Milk Protein Production in Cows Abomasally Infused with Casein or Glucose

Abstract

Lactating Holstein cows fed above standard allowances for energy and protein were abomasally infused with casein or glucose in four trials to study effects on milk protein production and related responses. Infusion periods were 5 to 7 days long, preceded and followed by a control period. Milk protein yields were increased by abomasal infusion of casein in three of four trials. Erratic intakes of feed masked treatment effects in one trial, but casein infusion still raised protein percent in milk. Higher milk yields and increased protein in milk both contributed to responses of milk protein. Isocaloric infusion of glucose or glucose plus casein raised protein production above controls but not as much as casein alone. Responses to casein infusion were similar when rations contained 14 or 38% of total nitrogen as nonprotein nitrogen. Nonprotein nitrogen in milk generally increased with protein infusion but represented only a minor part of the nitrogen increase in milk. Multiple regression analyses indicated that response to protein infusion depended more on milk yield than on amount infused. These studies demonstrate a specific response to abomasally infused protein in well-fed cows.