Effect of duodenal ornithine infusion in cows on milk production and blood plasma constituents at two levels of feeding

An ornithine solution, corresonding to 36.1 g ornithine/d, or an equivalent blank solution were each continuously infused into the duodenum of two lactating cows of the Friesian breed. The cows were fed either on a high energy and high protein diet (110% of energy and protein requirements) or a low energy and low protein diet (75% of requirements). Milk composition and milk yield were recorded, and blood plasma contents of amino acids, glucose, growth hormone (GH) and insulin were analysed. The procedure was repeated 1 year later. Ornithine had no effect on milk yields or blood components when thehigh energy and high protein diets were given. On the contrary, it significantly increased milk, lactose, and fat production with the low energy and low protein diets, while milk protein secretion was slightly affected. The levels of nonessential and essential amino acids in blood plasma were enhanced, except for histidine and methionine. The latter amino acids likely were limiting for milk protein synthesis. An increase in glucose supply in the underfed animals likely occured after ornithine administration, since there was a significant increase of plasma glucose and protein levels and total lactose production. Ornithine infusion increased the GH secretion only when the low energy and low protein diets were given. It was concluded that infusion of the non-casein amino acid ornithine into the duodenum increases milk secretion in underfed cows, and that GH likely is involved in this phenomen.