Effects of supplementing low-quality roughages with tree foliages on digestibility, nitrogen utilization and rumen characteristics in sheep: a meta-analysis.

Thirty-six published studies containing 219 treatment means and 1069 sheep were included in the dataset for meta-analysis to elucidate responses of foliage supplementation on digestibility, nitrogen balance and rumen parameters. Major predictive variables were percentages of foliages in diets (FL), crude protein (CP) in foliages (FCP), NDF in foliages (FNDF), NDF in basal forages (BNDF), CP in basal forages (BCP), CP in total diets (DCP) and foliage CP intake (FCPI). Organic matter digestibility increased linearly (p = 0.10) with increasing FL, but responded quadratically to DCP (p = 0.021) and BCP (p < 0.001), with maximal OM digestibility occurred at 14.6% of CP in diets (R(2) = 0.12). The digestibility of CP was affected by FL (p = 0.01), FCPI (p < 0.026), FCP (p < 0.001), FNDF (p < 0.001) and DCP (p < 0.032) positively and quadratically, and maximal response was at 14.1% DCP (p < 0.001), 51% FL (p < 0.001), 29% FCP (p < 0.001) and FCPI of 122 g/day (p < 0.001). The digestibility of NDF increased quadratically (p = 0.09) with increasing FL, peaking at 16% foliage levels, decreased quadratically (p = 0.001) with increasing FNDF and decreased linearly (p = 0.048) with increasing FCPI. Digestible OM and CP intakes were the highest at 37.2% and 42.1% FL, respectively. Fecal N excretion increased linearly with increasing FL (p < 0.001) and DCP (p < 0.001, R(2) = 0.66). However, FCPI (p = 0.082), FCP (p = 0.003) and BCP (p < 0.001) affected fecal N excretion positively and quadratically. Urinary N excretion increased linearly (p < 0.001) with increasing FL (R(2) = 0.45), FCPI (R(2) = 0.79), FCP (R(2) = 0.51), BCP (R(2) = 0.72) and DCP (R(2) = 0.77). Rumen NH(3) concentration was affected positively and quadratically (p = 0.023) by FL with a peak rumen NH(3) level of 160 mg/l at 17.3% FL. However, rumen NH(3) concentration (mg/l) increased linearly with increasing FCP (p = 0.001), FCPI (p = 0.01) and DCP (p < 0.001). In conclusion, catalytic foliage supplementation at low levels, preferably, at 16% may enhance nutrient utilization, while foliage levels up to 42% may result greater performance of sheep fed on low-quality roughages.