An Evaluation of the Supplementation of Rumen-Protected Lysine and Methionine on the Lactation Performance of Fall Parturition Grazing Holstein Cows in Southern Chile

SIMPLE SUMMARY: This study assessed the supplementation of two limiting milk production amino acids for dairy cows (methionine and lysine) on the lactational performance of fall-calving grazing dairy cows from Chile. Four groups were compared and 26 cows per group were randomly assigned to the following groups: (i) a control group without supplementation; (ii) a lysine group; (iii) a methionine group; and (iv) a lysine plus methionine group. Supplementation was conducted from 2 to 70 days postpartum. The group supplemented with both amino acids produced more milk protein and fat than the control group. In conclusion, the supplementation of methionine and lysine improved milk protein and fat yield in grazing dairy cows with fall parturitions; therefore, they can be used as a strategy to improve the performance of lactation in cows. ABSTRACT: The aim of this study was to evaluate the effect of the supplementation of rumen-protected (RP) methionine and lysine on milk yield, solids, and body weight over time on fall-calving grazing multiparous Holstein cows from Chile. Four treatment groups were studied and compared for the outcomes over time. The treatments were as follows: (i) CON: control (n = 26); (ii) RP lysine group (LYS; 20 g per cow per day; n = 26); (iii) RP methionine group (MET; 20 g per cow per day; n = 26); and (iv) LYS × MET (RP lysine and RP methionine 20 g of each amino acid per cow per day; n = 26). Data were analyzed with general linear mixed model ANOVAs for repeated measures to primarily test the main effects of each amino acid and their interactions. The supplementation of the amino acids was conducted from 2 to 70 days postpartum. Overall, milk production tended to be higher in the MET and in the LYS × MET group when compared to the control group. Furthermore, CON produced significantly less milk protein (kg) and milk fat (kg) than the LYS, MET, and LYS × MET groups. Milk urea tended to be lower during the entire study in the CON group than the rest of the groups. There was a trend for a reduction in the losses of postpartum body weight in the LYS × MET than the CON. In conclusion, RP methionine and lysine improved milk fat and protein yield in grazing multiparous cows with fall parturitions; consequently, both RP amino acids can be used as a strategy for improving grazing cows’ production performance.