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Abstract
Maximizing livestock production is of economic importance to the producer. Obtaining increased production through feeding high levels of crude protein (CP) has been extensively studied. The results of these studies are conflicting, while some show that high levels of CP are beneficial, others indicate it is detrimental to optimum reproductive performance. Many changes have been made within the last decade to increase livestock production and researchers have reevaluated methods of feeding livestock including protein recommendations (NRC, 1975). The dietary crude protein requirement for high producing dairy cattle (30 to 50 kg milk/ day) ranges from 14 to 22% of dietary dry matter (NRC, 1978). Feeding a 20% CP ration to dairy cows elevates blood ammonia and urea concentrations, compared to rations with lower CP and may be one mechanism by which high dietary CP adversely affects reproductive performance. The adverse effect may be due to the ammonia toxicity directly or to the secondary changes in intermediary metabolism. Ammonia and urea concentrations in excess of what the body can metabolize are toxic with a common symptom being abnormal behavior, which is theorized to be the result of interference with brain metabolism (Visek, 1979). Since manifestation of estrous behavior is critical to optimum reproductive performance this is also a possible mechanism by which the high CP acts (Jordan and Swanson, 1979). Still another way in which high dietary CP may exert a deleterious effect on reproduction is by interference with amino acid metabolism which may adversely affect brain energy metabolism thus interrupting the metabolic cycles of the body, including the reproductive cycles (Visek, 1979).
A factor which may alter the effect of CP levels on reproductive performance, is the form in which N is present in the diet. Preformed proteins are expensive and in great demand for the feeding of non-ruminants. Therefore, the capacity of rumen bacteria for de novo synthesis of protein from relatively cheap sources of non-protein nitrogen (NPN) has been utilized by feeding compounds like urea and biuret. These compounds tend to result in high rumen ammonia levels. The higher than normal ammonia levels in the blood, will induce higher than usual ammonia concentrations in the seminal fluids, which may produce a suboptimum environment for the spermatozoa and thus reduced fertilizing capabilities. Even though adverse effects of high dietary CP on reproduction have not been demonstrated consistently, reports have associated high dietary CP with increased services per conception and calving intervals (Jordan, et. al., 1983). Mechanisms for these effects include possible alteration of pituitary or ovarian function or changes in the uterine environment (Visek, 1984). Alterations of energy metabolism due to elevated nitrogen intakes may also be a factor. Protein also may interact with fiber components or with energy density of the ration to influence reproductive performance.
Resolving the conflicting results and identifying the effects of dietary protein on reproductive efficiency would permit balancing the dietary protein at a level optimal for both high production and efficient reproduction.
