Nitrogen Metabolism in Dairy Cattle. I. The Influence of Grain and Meadow Crops Harvested as Hay, Silage, or Soilage on Efficiency of Nitrogen Utilization

Efficiency of nitrogen utilization was studied in four experiments involving varying proportions of silage and hay, comparisons o£ silage and freshly cut meadow crop (soilage), effects of grain feeding, and feeding different ratios of roughage to grain.Feces and urine were collected separately and nitrogen balance and milk nitrogen determined.In all trials, lactating cows were allowed to eat roughage ad lib.No differences in nitrogen efficiency or urine losses were observed in cows fed different proportions of hay and silage.The nitrogen utilization response from freshly cut forages was different from that obtained with ensiled forages.Below 15% protein equivalent in the total ration, silage and soilage nitrogen were used with the same efficiency.Silage nitrogen declined in efficiency at higher levels, whereas soilage nitrogen was used as efficiently at the highest levels of protein equivalent measured as at the lowest.Adding grain to forage rations increased the efficiency of nitrogen utilization markedly, but increases in the grain content which raised the amount of grain from a 3:1 ratio of roughage to grain to a 2:1 ratio were without further effects.Efficiency of nitrogen utilization is of particular interest in dairy cattle feeding trials.Fundamentally, cost efficiency in feeding dairy cattle has been based on the utilization of forages as the principal sources of nitrogen and energy.That many dairymen were feeding higher levels of protein equivalent than could be used efficiently by dairy cows seemed probable, because of the continued trend toward increased production of high-nitrogen, more digestible roughages.This study was done with the present emphasis on efficiency of farm production in mind, in an effort to define more clearly the nitrogen status of dairy cows under conditions of free choice roughage consumption.Practical feeding experiments and nitrogen balance studies are the common methods used in evaluating the protein of feeds.Feeding experiments have had the practical advantage of measuring the end results in terms of a salable product (milk).The classical studies on protein requirements were made by this method (5, 8, 9).However, they required a relatively long period of time and provided no information on the metabolism of nitrogen.In nitrogen balance experiments the nitrogen consumed in the feed is determined along with the output nitrogen in the urine, feces, and milk in the case of lactating cows.These figures provide a measure of nitrogen metabolism and may be used to compute apparent digestibility and the efficiency with which nitrogen is used for productive purposes (4).Critical levels of nitrogen intake which will keep the test animal in approximate nitrogen balance must be used when comparing the protein value of different kinds of feeds, and the results apply only for the physiological state of the