Performance of lactating Simmental cows fed two diets differing in the content of digestible intestinal protein (PDI)

In each of two trials, two diets differing in intestinal digestible protein (PDI) content (LP: low protein, 70 g kg-1 dry matter; HP: high protein, 120 g kg-1 dry matter) were given to multiparous Simmental cows in early lactation. Effective rumen degradabilities of nitrogen and non-protein dry matter of feed ingredients, estimated by the nylon-bag technique, were used to calculate PDI contents of diets. In trial 1, two groups of six cows were fed, in two successive periods of 7 weeks each, the diets LP and HP allocated according to energy requirements (LPR and HPR). In trial 2, the diets LP and HP were fed in two periods of 6 weeks each to 24 cows (six cows/group) according to energy requirements (LPR and HPR) or ad libitum (LPL and HPL). In trial 1, total and energy-corrected milk (ECM) yields were significantly higher (P < 0.01) for cows receiving diet HPR (20.7 and 19.2 vs. 18.9 and 17.4 kg day-1), while milk fat and protein contents were similar. Live weight changes were different between groups (-126 and +39 g day-1 respectively, for LPR and HPR diets) but this difference was not significant. In trial 2, cows fed HPL diets had significantly higher milk yields compared with the LPL and LPR diets (20.0 vs. 17.7 and 17.4 kg day-1, respectively, P < 0.05), but not with the HPR diet (19.0 kg day-1). ECM yield was higher (P < 0.05) for cows receiving more protein both between restricted groups (18.4 vs. 16.4 kg day-1, respectively for HPR and LPR groups) and between ad-libitum groups (18.8 vs. 17.0 kg day-1, respectively for HPL and LPL groups). Milk composition was not affected by dietary treatments. Diets fed according to requirements caused negative, but differing, live weight changes (-783 and -44 day-1 respectively for LPR and HPR diets, P < 0.05). For ad-libitum treatments, LPL cows lost live weight while HPL cows gained live weight (-221 vs. 433 day-1, respectively, P < 0.05). In the present experiments, increasing the supply of dietary PDI improved milk production even if the recycled nitrogen and live weight change could have contributed to protein requirements.