Lactation performance and nitrogen efficiency of dairy cows fed increasing amounts of microencapsulated methionine

Amino acid requirements of high-producing dairy cows may be greater than what dietary and microbial crude protein can provide. Therefore, study objectives were to determine lactation performance of dairy cows fed rumen-protected methionine (MET; Timet®, VETAGRO, Italy). Individually-fed, multiparous Holstein cows (n = 48, 127 ± 41 DIM and 671 ± 8 kg BW) were used in a replicated 4 × 4 Latin square design with 28-d periods. Dietary treatments consisted of a basal TMR deficient in metabolizable MET (-10 g/cow/day): 1) Control (CON) with no supplement; 2) low methionine (LM) diet with 11 g/cow/d MET; 3) medium methionine (MM) containing 19.25 g/cow/d MET; and 4) high methionine (HM) diet plus 27.5 g/cow/d MET. Milk yield and feed intake data from the last 7 d of each period were used for analyses; BW, BCS and milk components were determined on d 27 and 28 of each period. Statistical analyses were performed using the MIXED procedures of SAS with square, period within square, and treatment as fixed effects and cow within square as a random effect. Linear, quadratic and cubic effects were also tested. No evidence of quadric or cubic effects was observed for any of the response variables. Dry matter intake was similar (P = 0.64) across treatments whereas milk production increased linearly from 38.0 ± 0.87 kg/d for CON to 39.7 ± 0.87 kg/d for HM. Milk fat concentration was similar across treatments (P = 0.83) averaging 3.69 ± 0.88% while milk protein concentration tended to increase linearly (P = 0.11) from 3.17 ± 0.04% for CON to 3.21 ± 0.04% with high MET supplementation. Overall, supplementation with MET resulted in greater (P ≤ 0.01) yield of milk protein and fat. Consequently, yield of energy-corrected milk (ECM) increased linearly; cows consuming the CON diet produced 38.3 ± 1.05 kg ECM/d whereas MET supplementation resulted in 41.4, 40.9 and 41.7 ± 1.05 kg ECM/d, for LM, MM and HM, respectively. Concentration of MUN averaged 13.5 ± 0.23 mg/dL across treatments (P = 0.58). These data suggest that the MET supplement increased the supply of metabolizable methionine resulting in increased yield of milk and milk components