초록
The feeding values of four indigenous Kenyan sorghum cultivars and the effects of polyvinylpyrrolidone (PVP) on the utilization of high tannin sorghum by broiler chicks were studied in two 3-week feeding trials. In Experiment 1, one hundred and five broiler chicks (initial average weight 97 g) were randomly assigned to each one of the seven grain-soybean meal diets. The diets consisted of maize [diet 1; no assayable tannin], white sorghum [diet 2; 0.59% catechin equivalents (CE)], cream sorghum [diet 3; 0.94% CE], light brown sorghum [diet 4; 2.71% CE] and dark brown sorghum [diet 5; 3.54% CE]. Diets 6 and 7 were included to test the possibility of overcoming the detrimental effects of sorghum tannins by adding PVP at 0.25% and 0.5% to dark brown sorghum, which resulted in dietary tannin levels of 3.46% and 3.38% CE respectively. In Experiment 2, the effects of tannin on dry matter digestibility (DMD) and nitrogen (N) retention were studied in a 3-week substitution assay in which high tannin sorghum (5% CE) was substituted for white maize at different inclusion levels. Ninety broiler chicks aged 7 days (initial average weight 102 g) were randomly assigned to each one of the six diets. The diets consisted of corn gluten meal and fishmeal as protein sources plus maize [diet 1] and high tannin sorghum at different inclusion levels [diets 2, 3, 4, 5 and 6], resulting in dietary tannin levels of 0, 1.25%, 1.66%, 2.08%, 2.5% and 3.2% CE respectively. Feed intake, feed efficiency and body weight gain were measured weekly. In Experiment 2, tannin absorption, DMD and N retention were measured on days 19, 20 and 21. The results of Experiment 1 showed that feed intake, feed efficiency and body weight gain were all affected by treatment (p<0.05). Diets 1, 2 and 3 gave similar body weight gains and all were better than diets 4 and 5 (i.e. 504, 517, 473 g, vs. 256, 267 g). Similarly, feed efficiencies were higher (p<0.05) for diets 1, 2 and 3 compared to diets 4 and 5 (0.4, 0.42, 0.39 vs. 0.21, 0.23). When 0.25% PVP was added to the dark brown sorghum (diet 6) there was no significant improvement in chick performance (p>0.05). However, addition of 0.5% PVP (diet 7) resulted in significant improvement (p<0.05) in body weight gain compared to the untreated dark brown sorghum. Overall, PVP did not completely overcome the deleterious effects of tannins. The results of Experiment 2 indicate that sorghum inclusion level and subsequent tannin level had no effect on feed intake, feed efficiency, weight gain, DMD and N retention. The above results suggest that tannin level should be limited to below 2.71% CE in broiler chick diets containing 20% CP and 0.4% methionine. However, in diets with 23% CP and 0.8% methionine tannin level of up to 3.2% will not affect performance. Consequently high tannin sorghum (5% CE) can be used to substitute for white maize by up to 100% in broiler chick diets.