Effects of soybean small peptides on nitrogen balance, nutrient digestibility and several indices in the portal veinous plasma of goats

To study the effects of soybean small peptides (SSP) on nitrogen balance, nutrient digestibility and concentrations of glucose, ammonia and amino acids (AA) in portal veinous plasma (PVP) of goats, eight Xiangdong black goats were used in a 4 × 4 Latin square design. Goats were divided at random into four groups (two goats per group). Each group received the following four treatments in a different order over four treatment periods: SSP in the basic diet by feeding (SSPF), free AA (FAA) in the basic diet by feeding (FAAF), SSP by infusing through the duodenum fistula (SSPI), or FAA by infusing through the duodenum fistula (FAAI). Results showed that nitrogen retention (NR) in SSPI and SSPF goats was greater than FAAF (P < 0.01) and FAAI (P < 0.05) goats, and NR in FAAI goats was greater than FAAF goats (P < 0.05). Apparent protein digestibility (aDN) in SSPI goats was significantly greater than FAAF (P < 0.01) and FAAI (P < 0.05) goats, and aDN in SSPF and FAAI goats was greater (P < 0.05) than FAAF goats, respectively. Moreover, apparent biological value of protein (aBV) in SSPI goats was greater by 48.71% (P < 0.01), 32.71% (P < 0.05) and 20.98% (P < 0.05) than FAAF, FAAI and SSPF goats, respectively, and aBV in SSPF goats was greater (P < 0.05) than FAAF goats. Regarding the digestibility of dietary nutrients, there were no significant differences in crude fat digestibility among the different treatments. However, ME digestibility in SSPI goats was greater by 14.59% (P < 0.05), 10.10% (P > 0.05) and 3.40% (P > 0.05) than FAAF, FAAI and SSPF goats, respectively. But, ME digestibility in FAAF goats was less (P > 0.05) than SSPF goats. Total AA digestibilities in SSPI, FAAI, SSPF and FAAF goats were 78.74%, 69.55%, 73.88% and 64.69%, respectively. By contrast, dietary crude fiber digestibilities in SSPF and FAAF goats were greater than SSPI or FAAI goats. Glucose levels in PVP of SSPI and FAAI goats were greater (P < 0.05) than SSPF and FAAF goats. Ammonia levels in PVP of SSPF and FAAF goats were greater (P < 0.05) than SSPI and FAAI goats. AA level in PVP of SSPI goats was significantly greater than FAAF (P < 0.01), FAAI (P < 0.05) and SSPF (P < 0.05) goats, and the levels in SSPF and FAAI goats were greater (P < 0.05) than FAAF goats. Our results suggested that SSP is more effective than FAA in promoting nitrogen balance and protein biological value, and increasing digestibility of dietary nutrients and levels of glucose and amino acids in PVP of goats. Supplying SSP or FAA by infusing through the duodenum fistula was more advantageous than that by feeding.