159 Grazing management effects on nitrogen utilization and carbon cycling in pasture-based systems.

Abstract Grazing cattle broadly affect the carbon cycle through consumption of plants containing carbon dioxide and release of carbon dioxide back to the atmosphere through respiration or excretion, primarily from feces. Methods to increase capture of soil organic carbon from grazing cattle have focused on management decisions for a “best” stocking method, stocking intensity, cover crop grazing, or various ways to improve uniform distribution of forage intake and fecal excretion. Rarely is it assessed through the lens of feedstuff selection or animal utilization of nutrients, which inherently affects enteric animal emissions. Stocker cattle and beef cows are frequently supplemented with mineral, energy, protein, or all three at some point during their grazing tenure that changes forage intake, plant selection, and nutrient excretion. The common occurrence of nitrogen delivered in a supplement makes it an important contributor to the carbon cycle. Less than a quarter of consumed nitrogen is retained in the animal; therefore, providing an opportunity to improve nitrogen utilization. This limits nitrogen loss from changing forms during the nitrogen cycle, providing incentive to retain fed protein contributing to animal product (milk, meat, etc.). The objective of this presentation is to examine methods of influencing nitrogen utilization on carbon cycling in pasture-based systems. Utilization of nitrogen by the animal can be altered by feedstuff type or an ionophore such as supplementing with dried distillers grains with solubles or monensin. Monensin specifically has been shown to spare protein in the rumen, potentially affecting bypass protein to the hind gut and fecal nitrogen excretion. Changing nitrogen utilization may also be accomplished by modifying metabolizable protein of the supplement relative to energy. Delivery of added nutrients in a supplement also encourages forage substitution that may slow rate of plant defoliation affecting the rate of both carbon and nitrogen cycling. If cattle are consuming low quality forages, supplying additional nitrogen will increase forage intake, negatively affecting the carbon cycle through increased methane emission. Pasture grazing management practices are highly variable through the U.S. due to differences in resources (forage, soil, cattle type, landmass, etc.) which makes it difficult to determine a best practice using environmental resources to optimize carbon cycle contributions. Nutrient supplementation practices that increase animal nitrogen retention may be counterproductive to increasing favorable nutrient excretion and subsequent soil organic carbon. The tradeoff is that supplementing may improve animal productivity to lesson grazing pressure days or reduce enteric emissions directly affecting the carbon cycle. Changing metabolism or nutrient retention in cattle receiving supplemental nutrients may positively affect cattle productivity with either useful or futile effects on the carbon cycling in pasture grazing systems.