Economic and Environmental Sustainability of Irrigated Grass-Legume Mixtures

View the project final report

• 2016 annual report

Commodities

• Agronomic: general hay and forage crops

Practices

• Education and Training: demonstration, extension, networking, on-farm/ranch research, workshop

Grass-legume mixtures are considered important alternatives to nitrogen (N) fertilized grass pastures. Unfortunately, limited information is available on effects of seed mass ratios on productivity, soil health, and economic returns. This study evaluated the effects of grass-legume seed mass ratios and N fertilizer rates on forage accumulation, nutritive value, soil properties and water use, and profitability. There were 15 treatments arranged in randomized complete block design with four replications. The treatments included four species (meadow bromegrass [Bromus biebersteinii Roem. & Schult], three legumes (alfalfa [Medicago sativa L.]; sainfoin [Onobrychis viciifolia Scop.]; and birdsfoot trefoil [Lotus corniculatus L.]), various seed mass ratios (100:0, 50:50, 70:30, 50:25:25, and 50:16.7:16.7:16.7), and three rates of N (0, 56, and 112 kg N ha^-1). Nitrogen was applied only to meadow bromegrass monocultures. The average annual forage accumulation of meadow bromegrass receiving 112 kg N ha^-1 was 8603 kg ha^-1 yr^-1 which was similar to the 30% alfalfa + 70% meadow bromegrass, 30% birdsfoot trefoil + 70% meadow bromegrass, 25% alfalfa + 25% birdsfoot trefoil + 50% meadow bromegrass, and 50:16.7:16.7:16.7 mixture treatments. Mixtures had greater nutritive value than N-fertilized meadow bromegrass. All treatments except 100% sainfoin and 50% sainfoin + 50% meadow bromegrass treatments were profitable. Soil water depletion (SWD) between April 4 and August 15, 2015 ranged from 301 to 318 mm. Birdsfoot trefoil monoculture (100% birdsfoot trefoil) depleted the highest amount (318 mm) of soil water. Water use efficiency (WUE) ranged between 44 kg DM mm^-1 water (100% sainfoin; 50% sainfoin + 50% meadow bromegrass treatments) to 74 kg DM mm^-1 water (30% alfalfa + 70% meadow bromegrass treatment). Levels of N and meadow bromegrass and alfalfa mixtures affected soil potentially mineralizable nitrogen (PMN) but did not affect potentially mineralizable carbon (PMC). Nitrogen levels and meadow bromegrass - legume mixtures also affected soil microbial biomass. Based on forage accumulation, nutritive value, and profitability, the 30% alfalfa + 70% meadow bromegrass and 30% birdsfoot trefoil + 70% meadow bromegrass seed mass ratios are simple mixtures which may be viable alternatives to 100% alfalfa and N-fertilized meadow bromegrass monocultures. Results of the study were communicated to producers and professionals through field days, meetings, presentations, bulletin articles, and scientific publications.

1. Assess the effects of different ratios of grass-legume mixtures and nitrogen fertilizer rates on forage yield and quality;
2. Determine the effects of grass-legume mixtures on the persistence of legumes;
3. Assess the efficiency of water and nitrogen fertilizer use in the grass-legume mixtures cropping systems;
4. Determine the economic profitability of sole grass stands fertilized with nitrogen, sole legume stands, and grass-legume mixtures;
5. Evaluate the effects of grass-legume mixtures on soil organic matter buildup, pH, and microorganism populations.