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.
- Assess the effects of different ratios of grass-legume mixtures and nitrogen fertilizer rates on forage yield and quality;
- Determine the effects of grass-legume mixtures on the persistence of legumes;
- Assess the efficiency of water and nitrogen fertilizer use in the grass-legume mixtures cropping systems;
- Determine the economic profitability of sole grass stands fertilized with nitrogen, sole legume stands, and grass-legume mixtures;
- Evaluate the effects of grass-legume mixtures on soil organic matter buildup, pH, and microorganism populations.
The study was established in September 2013 at the University of Wyoming Sheridan Research and Extension Center, Sheridan, WY. The experiment consisted of 15 treatments arranged in a randomized complete block with four replicates. The treatments comprised a perennial cool season grass (meadow bromegrass, cultivar ‘Fleet’), three legumes (alfalfa, cultivar ‘WL 363 HQ’; sainfoin, cultivar ‘Shoshone’; and birdsfoot trefoil, cultivar ‘Norcen’), three rates of N fertilization (0, 56, and 112 kg N ha-1) and five ratios of grass:legume mixtures (100:0, 50:50, 70:30, 50:25:25, and 50:16.7:16.7:16.7). Urea was used as the source of N (46% N) which was applied in two splits only to pure grass stands twice (April and October) in each year.
Treatment ratios were calculated based on pure live seed and recommended seeding rates. The calculated actual seed needed for each pure stand were 22, 39, 11, and 22 kg ha-1 for alfalfa, sainfoin, birdsfoot trefoil, and meadow bromegrass, respectively. The actual amount of seed needed for each pure stand was taken as 100% in calculating the mixture ratios. Plots were clipped two to three times each year depending on weather conditions. Clipped sampled were oven-dried and dry weights of samples were taken and expressed as weight per unit quadrat area. This was then used to calculate forage DM yield (kg ha-1). Soil moisture was monitored between April 4 and August 15, 2015 using a neutron probe (Hydroprobe Model 503DR, CPN International, Inc., Martinez, CA, USA). Soil water depletion was calculated by subtracting the total water transpired from total water input (irrigation + precipitation + residual soil moisture at the beginning of the growing season). Water use efficiency was calculated by diving forage DM yield by the total water transpired. Forage nutritive value was evaluated using near infrared reflectance spectroscopy (NIRS, Foss InfraXact analyzer, Silver Spring, MD).
Forage DM varied among treatments in 2014, 2015, and 2016 (Table 1). In 2014, meadow bromegrass receiving the highest N rate (100% meadow bromegrass + 112 kg N ha-1) produced the highest DM (5980 kg ha-1). In general, forage DM in mixture treatments were relatively lower than their respective sole stand treatments. However, an opposite trend in forage DM was observed in 2015. The highest yield (8580 kg ha-1) was observed in 25% alfalfa + 25% birdsfoot trefoil + 50% meadow bromegrass treatment (Table 1; Figure 1). Forage DM continued to increase significantly in 2016, except sainfoin monoculture, with the highest yield (16000 kg ha-1) in 50% alfalfa + 50% meadow bromegrass and 25% alfalfa + 25% birdsfoot trefoil + 50% meadow bromegrass treatments. Average forage yield was higher in 2016 than in 2015 and 2014. This was expected as the forage stands were well-established by 2015. Soil water depletion ranged from 301 to 318 mm (Table 1). Birdsfoot trefoil monoculture (100% birdsfoot trefoil) depleted the highest amount (318 mm) of soil water. Water use efficiency 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).
Forage nutritive value was higher in legumes (alfalfa, birdsfoot trefoil, and sainfoin) than meadow bromegrass (Table 2). This resulted in higher forage nutritive value in grass-legume mixtures than sole grass treatments. The nutritive value of birdsfoot trefoil was comparable to alfalfa in 2015 and 2016. For example, crude protein concentrations in birdsfoot trefoil were 231, 296, and 180 g kg-1 in 2014, 2015 and 2016, respectively which were similar to that of sole alfalfa (286, 292, and 176 g kg-1 in 2014, 2015, and 2016, respectively). Sainfoin although produced forage of higher nutritive value compared to meadow bromegrass, alfalfa and birdsfoot trefoil were of generally higher nutritive value than sainfoin. Nitrogen application to sole meadow bromegrass stands did not improve nutritive value.
There was significant influence (P = 0.0003) of levels of N and meadow bromegrass and alfalfa mixtures on soil potentially mineralizable nitrogen (PMN). Presence of alfalfa in grass sward significantly (P = 0.0003) suppressed PMN to 15 and 18 mg kg-1 soil compared to alfalfa monoculture which had 29 mg kg-1 soil (Figure 2). Grass – legume mixtures had similar effect on soil PMN as N fertilizers in grass monocrops (8 and 11 mg kg-1). There was, however, no significant difference (P = 0.429) in potentially mineralizable carbon (PMC) from different grass – legume mixtures and N rates (Table 3). The amount of labile carbon ranged from 433 to 705 µg CO2-C g-1 soil day-1.
There was significant treatment effects (P = 0.009) of varying levels of N applied to grass and meadow bromegrass – legume mixtures on soil microbial biomass (Table 3). Alfalfa monoculture (5796 ng g-1 soil) and meadow bromegrass receiving 112 kg N ha-1 (5332 ng g-1 soil) harbored similar amounts of microbes each of which was significantly greater than 70% meadow bromegrass + 30% alfalfa mixture (4047 ng g-1 soil) and meadow bromegrass receiving 56 kg N ha-1 (3861 ng g-1 soil). Meadow bromegrass – alfalfa (50:50) mixture and meadow bromegrass monoculture without any N had 4771 and 4617 ng of microbial biomass g-1 soil, respectively which was intermediate compared to the rest of the treatments. Treatments also significantly affected (P = 0.045) total bacterial biomass and similar effects as in total microbial biomass were recorded. Meadow bromegrass fertilized with 56 kg N ha-1 and the mixture of meadow bromegrass in a ratio of 70:30 significantly depressed (<0.05) bacterial biomass (2143 and 2252 ng g-1 soil, respectively) compared to monocultures. There was no significant change in fungal biomass as a result of different grass – legume mixture ratios and N applications.
Educational & Outreach Activities
Results of the study were presented in field days at Sheridan Research and Extension Center (2014, 2015, and 2016); in Western Alfalfa and Forage Symposium at Reno, Nevada (2015); Department of Plant Sciences Seminars and posters (2015 and 2016); ASA-CSSA-SSSA conference at Phoenix, Arizona (2016), and in Wyoming Forage Field Day at Sheridan (2016). The field plots were demonstrated to about 100 participants at the Wyoming Forage Field Day. Four field day bulletin articles and two proceedings article were published from the study. Additionally, one peer-reviewed journal article, a peer-reviewed bulletin, and two bulletin articles are in press.
Adjesiwor, A.T., Islam, M.A., Zheljazkov, V.D., Ritten, J.P., and Garcia y Garcia, A. 2017. Grass-Legume Seed Mass Ratios and Nitrogen Rates Affect Forage Accumulation, Nutritive Value, and Profitability. Crop Sci. 57:1–13 (in press).
Adjesiwor, A.T. and Islam, M.A. 2017. Establishing and managing of birdsfoot trefoil monocultures and mixtures in Wyoming. Field Day Bulletin, University of Wyoming Agricultural Experiment Station (in press).
Ashilenje, D.S. and Islam, M.A. 2017. Changes in Plant Community Structure Influence Forage Yield and Quality of Irrigated Meadow Bromegrass-Legume Mixtures in Wyoming. 2017 Field Days Bulletin, University of Wyoming Agricultural Experiment Station (in press).
Ashilenje, D.S. and Islam, M.A. 2017. Meadow Bromegrass in Mixture with Alfalfa Affects Light and Nitrogen Acquisition, Forage Yield, and Nutritive Value. 2017 Field Days Bulletin, University of Wyoming Agricultural Experiment Station (in press).
Islam, M.A. 2016. Managing Plant Diversity for Resilient Forage Systems and Environmental Benefits in the Mountain West. Proceedings of the ASA-CSSA-SSSA International Annual Meetings November 7-9, 2016 Phoenix, AZ (American Society of Agronomy, Crop Science Society of America, Soil Science Society of America).
Ashilenje, D.S. and Islam, M.A. 2016. Evaluation of Forage Productivity and Environmental Benefits of Meadow Bromegrass in Various Mixtures with Popular Legumes under Irrigation. 2016 Field Days Bulletin, University of Wyoming Agricultural Experiment Station, pp. 143-144. Available at http://www.uwyo.edu/uwexpstn/_files/docs/2016-field-days-bulletin.pdf.
Ashilenje, D.S. and Islam, M.A. 2016. Alfalfa Growth Forms, Light Capture, and Nitrogen Fixation Interact to Influence Durability of Legume in Meadow Bromegrass Mixtures. 2016 Field Days Bulletin, University of Wyoming Agricultural Experiment Station, pp. 27-28. Available at http://www.uwyo.edu/uwexpstn/_files/docs/2016-field-days-bulletin.pdf.
Adjesiwor, A.T. and Islam, M.A. 2015. Grass, legumes, and grass-legume mixtures: yield, nutritive value, and soil water use. In: Proceedings, 2015 Western Alfalfa & Forage Symposium Reno, Nevada December 3‐4, 2015. Western Alfalfa & Forage Symposium, 1521 I Street, Sacramento, CA 95814. (http://calhay.org/symposium/)
Adjesiwor, A.T., Islam, M.A., Jeliazkov, V., Ritten, J.P., and Garcia y Garcia, A. 2015. Productivity and Profitability of Irrigated Grass‐Legume Mixtures. 2015 Field Days Bulletin, University of Wyoming Agricultural Experiment Station, pp. 123-124. Available at http://www.uwyo.edu/uwexpstn/_files/docs/2015-field-days-bulletin.pdf.
Adjesiwor, A.T., Islam, M.A., Jeliazkov, V., Garcia y Garcia, A., and Ritten, J. 2014. Irrigated Grass-Legume Mixtures. 2014 Field Days Bulletin, University of Wyoming Agricultural Experiment Station, pp. 121-122. Available at http://www.uwyo.edu/uwexpstn/_files/docs/2014-field-days-bulletin.pdf.
Meadow bromegrass was generally compatible with all legumes evaluated in the study. In terms of forage accumulation in mixture treatments, meadow bromegrass was the dominant species, especially in mixtures with alfalfa and/or birdsfoot trefoil. Meadow bromegrass-legume mixtures, especially those containing alfalfa and/or birdsfoot trefoil, are potential substitutes for 100% alfalfa or N-fertilized 100% meadow bromegrass. The 100% sainfoin and 50% sainfoin + 50% meadow bromegrass treatments were not profitable after two years of establishment. Nitrogen fertilizer application to meadow bromegrass was profitable but might not be sustainable in the longer term. In general, grass-legume mixtures had lower SWD than sole legumes. On the other hand, legumes when mixed with grass improved WUE. 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.
All treatments were profitable two years after establishment except only the 100% sainfoin and the 50% sainfoin + 50% meadow bromegrass treatments (Table 4). The method used in estimating the value of birdsfoot trefoil and sainfoin hay may have underestimated the value of hay from these crops. Thus, a greater market value of sainfoin might improve the profitability of this crop. Mixtures, especially those containing alfalfa or birdsfoot trefoil, were profitable. Cost of stand establishment (field preparation, seeds, and planting) was the major cost factor in all treatments evaluated in this study. However, the crops evaluated are perennials and do not require annual re-establishment. Average annual production cost is, therefore, expected to decrease as stand life is increased. Stand life is, therefore, a major factor determining economic returns. Greater net incomes are therefore, expected if stands remain productive for longer periods, especially for the treatments receiving no annual N fertilization.
The study presented a major strategy to increase farm yield and reduce production cost. As legumes improved the quality of hay compared to sole grass, grass-legume mixtures produced higher yields than fertilized sole grass stands. This finding generated a lot of interest from producers. Through the outreach activities, such as agriculture research and extension field days, forage field days, conferences/presentations, producers gained insights into how to manage irrigation water and weed management to ensure optimum productivity. Benefit-cost analysis resulted in increased interest to producers for adoption the grass-legume practices with appropriate mixture ratios. As a result, a few producers in the region have started planting birdsfoot trefoil as monoculture as well as mixture with grasses.
A producer in Torrington, Wyoming adopted the birdsfoot practice in his farm and is very successful. Therefore, diffusion of the new practices and technologies is expected to increase because of producer-to-producer strong networking.
Some areas of additional studies include determining: change in plant community structure; indicators of crop persistence; carbon sequestration and N fixation using stable isotopes; trace gas emissions from grass mixtures and grass treated with different rates of N; soil moisture change; and in-depth microbial biomass. It is also important to conduct post-adoption surveys to measure percent increase or decrease in adoption.