Digestibility and protein utilization in wethers fed whole-crop barley or grass silages harvested at different maturity stages, with or without protein supplementation

Effects of whole-crop barley and grass silages harvested at different maturity stages, with or without protein supplementation, on intake, in vivo digestibility, feces characteristics, and protein utilization in wethers were evaluated. Whole-crop barley silage harvested at heading stage (BH) and at medium milk stage (BM), grass silage (GE) taken at the flag leaf-early heading stage, and grass silage (GL) taken at medium-late heading stage were fed to eight wethers in two 4 x 4 Latin squares. Wethers in one square were fed supplementary rapeseed meal. Experimental periods lasted for 4 wk and wethers were fed ad libitum during the first 3 wk, with intake recorded during the third week. During the fourth week, wethers were fed 80% of ad libitum, and feces and urine were collected during the last 4 d. The GE and BH diets had greater (P < 0.05) in vivo apparent digestibility of DM and its nutrients, lower proportion of fecal particle DM (PDM) with a greater proportion of small particles compared with GL and BM diets, respectively. The GE diet had greater (P < 0.001) in vitro OM digestibility and in vivo digestibility of OM and fibre, resulting in a smaller (P < 0.001) proportion of PDM with a greater (P < 0.001) proportion of small particles compared with the other diets. In vivo NDF digestibility was negatively related to fecal PDM across forage types (R-2 = 0.91, RMSE = 2.55). The GE silage had greater CP concentration, and animals fed the GE diet had greater intake of CP (P < 0.001) and sum of the degradable CP fractions A, B-1, and B-2 (P < 0.01), resulting in greater (P < 0.05) urinary nitrogen (N) excretion than when fed any of the other diets and a lower (P < 0.05) N retention compared with BH and BM diets. Microbial N supply tended to increase when animals were fed the BH diet (P = 0.10) and when rapeseed meal was added to the forages (P = 0.08). Increased N intake (P = 0.008) by rapeseed meal supplementation increased urinary N excretion in gram per day (P = 0.05). The strong relationship between in vivo NDF digestibility and fecal PDM indicates potentials for using PDM as a cheap method to predict NDF digestibility. Early harvest of the forages improved in vivo digestibility of nutrients, resulting in less fecal PDM with a greater proportion of small particles compared with late harvest within forage type. However, wethers fed the GE diet had greater urinary N losses compared with wethers fed the GL diet but this effect of maturity was absent when fed whole-crop barley silage.