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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

2012 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:
During FY-2012: A two-yr grazing study was completed evaluating the effects of finishing beef cattle grazed on tall fescue (TF) pastures without or with nitrogen (N) fertilization on growth performance, ruminal fermentation, and carcass characteristics. In response to N fertilization, overall average daily gain (ADG) was greater (P<0.05) and dry matter intake trended upward (P=0.07) in steers that grazed TF+NF pastures (P < 0.05). Regardless of N fertilization, ADG peaked between week four and eight (1.05 kg/d; P < 0.01), and then declined until week 16. Greater total volatile fatty acid (VFA) concentrations were detected in ruminal fluid of steers that grazed TF+NF (P < 0.01), but only a minor effect was shown on individual VFA (acetate, propionate, and butyrate) concentrations and acetate-to-propionate ratio. Back fat thickness, ribeye area, and intramuscular fat concentration did not differ between treatments. Generally, this study indicates that N fertilization on TF affected ruminal fermentation which positively influenced growth performances, but did not affect carcass characteristics of grazing beef steers. In addition, readily fermentable carbohydrate supplementation is needed to improve utilization of increased dietary CP due to N fertilization and consequently enhance growth performances of grazing steers. A peer-reviewed journal article on this study in the Professional Animal Scientist is in press. An in vitro continuous culture study was conducted to investigate energy supplementation strategies on pasture forages to assess effects of energy supplementation [no concentrate, 30% ground corn, or 30% dried distilled grains with solubles (DDGS)] with 4 pasture forages TF without N fertilizer (TF-NF), TF with N fertilizer (TF+NF), TF-alfalfa mixture, and TF-birdsfoot trefoil mixture (TF+BFT)] on vitro ruminal fermentation and N utilization. The results indicated that supplementing pasture forages with corn or DDGS enhanced microbial assimilation of ammonia-N and shifted metabolic pathways of microbial fermentation and methane gas production. Supplementation of corn in the TF+BFT elicited a similar ammonia-N concentration compared when corn was supplemented in the TF+NF. Therefore, grass-legume mixtures would be a sustainable component in grass grazing systems to improve N utilization efficiency with appropriate energy supplementation. This study was recently published in the peer-reviewed journal, The Professional Animal Scientist. The small plots of binary mixtures of five grasses [orchardgrass (OG), tall fescue (TF), meadow brome (MB), timothy, and perennial ryegrass (PR)] and three legumes [alfalfa (AF), birdsfoot trefoil (BF), and cicer milkvetch (CM)] were established and data collection initiated. Ratios in the mixtures included 0, 25, 50, and 75% legume composition. Tall fescue, OG, and MB grass-legume mixes averaged 6.0, 5.0, and 14.0% higher forage production than their respective grass monocultures. The highest seasonal forage production of TF combinations was 1.62 Mg/ha TF:AF (50:50), 1.63 Mg/ha TF:BF (75:25), and 1.64 Mg/ha TF:CM (75:25). Highest forage production of OG combinations was 1.10 Mg/ha OG:AF (50:50), 1.09 Mg/ha OG:BF (75:25), and 0.99 Mg/ha OG:CM (75:25). Highest seasonal forage production of MB combinations was 1.23 Mg/ha MB:AF (50:50), 1.25 Mg/ha MB:BF (75:25), and 1.11 Mg/ha MB:CM (75:25). These preliminary results suggest that grass-legume mixtures can be an effective strategy to improve pasture productivity. In addition, the nutrient cycling experiment is underway. The paddocks were established, and baseline soil samples were collected in the fall of 2011. Grazing and data collection began in 2012. Soil subsamples were analyzed for available nitrogen (ammonia and nitrate) and for total N by combustion. Leachate samples are being collected every two weeks during the growing season. Samples are analyzed for nitrate-nitrite. Plant samples, collected before and after each grazing event, will be used to determine the nutrients removed in the forage. A mass balance approach comparing total nitrogen outputs against total nitrogen inputs for each treatment will be utilized to estimate losses due to volatilization. The effect of tannins on nitrogen cycling will be examined.

4. Accomplishments

Last Modified: 06/27/2017
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