Protein value of diets for dairy cows with different proportions of crude protein originating from red clover silage versus soybean meal

The objective of this study was to evaluate the effect of replacing crude protein (CP) of soybean meal (SBM) with red clover silage (RCS) in total mixed rations (TMR) on ruminally undegraded feed CP (RUP) and intestinal digestibility (ID) of RUP. Four TMR were formulated to replace CP from SBM with RCS. The TMR was comprised of forage and concentrates (0.75:0.25), with ratios of RCS in TMR of 0.15, 0.30, 0.45, and 0.60 on a dry matter (DM) basis. The ruminal degradation of CP was determined using the nylon bag technique. For this, samples of TMR were incubated for 2, 8, 16, 24, 32 and 72 h. The RUP content of TMR was estimated using passage rates of 2, 5 and 8%/h. Additionally, samples of TMR were incubated in the rumen for 16 h, and their feed residues were used for determination of ID of RUP using the mobile-bag technique. For in situ ruminal degradation, increasing RCS and reducing SBM proportions increased linearly the rapidly degradable CP (a) from 26.2 to 212 g/kg CP (P <  0.001). The potentially degradable CP (b) decreased linearly from 810 to 676 g/kg CP (P < 0.001). Assuming a passage rate of 8%/h, RUP reduced linearly from 315 to 253 g/kg CP (P < 0.001). The ID of RUP reduced linearly from 760 to 574 g/kg CP (P < 0.001) with increasing RCS levels. Although differences in ruminal CP degradation and ID of RUP were apparently caused by differences in the proportion of RCS and SBM, neither linear nor quadratic effects on total tract digestible CP were observed (P = 0.22 and 0.21, respectively), and averaged 943 g/kg CP among all TMR tested. This suggests that increasing RCS and decreasing SBM proportion shifted the site of CP utilization. The CP degraded in the rumen increased, and the amount of intestinally absorbable RUP reduced linearly from 184 to 79 g/kg CP (P < 0.001). In general, replacing CP from SBM with RCS in the TMR reduced its protein value. It is concluded, that an almost complete replacement of CP from SBM by RCS in rations of high-yielding dairy cows cannot be recommended. However a replacement rate up to 0.30 does not lead to a substantial reduction in RUP supply and digestibility, and therefore considerably reduces the use of SBM.