Managing Nitrogen in Pastures
Nitrogen is the most limiting plant nutrient in agriculture due to the rapid turnover of nitrogenous compounds by soil microbial communities and limited storage availability within the soil matrix. With other macro-nutrients in check, the amount of nitrogen in the soil solution will determine seasonal forage dry matter production and thus beef production. Similar to field crop systems, producers and scientists alike have trouble to make forage crops take up even half of the applied N and convert it to plant protein. Nitrogen fertilizer is also very expensive to produce and it is unlikely that prices will come down anytime in the future. With these constraints in mind, what are feasible ways of improving the N-use efficiency in beef production systems?
There’s no simple answer, but a good start might be to reevaluate the entire production systems to achieve a certain goal. Slight changes of a single component will affect outcome and performance of the entire system. From a soil perspective, pH and fertility need to be optimized with regard to the desired forages grown. Improved forage varieties only function well in a narrow range of soil fertility. The time of fertilizer application also influences N utilization by plants. Annual and perennial forages have distinct seasonal growing curves, so N rates need to be adjusted for that.
All forages will readily respond to high N rates, but some forage crops are more frugal than others. Perennial forages are adapted to persist for several years with ever changing temperatures, solar radiation, and soil water status, and might be less sensitive to occasionally skipped N applications. Annual forages however, especially summer annuals such as pearl millet and sorghum varieties, are clearly more sensitive to low soil N and will not deliver their full yield potential if producers try to skimp with N fertilizer. In these cases, the overall inputs required for establishing summer annuals, including tillage, managing and harvesting them must not be limited by shortcomings in the fertilizer budget. Again, it is important to keep all other macro- (and micro-) nutrients in check to ensure optimum N uptake and utilization by plants.
With regard to forage legumes, they may or may not be the answer to increase N use efficiency in a beef production system or supply “cost-free” nitrogen. In general, N uptake by plants from decaying legume biomass or cattle feces is governed by the same biochemical principles and processes as the use of synthetic fertilizer. N compounds have to find their way into the soil nutrient solution before they can be taken up by other non-leguminous plants. Because of that, beef production systems containing forage legumes may not be any better in terms of N use efficiency than conventional systems, because the timing of N mineralization of legume biomass is difficult to manage. More importantly, the quantity of N accumulated in legume forage mass does not relate directly to the same hypothetical amount of synthetic N fertilizer applied and therefore cost-savings calculations are futile at best.
Grazing management is an area that can be constantly improved and adjusted, even in the presence of underlying natural laws of soil, plant, and animal biochemistry and behavior. Optimizing forage utilization in a grazing system with relatively high inputs in form of N fertilizer can be regulated with appropriate stocking methods. While rotational stocking allows for higher forage utilization than continuous stocking, it is not the only grazing method by any means with which the overall grazing system could be improved. Strip grazing is an excellent example that allows for high degrees of forage utilization such as required under the situation of stockpiled forage. Because most of the plant nitrogen taken up by cattle is excreted, stocking methods influence the distribution of feces and urine patches as well. Continuous stocking is the least advantageous here as animal will have more opportunities to congregate around water access points, shade areas, and feeding banks placed in a single pasture.
Dr. Dirk Philipp
Department of Animal Science, University of Arkansas—Fayetteville