NC1021: Nitrogen Cycling, Loading, and Use Efficiency in Forage-Based Livestock Production Systems (formerly NCT-196 and NC-189)
Statement of Issues and Justification
The amount of nitrogen (N) applied annually to forage production systems of the Midwest exceeds plant uptake (Mosier, 2001) and relatively little of the N consumed by grazing animals is removed from the ecosystem (Jarvis and Ledgard, 2002). 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.From an agronomic perspective, much is known about fertilizer type, amount, and timing of application for maximizing crop yields (Jenkinson, 2001). Likewise, forage quality and animal nutrition research are targeting ways of improving N use efficiency by plants (Singer and Moore, 2003) and livestock (Scholefield et al., 1991). The NC-189 Committee increased our understanding of protein utilization by grazing cattle (Klopfenstein et al., 2001; Johnson et al., 1998; Coblentz et al., 1998). Although mechanisms and pathways for N transformation and loss have been determined for grasslands and pastures (Ledgard 2001; Kroeze et al. 2003), major gaps exist in our knowledge of the relationships between management and harvest strategies and N pathways on farming and ranching enterprises (Mosier, 2001).
Agronomists have long studied the effects of N-laden soil amendments on plant growth, development, and production. Economical and efficient use of these soil amendments is critical to the long-term economic stability of many farming enterprises because commercial N fertilizers are among the most costly inputs for many production systems. Animal manures and other wastes are readily available alternatives to commercial N fertilizers in some areas; however, heightened environmental concerns demand responsible use of these by-products of confinement feeding operations. In addition, recent concerns about phosphorus (P)- loading may limit the potential to meet N requirements of growing crops strictly with animal manures and other wastes.
Traditionally, the efficiency of N use by forage plants or field crops has been measured by evaluating yield, total concentrations of N in the plant or in specific plant tissues, and N uptake. For forage crops this is often repeated over several harvests throughout the year (Cherney et al., 2002; Evers, 2002). Such evaluations are useful, and they clearly need to be continued in order to maximize the uptake and capture of N from applied soil amendments. However, these studies place little or no emphasis on whether ruminants can efficiently use these forms of N. In particular, there is a great void of information describing the effects of commercial N fertilizers and animal manures on the partitioning of N within plant fractions of nutritional significance to ruminants.
New feeding models for ruminant livestock, such as the Cornell Net Carbohydrate-Protein System (Russell et al., 1992; Sniffen et al., 1992; Fox et al., 1992) and the systems for dairy and beef production proposed by the National Research Council (NRC, 1989; 1996; 2001), require in-depth knowledge of the nutritional characteristics of forage proteins. This information was unnecessary with the old feeding systems that were based only on a measurement of total N (crude protein). Proper use of these new feeding systems returns economic benefits to dairy and beef producers through improved production and more efficient use of N from forage and concentrate feedstuffs. When highly degradable forage proteins are used more efficiently by ruminants, an additional benefit is realized because less N is returned to the environment via animal waste. In order to optimize the benefits of these new feeding models, accurate knowledge of the partitioning of N within the various fiber and cell-soluble fractions of the plant, and the relative degradability of forage N within the rumen is essential. This information can only be obtained by systematic evaluation of an enormous range of climatic, agronomic, and harvest management inputs that affect plant growth and development.
An important part of N use efficiency is the animal. Forages often contain more degradable protein than animals require but insufficient undegradable protein. The excess degradable protein is excreted in the urine. Strategies to increase forage production (N fertilization, interseeding legumes, etc.) often result in forages that contain N far in excess of animal needs. Identification of optimum grazing management and(or) supplementation strategies offer opportunities to increase N utilization. There is a voluminous literature on confinement feeding of beef and dairy cattle for maximum production. However, very little work has been directed toward developing strategies for precisely meeting animal requirements for metabolizable protein and amino acids without overfeeding crude protein in grazing situations. Low protein energy supplements may be an option, including byproduct sources of highly digestible fibers, and protein supplements that resist degradation and have amino acid patterns that are complementary to ruminal microbial protein also have promise.
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