Complementarity of bird-resistant and non-bird-resistant varieties of sorghum stover with cottonseed cake and noug (Guizotia abyssinica) cake when fed to sheep
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A study was conducted to examine whether oilseed cakes (cottonseed cake (CSC) or noug (Guizotia abyssinica) cake (NSC)) are complementary to the bird-resistant (BR) and non-bird-resistant (NBR) varieties of sorghum stover and whether sheep select against morphological fractions containing high concentrations of tannins or proanthocyanidins (PA). Either 24 (growth study) or 16 (metabolism study) Ethiopian Menz sheep were used in a randomized complete block design to study the interaction of variety of sorghum stover with type of oilseed cake on feed selection, intake, digestibility, microbial protein synthesis, nitrogen (N) retention and growth. The degradability of the feeds and refusals was also determined. The NBR variety was more degradable than the BR variety during the early hours of incubation. NSC degraded faster, although to a lesser extent than CSC. Stover variety had no effect on ruminal ammonia concentration (RAC) while CSC induced a higher RAC than NSC did. Sheep given the NBR variety had more dust, less heads, less leaf, less sheath and more stem in their ingesta than those fed on the BR variety. It was seen that the sheep discriminated against the stems, since the ratio of stem in ingesta to stem on offer was <1. Since PA are more highly concentrated in the sheath than in any of the other plant fractions, it appears that the choice of the morphological composition of the ingesta by sheep was not determined by PA. Sheep selected a diet which was 2-5% or 5-11% more degradable than the offered or refused feeds, respectively. Relative to NSC, CSC enhanced the intake of N, N bound to neutral detergent fibre (NDF), digestibility and N retention and tended to increase (P < 0.06) liveweight gain (LWG) and organic matter (OM) intake but had no effect on microbial protein synthesis. Compared with the NBR variety, the BR variety depressed N digestibility and N retention, had no effect on microbial protein synthesis and on LWG, but depressed the efficiency of LWG by c. 30%. Sheep given the BR + NSC diet had the lowest daily gain but excreted copious amounts of urine, probably in an attempt to eliminate toxic waste substances. Since sorghum is a crop of the semi-arid regions, it is quite likely that, although the bird resistance trait increases crop yield, it has resulted in a feed that is rather less suitable for livestock in this production environment, where water shortages are frequent. In-depth studies on the consequences of water shortage on the utilization of bird-resistant sorghum stover are required.
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