Browsing by Author "Marais, Johan Pieter."

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Effect of nitrate upon the digestibility of kikuyu grass (Pennisetum clandestinum)
(1985) Marais, Johan Pieter.; Dennison, Clive.
The factors affecting the accumulation of nitrate in kikuyu grass pastures and the effect of elevated nitrate levels upon digestion in the ruminant were investigated. A high potassium level in the soil seems to be the major factor stimulating the accumulation of excessive amounts of nitrate in kikuyu grass, when the nitrate content of the soil is also high. The continuous elongation of kikuyu grass tillers allows constant exposure of high nitrate containing stem tissue to the grazing ruminant. Digestibility studies in vitro showed that nitrite, formed during the assimilatory reduction of nitrate to ammonia, reduces cellulose digestion, but the degree of reduction also depends upon the presence of readily available carbohydrates and protein in the digest. Studies in vivo showed that the microbial population can adapt to metabolise high concentrations of nitrate (500 mg% N, m/m) in fresh kikuyu grass, without the accumulation of nitrite in the rumen. However, introduction into the rumen of nitrite in excess of the capacity of the nitrite reducing microbes, causes nitrite accumulation. Nitrite has no direct effect upon rumen cellulase activity. Due to the affinity of rumen carbohydrases for the substrate, attempts to isolate these enzymes by means of isoelectric focusing and other separation techniques met with limited success. Nitrite strongly reduces the xylanolytic, total and cellulolytic microbial numbers with a concomitant decrease in xylanase and cellulase activity of the digest. Decreased microbial numbers could not be .attributed to a less negative redox potential of the digest in the presence of nitrite, nor could the effect upon the cellulolytic microbes be attributed to an effect of nitrite on branched chain fatty acid synthesis required for cellulolytic microbial growth. A study of the effect of nitrite upon the specific growth rate of pure cultures of the major cellulolytic bacteria, Ruminococcus flavefaciens strain FDI, Butyrivibrio fibrisolvens strain Ce 51, Bacteroides succinogenes strain S 85 and Ruminococcus albus strain 22.08.6A and the non-cellulolytic bacterium Selenomonas ruminantium strain ATCC 19205 revealed the extreme sensitivity to nitrite of some of these bacteria and the relative insensitivity of others. Growth inhibition seems to depend primarily upon the extent to which these microbes derive their energy from electron transport-mediated processes.
The nutritive value of Italian ryegrass Lolium multiflorum) selected for high dry matter and nonstructural carbohydrate contents.
(2003) Hopkins, Cheryl.; Marais, Johan Pieter.; Dennison, Clive.
In traditional forage breeding programmes, breeders have spent decades improving the agronomic characteristics of grasses, such as herbage yield, persistence and resistance to diseases, without considering the nutrient requirements of the grazing animal. In an attempt to improve the nutritive value of Italian ryegrass, which is widely utilised for intensive dairy, lamb and beef production in South Africa, Enhancer ryegrass was developed from predominantly Italian types of Lolium multiflorum, with a minor Westerwolds component, by selecting for a higher concentration of total nonstructural carbohydrate (TNC) and lower moisture content than that currently available in commercial cultivars. The nutritional value of Enhancer was compared with Midmar ryegrass in a controlled environment study and in a grazing trial with weaned lambs; and with Dargle ryegrass in a grazing trial with Holstein dairy cows. Neutral detergent fibre, acid detergent fibre, lignin, nitrogenous compounds, mineral content and in vitro digestibility were also investigated as parameters of nutritive value. The anatomical features of Enhancer and Midmar were studied to determine possible structural differences. Weaned lambs grazed Enhancer and Midmar in an eight-paddock rotational grazing system, with 3.5 days spent in each paddock, allowing a 24.5 day regrowth period for the pastures. Holstein dairy cows grazed Enhancer and Dargle which were established on 16 and 19 hectare pastures, respectively. The n-alkane technique was used to estimate dry matter intake (DMI) in both grazing trials. Results from the controlled environment study suggest that the differences in the dry matter and TNC concentration of Enhancer are not positively linked to anti-quality factors associated with forage species, but can be attributed to genetic differences between the two grasses. Despite the significantly higher (P < 0.01) DMI of weaned lambs grazing Midmar compared with Enhancer, the lambs on Enhancer outperformed those on Midmar in terms of liveweight gain and carcass quality. The superior animal performance on Enhancer is likely due to an improvement in the readily digestible energy to protein ratio as a result of its significantly higher (P < 0.001) concentration of TNC compared with Midmar. Milk yield for cows grazing Enhancer in period 1 of the cross-over study was significantly higher (P < 0.05) than for cows grazing Dargle, despite the significantly lower (P < 0.05) DMI of animals on Enhancer. The higher TNC concentration relative to the true protein content of Enhancer would suggest that the protein metabolism in the rumen can be enhanced.