Response of cows to rumen-protected lysine after peak lactation

Thirty Holstein cows (111 ± 18 DIM, 37.9 ± 5.6 kg/d of milk) were used in a crossover design trial with 2 periods of 3 wk each. Treatments included diets supplemented with 0 (CONT) or 37 g (LYS +) of rumen-protected Lys provided by Metabolys (H. J. Baker & Bros. Inc., Westport, CT). Diets were based on corn silage and formulated to provide adequate amounts of rumen-protected Met. No differences were observed in DMI, milk yield, or concentration of fat, lactose, or nonfat solids, but cows fed supplemental LYS + had a greater concentration of milk protein and tended to have higher yields of milk fat, protein, and energy-corrected milk (ECM) than cows fed CONT. Cows fed LYS + and producing more than 36 kg/d just before beginning the trial had greater concentrations of milk fat and yield of milk fat and ECM and tended to have higher milk protein concentrations compared with CONT. Cows producing less than 36 kg/d just before beginning the trial tended to have higher concentrations of milk protein when fed LYS + compared with CONT, but no other differences were observed. The results of this trial with cows after peak lactation suggest feeding supplemental rumen-protected Lys to cows producing more than 36 kg/d can result in improved concentrations of fat and protein and yield of ECM, but cows producing less than 36 kg/d do not respond positively.