Innocent.; Greenfield, Peter L.
The study was undertaken to evaluate the interference of Cyperus esculentus (L.) with growth and quality of irrigated sugarcane (Saccharum officinarum (L.)) in northern Swaziland, and to quantify or characterise yield loss in relation to C. esculentus population and its control. The study comprised two field experiments on different soil types on plant, 1st, 2nd and 3rd ratoon sugarcane located at Mhlume (Swaziland) Sugar Company during the period 1988 to 1991. Three C. esculentus population levels were established at 1680 to 1833 plants m(-2) (heavy), 1110 to 1205 plants m(-2) (medium), and 550 to 582 plants m(-2) (light). Medium and light infestation treatments were instituted through thinning by hand to populations of 67% and 33% of the original populations respectively. Four weed control methods were superimposed. These comprised two controls; a) no weed control (C. esculentus was left undisturbed throughout the growth of the crop), b) complete weed control by hand weeding throughout the season; and two levels of herbicide application rates, a recommended estate level, which was a mixture of 1.6 L MCPA (a.i.) ha(-1) (2-methyl-4-chlorophenoxyacetic acid) (400 g a.i. L(-1) ) with 1.5 L ametryn (a.i.) ha(-1) (2-methylthio-4-isopropylamino-s-triazine) (500 g a.i. L(-1) ) and one-half the recommended herbicide application. A surfactant was added to the mixture at 0.5 L ha(-1). Both trials had 12 treatments arranged in a 3x4 rectangular lattice with five replications. Significant sugarcane yield responses to weed control method were obtained on plant and 3rd ratoon sugarcane. For the plant sugarcane crop, the institution of the recommended herbicide rate and complete hand weeding gave yield increases of about 14% and 24%, respectively, compared to the no control treatment. The plant crop sucrose yield was significantly affected by weed control methods with the recommended herbicide and complete hand weeding giving 15% and 26% increase in sucrose yield, respectively, compared to the no control treatment. In the 3rd ratoon crop complete hand weeding gave an increase of 26% and 28% sugarcane and sucrose yield, respectively. Indications were that 1st and 2nd ratoon sugarcane displayed the most vigorous growth, and hence was highly competitive against the C. esculentus and suffered no yield or quality loss due to weed populations. The possible reason was that the 1st ratoon sugarcane crop grows more vigorously than the plant crop and would therefore achieve canopy earlier than in the plant sugarcane crop. Even though results under the environmental conditions and time of regeneration of young ratoons used in this study indicated little, if any, benefit of weed control, this aspect would need further study at other times of the year before no weed control on young, vigorously growing ratoons could generally be recommended. As sugarcane yield and quality were not affected by C. esculentus populations, it was not possible to establish a population level for economic control of the weed. However, regeneration of C. esculentus in subsequent years was shown to be a function of previous years' populations and control method imposed on that population. Final populations in the subsequent year were lower where weed control was instigated than where there was no weed control. The conclusion reached in this study is that weed control will not only affect competitive abilities of current C. esculentus, but also reduce future population levels of the weed. The lack of sugarcane yield and quality response to different levels of C. esculentus population made it impossible to conclusively determine the economic threshold of C. esculentus which caused sugarcane yield and quality loss in irrigated agriculture on these soils.
Nutrient studies in potatoes (Solanum tuberosum)
(1997) Ahmadi, Ramtin.; Cairns, Andrew Lawrence Patrick.; Greenfield, Peter L.
One of the biggest problems facing potato (Solanum tuberosum) production in Kwazulu-Natal is acidic soils with high a aluminium content. Traditionally, such soils were ameliorated using lime, thus increasing soil pH, Ca and Mg availability, and reducing Al availability. This study aims to determine the extent to which lime could be replaced by Calmag+B (a Ca source with little ameliorative capacity). The Calmag+B fertiliser increases the soil's Ca content appreciably, but does not alter soil pH and Al availability to the same extent as lime. Pot trials were carried out to determine the effects of liming a highly acid soil, resulting in four levels of amelioration. At each level of amelioration, three levels of Calmag+B were applied to determine whether an optimised yield response would be attained through these applications. Plant emergence and subsequent development was shown to be poor, and in extreme cases absent, under highly acidic soil conditions. Soil amelioration using lime greatly improved plant emergence, development and tuber yield, whereas Calmag+B applications were unable to improve plant emergence, development or tuber yield. The effect of both applications of substantial quantities of Calmag+B to a highly acid soil, and of the dipping of mother tubers in a nutrient solution prior to planting was investigated. The Calmag+B soil applications were unable to improve plant emergence and development. The dipping of the mother tubers in nutrient solution, however, resulted in greatly increased seedling emergence and plant development. This was due to increased nutrient uptake from the mother tuber, and not through increased root development and subsequent nutrient uptake. There were unsubstantiated claims by the manufacturers of Calmag+B that the Mg, NO(3) and B components of the fertiliser would give rise to increased uptake of the Ca component, thus enhancing the efficacy of the fertiliser. Pot trials using a sand medium were employed to test this claim. The trial consisted of interactions of different levels of Ca(2+), Mg(2+), NO(3-), and BO3(2-) applied to the sand medium in the form of a nutrient solution. None of the treatments (barring Ca itself) led to increased Ca uptake by the potato tubers. This would indicate that the claim that the Mg, NO(3) and B components of Calmag+B fertiliser would enhance the uptake of the Ca component are unfounded. It has been suggested that one of the main factors limiting potato production in Kwazulu-Natal has been inadequate mineral nutrition. Certain fertiliser distributors claim that fertiliser application over and above the levels recommended by the Kwazulu-Natal Department of Agriculture Fertiliser Advisory (KDAFA) is the solution to the problem of below potential yield and tuber quality in the province. A field trial was carried out in New Hanover (Kwazulu-Natal), using different levels of Calmag+B and Agrifos, as well as one level of application of KNO(3). All treatments were applied after KDAFA fertiliser recommendations had been fulfilled. At the 95% level of significance, the treatments did not give rise to increased yield and tuber quality. At the 80% level of significance, however, the results indicated that applications of 100kg ha -1 of Calmag+B and KNO(3) would give rise to increased tuber yield.
Intercropping of maize and dry beans for the Vulindlela district of KwaZulu-Natal.
(1997) Liebenberg, Benjamin Christiaan.; Cairns, Andrew Lawrence Patrick.
The use of a maize/bean intercropping system to improve land productivity was investigated after limited land availability had been identified (Liebenberg, 1993) as a major constraint to crop production in the Vulindlela area of the KwaZulu-Natal province of South Africa. The objective of this study was to develop an intercropping system that would: a) Give an intercrop bean yield approximately equal to that of the sole crop yield, b) Give a maize yield acceptable to the farmer (needed mainly for green maize). c) Produce a land equivalent ratio (LER) greater than one. In order to ensure high bean yields, maize dominance was reduced by lowering the normal maize population of the intercrop by 50% and by using a tramline row arrangement instead of evenly spaced rows. Two bean cultivars namely Mkuzi (carioca) and Umlazi (speckled sugar) and two maize cultivars namely Kalahari Early Pearl (KEP) (an open pollinated cultivar) and SR 52 (a hybrid) were used. Single trials were planted at four localities spread over three seasons i.e. Vulindlela and Ukulinga (1992/93), Cedara (1995/96) and Makhathini (1996). The treatments included varying bean densities, bean planting times and maize harvesting stages. These treatments were compared to maize and bean sole crop controls. High maize yields led to low bean intercrop yields. However, there was little or no difference between sole bean yield and intercrop bean yields associated with lower maize yields. Intercrop maize yields were 50% of the sole maize yields at all the sites. The mean LER's for the Vulindlela and Ukulinga trials were 1.04 and 0.96 respectively while the mean LER's for the Cedara and Makhathini trials were 1.34 and 1.31 respectively. Only the latter two trials displayed significant improvements in land productivity. Mkuzi was more affected by intercropping than Umlazi while KEP competed less with the beans than SR 52 and gave higher yields under less favourable growing conditions. Yield component studies indicated that stress during the vegetative, pod formation, and pod filling stages led to yield reduction in the dry bean crops. Light and leaf nutrient level studies suggested that the yield reduction resulted from competition for nitrogen and light. There was no competition for phosphate and potassium. The study indicates that the intercropping system did meet the desired requirements under conditions that are less than ideal for maize production, such as low soil fertility, water stress and cool temperatures.
Towards the achievement of environmental standards in the South African sugar industry : the role of GIS.
(1999) Wallace, Michael Grant.; Breen, Charles Mackie.
The South African Sugar Industry is a major land user in the South African provinces of KwaZulu-Natal and Mpumalanga. Although offering substantial economic benefits in these regions, monocultural sugarcane production has had a fundamental impact on the natural environment in which sugarcane is produced. Attention was focused on the growing sector of the industry after flood events during the previous decade resulted in major soil erosion of sugarcane land. Widespread intentional cane burning is attracting increasing societal and regulatory pressure. New national environmental legislation in the spirit of the United Nations Conference on Environment and Development (UNCED) Agenda 21 and various other international agreements, demand that industries - including agriculture - demonstrate sustainability in their use of environmental resources. National law now more rigorously addresses biodiversity and wetland environmental issues. New water laws will fundamentally alter the existing water-use paradigm in sugarcane production. These issues are not unique to South Africa, having much in common with those faced by other major sugar producing countries. In order to effectively manage the impacts of production processes on the environment, organisations are turning to internationally accepted environmental management standards, such as the ISO 14000 series, in order to demonstrate their environmental responsibility to government and society, whilst promoting their acceptability to consumers. The SA Sugar Industry is in the early stages of investigating appropriate environmental management systems. The natural resources required for - and impacted upon - by sugarcane production are variable in space and over time. Effective and responsible environmental management must make optimum use of appropriate technology to effectively utilise the large volumes of often complex data pertaining to these resources and associated environmental processes. Geographic Information Systems (GIS) are computer systems designed for the capture, analysis, storage and display of spatial data and attribute data related to location. Whilst not a new development, recent advances indicate that GIS has substantially matured as a decision support technology and as such is being used successfully by many organisations involved in environmental management, where its use offers unique benefits at a variety of decision levels and spatial scales. GIS is applied at many complexity levels, from simple thematic map production to complex spatial analysis. The major advantage of GIS is considered by some to be its ability to spatially model environmental scenarios, producing graphic results (usually maps). As such, GIS has considerable value in formal Decision Support Systems. The major environmental issues facing South African sugarcane producers are fundamentally spatial in nature. The development and incorporation of environmental GIS capacity into their proposed environmental management system is indispensable in addressing these issues and moving towards achieving and maintaining acceptable environmental standards in the SA sugar growing sector.
Modelling the production potential of land for sugarcane in the KwaZulu-Natal Midlands sugarcane belt.
(2000) Brüggemann, Edgar Alfred.; Klug, John R.; Greenfield, Peter L.; Dicks, Harvey M.
Commercial sugarcane records for 19 seasons from 146 fields were obtained from selected estates in the KwaZulu-Natal Midlands. The estates, located at Kranskop, Umvoti and Richmond, are representative of the higher-potential rainfed sugarcane production region of the Midlands sugarcane belt. Extensive editing and cleaning of the agronomic records was required. Regression models were developed to determine which parameters of the field records were consistently associated with sugarcane yield (TCH) and could be used for yield predictions. Depending on the predictor variables selected, the best models based on 535 crop cycles accounted for 55% and 43% ofthe observed yield variation respectively. Linear regression was an appropriate analytical technique since the assumptions of normality and homoscedasticity were upheld and multicollinearity was not a problem in the models. The models were validated using an independent data set of 47 observations and satisfactory performances were confirmed. The 95% confidence limits of yield predictions for the population mean lie within 10% of long-term mean yields. These predictions could be useful for estate resource allocation and harvest planning. Key physical field attributes associated with sugarcane yield were locality, aspect, altitude, soil type and effective rooting depth. Season and rainfall were important climatic variables. Ofthe factors influenced by management, sugarcane variety, plant / ratoon status, crop cycle, N and K nutrition and the topsoil Ca:Mg ratio were important yield predictors, depending on the equation used. The relative importance of individual predictors varies with the specific combination of resources for a particular observation. The models were linked to a geographic information system to demonstrate an application ofthe models for yield prediction in response to spatial variables. These predictions showed that the models could be used at a general scale within estates to identify areas of differing production potential. Reliable yield predictions could not be made for individual fields and within-field resource variations could not be adequately accounted for.
Distribution and severity of herbicide resistance in the Republic of South Africa.
(2001) Smit, J. J.
No abstract available.
Effect of irrigation water regimes applied via subsurface drippers on soil water distribution and on sugarcane (Saccharum officinarum L.) growth.
(2002) Gama, Mandla Ernest.; Greenfield, Peter L.
The effects of different water regimes on sugarcane growth were investigated for subsurface drip irrigation over three seasons. Arbitrary factors (0.5, 0.75, 1.0 and 1.2 of ETc), were applied to adjust the daily estimates of Penman Monteith evapotranspiration (Et). These drip treatments were compared to an overhead sprinkler irrigated (1.0 ETc) block of sugarcane. In the plant and first ratoon crops, the drip plots were irrigated to field capacity, by replenishing an allowable deficit of 10 mm. In the second ratoon however, the soil was irrigated to a 50% total available moisture (TAM) level. Soil water movement and root distribution adjacent and below emitters was measured. There was an increase in vertical and lateral soil water movement with increase in irrigation water regime. The rooting density increased with increase in soil water content. However, the root density decreased slightly in the 1.2 ETc treatment below the emitters in the wetter season. There were no differences in cane growth among the drip treatments in the first two seasons, mainly due to the wet weather conditions. In the second ratoon the 1.2 ETc treatment had the tallest stalks, which resulted in the highest cane and sucrose yields (t ha-1). There were no differences in cane and sucrose yields among the other treatments. There were no consistent differences in the plant canopy cover among the drip treatments. The cane growth in the sprinkler block was poor in the plant and first ratoon crops, probably due to the leaching of N, and it was comparable to the drip block in the second ratoon. The crop water use efficiency (CWUE) decreased with an increase in irrigation water regimes, and all drip CWUE treatments were better than that of the sprinkler block in all years. The study has shown that, a) the estimated daily ETc could be reduced by 50 % while still achieving the Mhlume estate average cane yield of 95 t ha-1, b) rainfall use efficiency, cane and sucrose yields could be increased by using 1.2 ETc at the 50% TAM level, c) there is a need to re-examine N applications under drip irrigation, particularly where heavy rains occur after leaf samples have been taken.
Nitrogen in the soil-plant system of successive rainfed wheat crops under conventional cultivation.
(2002) Otto, Willem Morkel.; Haynes, Richard John.; Greenfield, Peter L.
Soil mineral N and soil water content at planting, biomass accumulation, yield and grain quality parameters (hectolitermass and protein percentage) were measured on an unfertilized and recommended-N-application treatment during two consecutive growing seasons (1997-1998). The trials were planted in a fallow-wheat-wheat cropping system at three representative localities in the summer rainfall region of South Africa. High levels of available soil water and mineral N were measured following the fallow period preceding the start of the trials in 1997. For example, soil water content was 81.7%, 69.6%, and 78.2% of DUL at Bethlehem, Kroonstad and Petrusburg respectively. Although comparable total soil profile water contents to 1997 were measured in 1998 at all three sites, the cultivation zone (0-400 mm) had a substantially lower soil water content. This was due to erratic rainfall distribution during the fallow period, which prevented effective soil cultivation management, subsequent soil water conservation and residue decomposition. Undecomposed residue in the cultivation layer at planting appeared to affect availability of soil mineral N to the growing crop. At planting in 1998, undecomposed crop residue amounted to 53.6% at Bethlehem, 32.5% at Kroonstad and 46.9% at Petrusburg of that added at harvest in 1997. Soil mineral N was lower at planting in 1998 compared to 1997 due to decomposing residue (C:N ratio of above 73) in the cultivation zone immobilizing soil mineral N. This reduced initial growth, N accumulation, yield, and grain protein percentage without additional fertilizer N. Distribution of soil mineral N showed notable amounts in the 600-1200 mm soil layers, with limited changes over the trial period. This was linked to low root exploration of these soil layers (10-15% of total root distribution). The ratios of soil mineral NH(4+):N0(3)- for the different soil layers indicated similar values over the trial period. Climatic data for the localities indicated differences in the amount and distribution of rainfall and temperatures during the study period, which influenced crop development, yield and grain protein percentage. At Bethlehem above average in-season rainfall was measured during 1997, at Kroonstad average rainfall and at Petrusburg below average in-season rainfall. Response to applied N at the localities varied in magnitude during 1997. Nitrogen application significantly increased N concentrations of plant components, N uptake, yield and grain protein percentage, although values for all these parameters were lower in 1998 than in 1997. Indeed higher yields were produced in 1997 (mean=1.838 t ha(-1)) compared to 1998 (mean=0.980 t ha(-1)). A significant yield response to applied N was measured at the two higher yielding localities in both cropping years, but there was no significant response at the lower yielding locality. The limiting factors appeared to be the availability of soil water and residual soil mineral N. From the calculated response functions, the variables soil water content at planting, soil mineral N content at planting, in-season rainfall, and added fertilizer N explained the bulk of the variations in grain protein percentage, plant N uptake, and yields. It was concluded that the present fertilizer N recommendation system for dryland wheat production, which is based on fertilizer response curves for specific yield potentials, should be augmented by using initial soil mineral N and water contents in the profile measured prior to planting.
Comparative responses of fodder and grain teff (Eragrostis tef (Zucc.) Trotter) cultivars to spatial, temporal and nutritional management.
(2002) Kassier, Sigrun Barbara.; Greenfield, Peter L.
Teff has its origin in Ethiopia as grain crop, while in South Africa it is primarily a forage crop for hay and recently as summer grazing pasture. The response of teff herbage and grain production to planting date, growth stage at cutting, seeding rate and N fertilizer application was studied. Previously limited research data were available for teff production in South Africa. Spring plantings (September to October) are required to maximise total herbage yield with 9.40, 8.48 and 7.64 t DM ha -1) recorded for 1996/97, 1997/98 and 1998/99 respectively. Summer plantings (November to December) give maximum herbage yield from the first cut, yielding 4.42, 4.72 and 3.78 t DM ha -1) for 1996/97, 1997/98 and 1998/99 respectively. The exact planting date is season dependent. Temperature and rainfall determine the beginning of the growth season regarding favourable conditions for teff germination and growth. Herbage yield of cut 1 increases with advancement in growth stage at cutting. Cutting at the vegetative and piping stages gives most number of cuts , up to five yielding 7.45 t DM ha -1) (1996) while the full flowering stage gives the least (one or two cuts , 4.75 and 7.72 t DM ha -1 in 1996 and 1997 respectively). Yield is also affected by environmental conditions influencing germination, biomass accummulation and regrowth after cutting and by lodging. A trade-off results between herbage quantity and quality. Yield increases while quality decreases with advancing phenological stage, resulting in reduced digestibilty and CP and increased fibre content. Seeding rate differences were manifested primarily in weed infestation level, which varied between cultivars depending on leafiness and associated sward density. Nitrogen application levels gave maximum response between 75 and 150 kg N ha -1, with some cultivar differences. Split N application according to expected yield distribution related to planting date is recommended. Grain yield response to seeding rate and N fertilization levels could not be established. Heavy grain losses through thunderstorms and wet conditions at grain maturity precluded yield measurements. Teff yield responses are influenced by day length, environmental factors, such as temperature and rainfall. and phenological stage at cutting. These variables influence biomass accumulation and regrowth.
The effect of different furrow irrigation regimes on infiltration and sugarcane (Saccharum officinarum L.) yield at Ubombo Swaziland.
(2003) Mazibuko, Njabulo W.; Greenfield, Peter L.
In surface irrigation, the soil serves as a medium for infiltration and for conveying water from the upstream to the downstream end of a field. Soil infiltration characteristics are therefore extremely important for surface irrigation design and management. In this study, the infiltration characteristics of the Sibaya (Si) soil type (Glenrosa, in the South African soil classification system) was determined by a volume balance method using a two-point approach technique. The infiltration model adopted was that of Kostiakov. The purpose of the study was to examine the effect of different irrigation scheduling on infiltration characteristics, and on irrigation performance. A trial was conducted on a field with predominately Rondspring and Sibaya soils from 1999 to 2001. The five irrigation treatments were the Ubombo method, Penman-Monteith (PM) derived irrigation scheduling factors of 1.25, 1.00 and 0.75, and alternate row irrigation using Ubombo scheduling and 1.00 x PM on plant and first ratoon cane, respectively. Treatments were arranged in a randomised complete block design with five replication. The Ubombo scheduling method had the highest number of irrigation events followed by the 1.25 x PM, whilst the 0.75 x PM had the least. The infiltration variables indicated that, for the Ubombo and 1.25 x PM treatments, irrigation often occurred when the soil water content was still less than 50% depleted plant available water (DPAW). This was in agreement with the tensiometer and neutron probe data. The tensiometer readings ranged from -55 to -75 kPa, -50 to -65 kPa, and -8 to -12 kPa at 0.15 m, 0.30 m, 0.45 m soil depth respectively. Likewise, 0.75 x PM was irrigated when the soil water content was greater than 50% (DPAW). Tensiometer readings would nearly always read above -80 kPa at both 0.15 m and 0.30 m, and above -75 kPa at 0.45 m. Further examination of the tensiometer and neutron probe data suggested that irrigation scheduling determined the preferential depth of water uptake by the crop. Frequent irrigation resulted in the crop depleting soil water predominately at the 0.15-0.30 m soil depth and hardly any at 0.45 m and below, particularly when the crop was young. There were no significant differences in yield among any of the treatments in the plant or ratoon crops. The plant crop consistently recorded higher yields than the first ratoon in all the treatments. Ubombo scheduling recorded the highest sugarcane yield in both seasons at 84 tha(-1) for the plant and 82 tha(-1) for the first ratoon cane. The 0.75 x PM had the lowest yield (78.3 tha(-1)) in the plant crop as well as in the first ratoon (74 tha(-1)). The volume balance approach provided a reliable and convenient way of assessing surface inigation systems to identify alternatives that may be effective in improving the system performance, and in assessing different irrigation schedules. Sound management which comes about by selecting the most efficient stream size, length of field, and set time, and also a suitable irrigation schedule for that soil type depends on detailed knowledge of the infiltration rate of a particular soil. Information on infiltration constitutes the basis for establishing the necessary design, evaluation criteria and operational management system in irrigation.

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