Effect of additives (brewer’s grains and biochar) and cassava variety (sweet versus bitter) on nitrogen retention, thiocyanate excretion and methane production by Bach Thao goats
This experiment aimed to measure N retention and rumen methane production when goats had access to mixed sweet and bitter varieties of cassava foliage (SW-BI) compared with the sweet variety alone (SW). In each feeding system, biochar (1% of diet DM) and brewers’ grains (4% of diet DM) were supplied singly or in combination or not at all. Eight Bach Thao goats (LW 16.4 ± 2.03 kg) were allocated to a double 4*4 Latin square design, with one square for each of the main effects of cassava variety. The four combinations of additives were the treatments within each of the Latin squares.
Levels of condensed tannins were similar in leaves of sweet and bitter varieties of cassava and were higher in leaves than in petioles. Leaves of the bitter variety had 72% more precursors of HCN than leaves of the sweet variety. HCN precursors were higher in leaves than in petioles.
The goats with free access to both cassava foliage varieties (BI-SW) consumed equal parts of each, but the overall intake of DM was 25% higher and the N retention was 23% higher compared with feeding the sweet variety alone. N retention was increased when either brewers’ grains or biochar or both were added to the diet. The combined effect of mixed cassava foliage with brewers’ grains and biochar represented a 58% increase in N retention compared with sweet cassava alone and no additives. Rumen ammonia and methane were lower when the goats had access to both sweet and bitter cassava foliage. The reduction in rumen methane was positively correlated with N retention. Dietary treatments had no effect on the pattern of rumen VFA .
It is proposed that HCN precursors present in higher concentrations in the leaves of bitter compared with sweet cassava leaves induced a partial shift in digestion of nutrients from the rumen to the lower digestive tract facilitating more efficient use of dietary protein by enzymic digestion in the small intestine and reducing the formation of methane which is not produced when fermentation takes place in the cecum-colon.
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