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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):
(1) Compare livestock performance, economics, and subsequent carcass characteristics from beef grazing grass monocultures and low- and high-tannin grass-legume mixtures versus traditional feedlot-based finishing; (2) Determine best possible grass-legume mixtures and plant densities that maximize pasture productivity and nutritional quality; and (3) Determine the effects of tannins on nutrient cycling in grazing systems.


1b.Approach (from AD-416):
To compare livestock performance, economics, and subsequent meat quality of beef produced from grass monocultures versus low- and high-tannin grass-legume mixtures.

(1) Determine if tall fescue (TF)-legume mixed pastures will enhance livestock performance (ADG, feed efficiency, intake, and nutrient digestibility) and to compare if the animal performance will be further improved when animals are grazed on TF-birdsfoot trefoil (BFT) mixed pasture; (2) Investigate in vitro fermentation characteristics of forage pastures used in grazing study with emphasis on microbial protein synthesis and ruminal fatty acid (FA) composition; (3) Assess how different compositions of pasture alter carcass characteristics and beef quality; (4) Determine the differences in the FA composition of beef from pasture-finished vs. feedlot-finished cattle and the time required for changes in FA composition to occur; and (5) Assess the economics and barriers to adoption of grazing-based beef production, develop an effective extension program to facilitate the adoption of grazing management for beef production, and create an awareness of the beneficial nutrient management and other environmental impacts of such an exchange.


3.Progress Report:

During FY-2012: A two-yr grazing study was completed, and animal performance results have been published. The purpose of the study was to evaluate 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 each grazing season, 18 Angus crossbred steers were arranged in a completely randomized design with repeated measures on the following two treatments: TF without N fertilizer (TF–NF) and TF with N fertilizer (TF+NF). Three replicated pastures with 3 steers per replicate were assigned to each treatment. A total of 168 kg/ha N fertilizer was applied in three split applications at 56 kg/ha each to the TF+NF in each grazing season. Steers rotationally grazed on 0.47-ha pasture for 7 d with a 28-d rotation interval for a total of 16 wk. Body weight data and pasture forage samples were collected every four weeks, whereas ruminal fluid was collected at weeks 4, 10, and 16. After the completion of 16-wk grazing, ultrasound measurement was performed to assess carcass characteristics. In response to N fertilization, greater CP concentration was detected on TF+NF compared with TF–NF (P < 0.01). 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 4 and 8 (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. Ruminal ammonia-N (NH3-N) concentration increased (P < 0.01) with N fertilization, while NH3-N:total VFA increased (P < 0.01) with the progression of grazing seasons. 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 accepted and in press.

An in vitro continuous culture study was conducted to investigate energy supplementation strategies on pasture forages. Eight dual-cow continuous culture fermentors (700 mL) were used to assess effects of energy supplementation [no concentrate, 30% ground corn, or 30% dried distilled grains with solubles (DDGS)] with 4 pasture forages [tall fescue (TF) without N fertilizer (TF-NF), TF with N fertilizer (TF+NF), TF-alfalfa mixture, and TF-birdsfoot trefoil mixture (TF+BFT)] on in vitro ruminal fermentation and N utilization. Twelve dietary treatments were tested in a completely randomized design with a 3 (energy supplements) × 4 (pasture forages) factorial arrangement. Forages were supplied in four equal portions at 0600, 1200, 1800, and 2400 h, while energy supplements were fed in two equal portions at 1200 and 2400 h. Three replicated runs lasted 10 d each, with the first seven days allowed for microbial adaptation to the diets, and three days for sampling. Mean culture pH was affected by energy supplementation (P < 0.01), but not by forage, ranging from 5.94 to 6.44. Total VFA concentration increased by corn, but decreased due to DDGS supplementation (P < 0.01). As a result of interaction between energy supplements and pasture forages, the greatest VFA concentration was achieved when corn was added in the TF+NF (P < 0.01). Decreases in ruminal ammonia-N concentration, methane production, and acetate-to-propionate ratio were observed when corn or DDGS was added in the diets (P < 0.01). These results indicate that supplementing pasture forages with corn or DDGS enhanced microbial assimilation of ammonia-N and shifted metabolic pathways of microbial fermentation. 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 was begun. Ratios in the mixtures included 0, 25, 50, and 75% legume composition. Seasonal forage production of unfertilized TF, MB, and OG monocultures was 1.45, 1.02 and 0.96 Mg/ha respectively. 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). Individual harvests showed a similarly higher yield of the mixtures over the monocultures. Relative species composition had an overall effect on total forage yield. Mixtures with CM and BF were most productive when they constituted 25% of the mix, and AF at 50% of the mix. 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 have been reestablished, and baseline soil samples were collected in the fall of 2011. Grazing and data collection began in 2012. Soil subsamples are 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 were analyzed for nitrate-nitrite. Plant samples, collected before and after each grazing event, were used to determine the nutrients removed in the forage. Herbage dry matter analyses and total nitrogen (N) were analyzed. 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.


Last Modified: 9/1/2014
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