This thesis offers a solution for protein deficiency in human diets of rural people in developing countries. Dairy goats can provide locally produced protein rich foods. However, the low production of goats in these countries is often owing to low dietary intake of nitrogen and energy associated with low quality feeds. A combination of a cheap nitrogen (N) source and a readily available dietary energy source such as grain has been suggested to improve the productivity of milking goats.
The purpose of this thesis work was to determine if the amount and type of dietary N, and the ratio between N and available energy in the diet are important for enhancing milk protein and production in dairy goats.
In the first three feeding trials, the effects of feeding barley meal supplemented with either soybean meal (SBM), cottonseed meal (CSM) or urea on nutrient intake, nutrient digestibility, nitrogen balance (NB), microbial nitrogen synthesis (MNS), growth rate and milk protein and production were investigated using cross-bred (Anglo Nubian x Angora) growing goats and milking Saanen goats. There were no significant differences in hay intake, metabolizable energy (ME) intake, growth rate, nitrogen balance (NB), total ruminal volatile fatty acid (VFA) concentration, MNS and efficiency in growing dairy goats offered different dietary nitrogen sources, although the digestibility of nutrients was significantly (P< 0.01) improved. Nitrogen balance was significantly affected by N supplementation for milking goats, but the total nutrient intake, nutrient digestion, milk-feed efficiency, milk protein and production were not affected.
In experiment 4, milking Saanen goats were used to examine whether increasing the levels of N in the isocaloric diets could enhance nutrient digestibility, NB, milk protein and yield. No improvement was observed in nutrient intakes, milk-N and production, milk-feed efficiency, digestibility of dry matter (DM), organic matter (OM), neutral detergent fibre (NDF) and energy (DE) and NB. Digestible organic matter intake (DOMI) had more effect on milk production than dietary N intake. Any excess of dietary N was excreted in the urine. There was no benefit in feeding dairy goats a diet containing more than 2.4% N (15.3% protein). The nutrient complementary effects of nitrogen sources for the goats did not affect N metabolism presumably because the energy content of the diet was the limiting factor.
In experiment 5, milking Saanen goats were offered nine combinations of dietary urea (Ul= 0 g, U2= 80 g, U3=160 g) and energy (El= 1000 g, E2= 1200 g, E3= 1400 g barley meal) supplements. The goats were fed a basal diet of barley straw. Increasing dietary urea and readily available dietary energy did not significantly increase milk protein and milk yield, milk production efficiency or the total nutrient intakes or digestibility. Some combination of urea and readily available energy level gave significant responses. The U1E3 and U2E3 improved milk yield; U2E1 and U3E1 improved straw intakes; U2E2 and U3E3 improved nutrient digestibility. The U1El treatment gave significantly lower rates of MNS than other combination treatments. The highest rate of MNS was recorded in goats fed the U2U3 treatment, although this response was not statistically significant.
Significant relationships were observed between rate of MNS and digestible organic intake (DOMI) (r = 0.73; P<0.0001), gross energy intake (GEI) (r = 0.67; P<0.0001) and digestible energy intake (DEI) (r = 0.59; P<0.0001). The relationship between MNS and N intake was slightly weaker (r=0.55; P<0.0001). However, no relationship was observed between MNS and total VFA, rumen ammonia-N, or plasma urea concentration.
The daily MNS in dairy goats could be estimated using either of these following equations:
MNS (g/day) = 1.16 + 0.046 DOMI (g); (R2= 0.73; P<0.0001).
MNS = 0.116 + 0.996 GEI + 0.005 GEI x NX
(R2 = 0.78 and P< 0.0001).
It was concluded that the total ruminal VFA and ammonia-N concentration were responsible for the response of milking goats to the dietary supplement. Also of important was the synchronisation of the diumal mminal ammonia-N concentration with the diurnal variation of readily available energy.
Overall, the results suggest that urea is as good as soybean meal and cottonseed meal for growing and milking goats if sufficient dietary energy is readily available. However, to prevent ammonia loss from urea before it can be incorporation into microbial N, the intake of fermentable carbohydrate must be sufficient. Thus, urea remains a promising source of N for improving the production of milk from goats in developing countries. The effectiveness of dietary urea supplementation is dependent upon the concurrent supplementation of the diet with readily available energy sources. Urea plus grain supplementation is effective and cheaper than providing dietary amino-protein sources to milking goats.