The effect of nitrogen fertilization and stage of re-growth on the nutrititive value of kikuyu in the Midlands of KwaZulu-Natal.
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Date
2011
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Abstract
Kikuyu pasture was fertilized at low and high levels of nitrogen (N), namely 50 and 200 kg N/ha, after
mowing and clearing the plots, to induce low and high levels of N in the herbage. The subsequent growth
was harvested at 20-, 30- and 40-d re-growth. These treatments were conducted in spring, summer and
autumn. Treatments included level of N, stage of re-growth and season as variables in digestion trials using
sheep and voluntary feed intake (VFI) trials using long yearling heifers in pens equipped with Calan gates.
Nitrogen fertilization level had no impact on herbage dry matter digestibility (DMD). Stage of re-growth
influenced digestibility in the spring and summer, the highest values recorded in the 30-d treatment.
However, in the autumn, the 20-d re-growth recorded the greatest digestibility. Digestibility declined as the
season progressed. Digestibility was not correlated to any of the chemical fractions measured in the
herbage, including in vitro DM digestibility (IVDMD). Voluntary feed intake (VFI) followed a similar
trend to digestibility, with peak values recorded for the 30-d treatment in the spring and summer, while the
20-d material induced the greatest intake in the autumn. Nitrogen fertilization had a negative impact on
VFI over all seasons. Similarly to digestibility, VFI was not correlated to any of the chemical fractions
measured, but was correlated to digestibility and moisture concentration of the herbage.
Nitrogen degradability was determined using the in situ bag technique. Differences (p<0.05) were
recorded for the quickly degradable N (a) and potentially degradable N (b) fractions within season, but not
for the degradation rate of the slowly degraded fraction (c) per hour. The effective degradability (dg) was
not influenced by N fertilization level in the spring, while N fertilization increased the dg values in the
summer and autumn. Stage of re-growth exerted a positive effect (P<0.05) on the dg values.
Rumen pH, rumen ammonia and blood urea nitrogen (BUN) levels were measured in rumen fistulated
sheep. Rumen pH increased also with increasing level of N fertilization and declined with advancing stage
of herbage re-growth in the autumn. Rumen ammonia increased with time of sampling post feeding to 4 hrs
and then tended to decline by 6 hrs. Nitrogen fertilization level influenced rumen ammonia levels (p<0.05),
with the low N level producing the lowest rumen ammonia levels. Rumen ammonia levels were highest at
20-d re-growth stage in summer and at the 40-d re-growth stage in autumn. DM concentration of the
herbage had an inverse relationship with rumen ammonia. BUN levels were increased by high N
fertilization and were positively correlated to rumen ammonia levels.
Five years of digestibility data (82 digestion trials) and three years of intake trials (38 trials) data was
pooled. These data, chemical composition of the herbage and the daily maximum temperatures, rainfall and
evaporation recorded at and prior to the digestion and intake trials at Cedara were analysed using multiple
regression techniques. Rainfall and temperature in the period of cutting and fertilization had a negative
effect on digestibility, irrespective of the stage of re-growth at harvesting, 20, 30 or 40 days later, and a
combination of the two proved significant, accounting for the most variance in DDM. Temperature
depressed DMD by 11.4 g/kg DM per degree rise in temperature (Degrees C). Temperatures recorded during the
cutting and fertilization phase were highly negatively correlated to VFI, irrespective of stage of re-growth.
The DM concentration of the herbage as fed accounting for 32% of the variance in DMD, the NPN content
of the herbage accounting for only 12.2% of the variance and the ash concentration of the herbage
accounting for 15.9% of the variance in digestibility. Non-protein nitrogen was negatively correlated to
VFI. Both DMD and VFI were highly negatively influenced by the moisture concentration of the herbage.
Overall, the results of these trials demonstrated that environmental factors such as rainfall and temperature
had a far greater impact on the digestibility of kikuyu herbage than the chemical composition, which had a
minimal effect. Nitrogen fertilization did not influence herbage digestibility overall, but exerted a highly
negative impact on voluntary intake.
Description
Thesis (Ph.D.Agric.)-University of KwaZulu-Natal, Pietermaritzburg, 2011.
Keywords
Kikuyu grass--KwaZulu-Natal., Kikuyu grass--Fertilizers., Pastures--Fertilizers., Nitrogen fertilizers., Plants--Effect of nitrogen on., Kikuyu grass--Growth., Digestion., Feeds--Evaluation., Theses--Animal and poultry science.