The effects of intraruminal infusions of urea on the voluntary intake and milk production of cows receiving grass silage diets

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Abstract
Responses of dairy cows given silage diets to the intraruminal infusion of urea in progressively increasing doses were studied in four experiments, two with non-lactating cows and two with lactating cows. No clinical symptoms of NH3 toxicity were observed in any of the experiments. When urea was infused continuously, silage intake was depressed (P < 0·05) when the total supply of N exceeded the equivalent of 250g crude protein (CP)/kg DM in the total diet. However, when the urea load was administered twice daily, as opposed to continuously, intake depression (P < 0·05) occurred at the equivalent of 170g CP/kg DM. At the higher doses of urea, concentrations of NH3 in peripheral blood increased and were accompanied by increased concentrations of glucose and reduced levels of insulin in plasma. In general, responses of milk production followed those of silage intake but there was evidence of greater proportional reductions in the yield of lactose relative to that of fat and protein. It is concluded that the voluntary intake of high-protein silages may be depressed by factors associated with high rates of absorption of NH3 from the rumen.