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United States Department of Agriculture

Agricultural Research Service

Research Project: Grass-Legume Mixtures to Enhance Sustainability of Irrigated Pastures

Location: Forage and Range Research

2011 Annual Report

1a. Objectives (from AD-416)
Recent high costs of nitrogen fertilizer and the need for increased environmental stewartship necessitate a renewal of the mixed grass-legume pasture. Past research was not indicative of the irrigated, rotational grazing systems common to the western U.S. There are few guidelines on which species and grass-legume ratios optimize economic and environmental sustainability. This experiment proposes to (1) compare livestock performance, economics, and subsequent meat quality of beef produced from grass monocultures versus low- and high- tannin grass-legume mixtures, (2) determine best possible grass-legume mixtures and plant densities that maximize pasture productivity, and (3) determine if high-tannin legumes can reduce potential nitrogen-based environmental impacts in grass grazing systems. Overall this research will evaluate the economic and environmental benefits of grass-legume pasture mixtures as compared to using commercial nitrogen fertilizer.

1b. Approach (from AD-416)
Objective 1. Livestock performance and carcass characteristics will be evaluated using thirty six Angus crossbred steers grazing the following four treatments: 1) tall fescue with no fertilizer, 2) tall fescue with fertilizer, 3) tall fescue-alfalfa mixture, and 4) tall fescue-birdsfoot trefoil mixture. Treatments will be arranged in a randomized complete block design with three pasture replicates, four paddocks per pasture, and three steers per pasture. Grazing will be for 7 days per paddock on a 28-day rotation interval. Forage samples will be obtained and steer body weight will be recorded at 28-d intervals throughout the study. Forage samples, and ruminal fluid and blood samples will be obtained on day 0, 28, 84, and 140 for analysis of ruminal fermentation, metabolism, and fatty acid (FA) composition. At the end of the grazing period, steers will be slaughtered at a commercial meat plant, and carcasses will undergo FA analysis, tenderness, sensory panel evaluation, and lipid and color stability measurements. A fifth treatment consisting of a typical feedlot-based diet will be included in order to compare pasture- and feedlot-finished cattle. All procedures and data collection will be similar to those described in the grazing treatments. Objective 2. Optimum grass-legume mixtures will be determined by evaluating tall fescue, meadow brome, orchardgrass, alfalfa, birdsfoot trefoil, and cicer milkvetch in grass/legume binary mixtures. Legume plant densities of 0 (with N fertilizer), 0 (without N fertilizer), 25, 50, 75, and 100 percent will be tested with each grass for a total of 48 treatments. Grazing pressure will be applied to the entire experiment for 7 days on a 28-day rotational interval. Immediately prior to each grazing period, one-half of each plot will be harvested with a forage plot harvester. Forage production and forage quality parameters including crude protein, neutral detergent fiber, acid detergent fiber, acid detergent lignin, and in vitro true digestibility will be evaluated. Objective 3. The effects of tannins on nutrient cycling will be evaluated for the plant, soil, and soil water phases. Plant samples will be collected before and after each grazing event and herbage dry matter and total nitrogen (N) will be analyzed to determine the nutrients removed in the forage. Soil samples will be collected in the spring, prior to grazing, and in the fall after the growing season to a depth of 1.5 meters. Four soil cores will be taken in each plot and divided into three subsamples: 0-30 cm, 30-60 cm, 60-152 cm. Composite soil subsamples for each depth will be analyzed for available nitrogen (ammonia and nitrate) and for total Nitrogen. Soil water (leachate) nitrogen will be monitored by means of zero-tension lysimeters that were previously installed to a 120 cm depth. Leachate will be collected from the lysimeter collection basin every two weeks during the growing season and winter months. Samples will be analyzed for nitrate-nitrite. A mass balance approach comparing total nitrogen outputs against total nitrogen inputs for each treatment will be utilized to estimate losses due to volatilization.

3. Progress Report
The objective of this project was to determine the production, environmental, and economic potential of grass-legume pastures in comparison to fertilized and unfertilized grass monocultures. The project has three components including evaluating livestock performance and carcass quality when grazing mixtures, determine optimum grass-legume mixtures, and determine the effect of high-tannin legumes on nitrogen-based environmental impacts. Livestock performance was evaluated using large plots (0.5 ha) of tall fescue monocultures, tall fescue-legume mixtures (alfalfa and birdsfoot trefoil), and feed-lot. Treatments to determine mixtures for optimum pasture productivity included small plots of binary mixtures of grasses and legumes. During FY-2011: The work outlined in the SOW was initiated. It was discovered that legumes had died out of previously established pastures. These tall fescue-legume (alfalfa and birdsfoot trefoil) pastures were re-established in 2011. However, the graduate student on this project used the existing pastures to collect a second year’s data on livestock intake and gains, fatty acid profiles, ruminal pH and ammonia, and forage production and quality comparing tall fescue with and without N-fertilizer and feedlot treatments. Preliminary results were presented at the Annual Meeting of Western Section, American Society of Animal Science in June 2011 and at the Joint Annual Meeting of ADSA-ASAS in July 2011, and indicated that applying N fertilizer to tall fescue did not influence growth performance or carcass characteristics. Additional research initiated during FY2011 included installation of lysimeters during the re-establishment of the tall fescue-legume pastures, which were used to determine the initial nitrogen in the leachate. These pastures will be grazed next year. The small plots of binary mixtures of five grasses (orchardgrass, tall fescue, meadow brome, timothy, and perennial ryegrass) and three legumes (alfalfa, birdsfoot trefoil, and cicer milkvetch) were established and data collection was begun. Ratios in the mixtures included 0, 25, 50, and 75% legume composition. Samples from these plots were used to initiate development of an NIRS equation that can determine composition of legume in mixture plots.

4. Accomplishments

Last Modified: 06/27/2017
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