Nutritive value of Mucuna pruriens and effects of replacing soybean meal with mucuna on in vitro rumen fermentation, lamb performance and meat safety.

摘要

An experiment was conducted to monitor maturity-related changes in the nutritive value of velvet bean (Mucuna pruriens) and this was followed by three experiments aimed at determining how mucuna supplementation affects nutrient digestion, animal performance and safety of meat from sheep. Experiment 1 evaluated the stage of maturity at which the dry matter (DM) yield and nutritive value of mucuna is optimized. Mucuna pruriens was harvested and analyzed for DM yield, chemical composition and in vitro digestibility (IVDMD) at 77, 110 and 123 days after planting (DAP). As the plant matured, DM yield increased linearly, IVDMD remained unchanged and yield of digestible DM increased linearly. Therefore, the ideal maturity at harvest for optimizing biomass yield and nutritive value was approximately 123 DAP. Experiments 2 and 3 determined the fermentability of mucuna seeds (M), soybean meal (SB) and SB treated with 138 g/kg DM of L-dopa (SBD) using the in-vitro consecutive batch culture (CBC) technique and automated, wireless gas production system respectively. In Experiment 2, DM digestibility (g/kg) and gas production (ml/g DM) from M were higher than those from SB and SBD, but SB and SBD had similar DM digestibility. Therefore, like SB, mucuna seeds were readily fermentable in the rumen. Adding L-dopa to SB did not affect the extent of digestion or composition of fermentation products. Experiment 3 showed that the total gas production (ml/g DM), immediately fermentable fraction and slowly fermentable fraction from M were higher (P < 0.05) than those from SB. The addition of L-dopa increased the slowly fermentable fraction and total gas production of SB. Therefore, adding L-dopa to SB increased the extent of SB fermentation. Over the 24-h fermentation period, the concentration of mucuna L-dopa increased initially and then decreased, while that of added L-dopa linearly decreased. Therefore, this study shows for the first time that L-dopa is physically degraded during ruminal fermentation. Experiments 4 and 5 determined the effect of replacing SB with mucuna seeds on the performance of lambs and investigated the fate of the ingested mucuna L-dopa in the circulatory and digestive systems and tissues of the lambs. The treatments were supplements formulated by substituting 0 (SBM), 33 (Lo), 67 (Med) or 100 (Hi) % of SB with rolled mucuna seeds (M). Lambs were fed a basal diet of corn, cottonseed hulls, molasses, and urea (Experiment 4) or a basal diet of coastal bermudagrass (Cynodon dactylon) hay, corn, and molasses (Experiment 4). Dry matter intake was greater in sheep fed SBM than those fed mucuna diets, but N intake, DM digestibility, N retention, microbial protein yield, blood urea N (BUN) and blood glucose concentrations were similar in mucuna and SB diets. Among sheep fed mucuna diets, N retention, microbial protein yield and the efficiency of microbial protein synthesis increased with level of mucuna inclusion. Experiment 5 showed that Rambouillet and Florida native sheep gained more weight on the SB diet than on diets containing mucuna. However, final BW, hot carcass weight and dressing percent and concentrations of blood glucose, L-dopa and its metabolites in blood and muscle tissues, and acute phase proteins were not affected by mucuna inclusion. Muscle L-dopa concentrations of all lambs were low and within the normal range (< 5 ng L-dopa/g), hence ingested mucuna L-dopa did not accumulate in muscle tissue. These experiments indicate that replacing SB with mucuna did not adversely affect the performance and welfare of sheep and that mucuna seeds can safely be used as a protein supplement for ruminant livestock. The meat from ruminants fed mucuna diets in the manner described in this study does not contain toxic levels of L-dopa; therefore it is safe for human consumption.