Milk performance and oxidative status responses to rumen protected methionine supplementation in genotyped α-S1 casein lactating dairy goats fed two levels of metabolizable protein diets

•Rumen-protected methionine (RPM) increased raw milk production in goats fed low protein diets (14 % CP).•RPM increased milk protein content in goats fed high protein diets (16 % CP).•These differences in productive responses were affected by the genetic variant for alpha-S1 casein content in goat milk.•Goats fed RPM tended to have a higher plasma oxidative status than those fed no RPM.•Specific requirement for dietary methionine have to be studied for dairy goats relatively to their genetic variant for alpha S1 casein in milk.