Milk production in Holstein cows supplemented with different levels of ruminally protected methionine A. Lara a,c , G.D. Mendoza b, * , L. Landois c , R. Barcena c , M.T. Sa ´nchez-Torres c , R. Rojo c , J. Ayala a , S. Vega b a Universidad Auto ´noma Chapingo, Estado de Me ´xico, Mexico b Universidad Auto ´noma Metropolitana, Xochimilco, Departamento de Produccio ´n Agrı ´cola y Animal, Mexico c Colegio de Postgraduados, Montecillo, Estado de Me ´xico, 56230, Mexico Received 25 April 2006; received in revised form 25 April 2006; accepted 27 April 2006 Abstract Forty multiparous cows (599 kg F 18 kg BW) were fed with a basal diet (19.6% CP, 35% RUP, and 1.7 Mcal kg À 1 NEl) with alfalfa, corn silage and concentrate (49% forage: 52% concentrate). After calving, cows were randomly assigned to the treatments, which consisted in four levels of ruminally protected methionine (RPM): 0, 8, 16 and 24 g d À 1 of MepronRM85 (Degussa Co.). Experiment was conducted for 120 days with measurements of milk production, composition, body weight, body condition score and DM intake every 15 days (3 consecutive days) starting on day 5 postpartum. Data were analyzed with the repeated measures model (four treatments in 8 periods through lactation). No treatment effects were detected on DM intake (20.38 F 2.51 kg d À 1 ), body weight (599.78 F 19.78 kg), body condition score (2.51 F 0.19 units) and milk fat. However, milk production and protein yield were increased with addition of RPM ( P b 0.01). Milk production responded quadratically to methionine level. Holstein cows with a mean production of 35 kg d À 1 milk require addition of ruminally protected methionine (16 g d À1 ) to improve milk production. D 2006 Elsevier B.V. All rights reserved. Keywords: Methionine; Milk composition; Milk production; Dairy cow 1. Introduction It is well known that the high producing cow needs bypass protein to meet essential amino acid require- ments to maximize milk production. Methionine and lysine have been considered as the main limiting amino acids for milk protein synthesis (Schwab et al., 1992; Rulquin and Delaby, 1997). Ruminally pro- tected amino acids have been used to increase duodenal flow of essential amino acids to improve milk production. There are several studies with ruminally protected methionine (RPM) which show increments in milk production (Ferguson et al., 2000), 1871-1413/$ - see front matter D 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.livsci.2006.04.032 * Corresponding author. Tel.: +52 595 95 2 02 00; fax: +52 595 95 2 02 79. E-mail address: gmendoza@correo.xoc.uam.mx (G.D. Mendoza). Livestock Science 105 (2006) 105 – 108 www.elsevier.com/locate/livsci