Supplementing dairy cows with low in protein diets with specific amino acids is a promising strategy to counteract the potential negative effect of metabolizable protein deficiency on productivity. The objective of this study was to quantify the productive performance of lactating dairy cows and their milk quality when they are offered two diets with different crude protein (CP) concentration and when they are supplemented with methionine hydroxy analog (HMTBA). This study was carried out in a commercial dairy farm located in Alfaro Ruiz-Alajuela, Costa Rica, during the first semester of 2014. Twenty multiparous Holstein cows under a grazing system were grouped according to days in lactation and calving number. Two diets with different concentrations of CP (high 16.6% and 15.8% standard), with or without inclusion of HMTBA (25 g/TM) were studied in a 2x2 factorial arrangement. No significant effects (P>0.05) on milk production, 4% fat corrected milk, milk fat, milk protein, lactose, total solids or milk urea nitrogen (MUN) were found when including HMTBA. Feeding cows with high CP concentration significantly increased (P<0.05) MUN concentration from 18.33 to 20.70 mg/dl. Supplementing HMTBA to grazing cows did not have a significant response (P>0.05) regarding the different variables, and no financial return was found due to the fact that neither total milk production nor total solids were increased.

Keywords: essential amino acids, methionine analog, milk solids, milk production.