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Nitrogen Cycling, Loading, and Use Efficiency in Forage-Based Livestock Production Systems
Department of Animal and Food Sciences
The amount of nitrogen (N) applied annually to forage production systems of the Midwest and Eastern U.S. exceeds plant uptake and relatively little of the N consumed by grazing animals is removed from the ecosystem. Significantly greater N is removed via mechanical harvesting for feed but the same problem occurs when the forage is fed. Nitrogen fixation by legumes and purchased feed supplements contribute additional N to the system. There is little potential for highly mobile N to accumulate in soils, so the fate of this surplus N is waterbodies, groundwater, the atmosphere, and adjacent terrestrial ecosystems where it can have undesirable effects.
The purpose of this research is to determine the potential for dietary carbohydrate to alter N recycling in forage-fed ruminant animals, and ultimately, to affect efficiency of N use by the animal. The use of both ruminal and postruminal carbohydrate infusion will allow us to predict the effects of different carbohydrate sources or processing methods which are known to influence the ruminal fermentability of dietary carbohydrates. Results from this research will aid livestock producers in making management decisions that can simultaneously improve forage-based livestock productivity and decrease negative effects on the environment.
2009 Project Description
Results have been disseminated through refereed journal articles and in presentations at national meetings of the American Society of Animal Science and at annual multistate research committee meetings.
Stocking rate studies provided data necessary for determining economically optimum stocking rates for stocker producers grazing endophyte-infected fescue in the transition zone. These data show that systems which result in the maximal conversion of exogenous N into animal product are not those which are considered economic optima under current marketing and regulatory conditions.
Studies evaluating movements of nitrogen through various metabolic pools within forage-fed steers indicated that we have minimal ability to influence overall efficiency of N utilization by manipulating site and/or quantity of carbohydrate infusion into the GI tract of beef cattle. However, of particular importance with respect to managing N fluxes in pastoral environments with beef cattle is the elasticity demonstrated in the form of N output. This work demonstrated shifts in the proportion of total excreted N that was in urine subsequent to a fairly modest shift in source of dietary energy. Our maximum response, a 28% decrease in urinary N excretion (countered by a concomitant increase in fecal N excretion) was effected with ruminal infusion of starch hydrolysate equivalent to 20% of the dietary ME.
The urea kinetic data generated in this work can be used to provide inputs for refinement of current prediction models and development of novel models and to define the putative mechanisms limiting whole body nitrogen utilization in ruminants consuming high quality forages. In turn, this will provide opportunities to manipulate N utilization efficiency in order to minimize N excretion into the environment and maximize incorporation into animal products.
Additional grazing studies on tall fescue-based pasture confirmed that considerable variation in fertilizer nitrogen extraction efficiencies existed between pastures and between years. The most significant influencing variable between years was that the use of lower N application rates resulted in nearly a doubling of the fertilizer nitrogen extraction efficiency.
The use of growth promoting implants has come under increased scrutiny in the U.S. and some emphasis has been placed on growing beef cattle without the use of hormonal implants. However, the increases in efficiency of animal growth consequent to the use of implants translates to increasing the efficiency of N use within the pasture ecosystem.
El-Kadi, S. W., K. R. McLeod, N. A. Elam, S. E. Kitts, C. C. Taylor, D. L. Harmon, B. J. Bequette, and E. S. Vanzant. 2008. Nutrient net absorption across the portal-drained viscera of forage-fed beef steers: Quantitative assessment and application to a nutritional prediction model. Journal of Animal Science 86: 2277-2287.
Liao, S. F., E. S. Vanzant, D. L. Harmon, K. R. McLeod, J. A. Boling, and J. C. Matthews. 2009. Ruminal and abomasal starch hydrolysate infusions selectively decrease the expression of cationic amino acid transporter mRNA by small intestinal epithelia of forage-fed beef steers. Journal of dairy science 92: 1124-1135.
Taylor-Edwards, C. C., N. A. Elam, S. E. Kitts, K. R. McLeod, D. E. Axe, E. S. Vanzant, N. B. Kristensen, and D. L. Harmon. 2009a. Influence of slow-release urea on nitrogen balance and portal-drained visceral nutrient flux in beef steers. Journal of Animal Science 87: 209-221.
Taylor-Edwards, C. C., G. Hibbard, S. E. Kitts, K. R. McLeod, D. E. Axe, E. S. Vanzant, N. B. Kristensen, and D. L. Harmon. 2009b. Effects of slow-release urea on ruminal digesta characteristics and growth performance in beef steers. Journal of Animal Science 87: 200-208.