UNL Researchers Examine Use of Cover Crops in Semi-Arid Conditions
Figure 2. Wheat yield following a variety of cover crops (colored bars) compared with yield following fallow (pink bars on the right) for both dryland and average precipitation (irrigated).
Figure 3. Winter wheat yield vs. water use at Akron, Colo., and Sidney sites in 2013. There were no significant differences between the sites in wheat yield response to a preceding cover crop under a wide range of water availabilities.
Potential Benefits Associated with Cover Crops
- Increased organic matter in the surface soil layer
- Improved soil structure and infiltration
- Reduced surface soil water evaporation
- Protection from wind and water erosion
- Increased snow catch
- Increased soil biological activity
- Increased nutrient availability
- Reduced nutrient loss
- Alleviating compaction
- Suppression of weeds, insects, and diseases
Research in western Nebraska and eastern Colorado suggests that dryland farmers in the semi-arid Central Great Plains should approach the use of cover crops with some caution.
Many soil health benefits have been associated with cover crops; however, there is little long-term university research data about how well they perform in semi-arid regions such as western Nebraska.
Many of the benefits have been reported in areas that receive more precipitation and have greater humidity than the High Plains region.
To evaluate cover crop use under semi-arid conditions, a two-year study (2012-2013) was conducted at UNL's High Plains Ag Lab near Sidney and at a USDA research station at Akron, Colo. Lead investigators were Gary Hergert, soil and nutrient management specialist, UNL Panhandle Research and Extension Center; David Nielsen, research agronomist, USDA Central Great Plains Research Station at Akron, Colo.; and Drew Lyon, former cropping systems specialist at the UNL Panhandle REC and now professor, Crop and Soil Science Department, Washington State University, Pullman, Wash.
At Sidney cover crops were spring-planted (mid-April) into proso millet stubble in a no-till system under two water availability conditions: rainfed and "average" rainfall. The "irrigated-to-average" rainfall treatments were irrigated every two weeks if additional water was needed to bring the total amount of precipitation for the season up to the 30-year long-term average precipitation.
The cover crop treatments included a 10-species cocktail mix of spring forage peas, lentils, common vetch, berseem clover, oats, spring barley, rapeseed, flax, phacelia, and safflower. For comparison purposes, we chose one species from each of the seed types, that is, legumes (spring forage peas), grasses (oats), Brassicas (rapeseed), and other broadleaves (flax), to plant as single-species cover crops along with the mixture to evaluate the claim that mixtures use less water than single-specie planting. We also included a no-till fallow as a check treatment. The cover crops were terminated (killed with herbicide) around June 20 to allow some soil moisture storage before wheat planting in mid-September.
- The water use and yield of wheat following cover crops were measured under two water regimes: rainfed and average rainfall. All cover crops (single species and mixtures) were found to reduce available water for the subsequent wheat crop (Figure 1), resulting in decreased wheat yield compared with wheat following fallow (Figure 2).
- Both at the HPAL and Akron, Colo. sites, cover crops used more water during the growing season than the amount of water that evaporated from the fallow plot. Water use for the subsequent wheat crop was reduced following cover crops, resulting in decreased yield compared with wheat following fallow.
- The multi-species cover crop mixture did not use less water than the single-species plantings.
- Cover crop water use reduced available soil water at wheat planting. Subsequent wheat yield was reduced by 5.94 bushels per acre for every inch of water used by the cover crop that was not replenished prior to wheat planting. Even when a cover crop was terminated early, yield loss was significant.
- Soil samples from both sites were analyzed for 16 microbiological populations and residual soil nitrate. As expected, both the numbers of total fatty acids and mychorrhyzal fungi were greater with cover crops where there was a growing crop with living roots compared with the fallow plot. The biological activity was similar between single-species and multi-species cover crop plantings.
- In addition to water, the cover crops used soil nitrogen (and other nutrients not measured) during growth. Soil nitrate to a 4-foot depth was 30 to 50 pounds less under cover crops than it was in fallow at wheat planting.
- The costs of the cover crop seeds used in this study ranged from $6 per acre to $40 per acre. The cost of the cover crop mixture was about $35 per acre, comparable to most single-species legume cover crops. Additional costs would include planting costs, seed shipping costs, cover crop termination costs, and loss of income due to yield reduction caused by the cover crop water use. Some of these costs would be offset if the cover crop could be grazed or a portion of it harvested and sold. However, grazing would likely reduce some of the cover crop benefits.
Apply Research Results to Your Operation
If you're farming in a semi-arid environment, are cover crops a good fit for your operation? With cover crops as with any new practice you're considering, we recommend you consider factors specific to your situation: environment, annual precipitation, soil type, cropping practices, management style, and variety selection as well as current research that relates to your farming environment.
Based on this research, the authors recommend that dryland farmers in the semi-arid Central Great Plains consider the following:
- All of the beneficial effects of cover crops that are reported from their use in more humid environments may not be observed in this region due to the greater evaporative demand.
- Findings from this research indicate that there will likely be a significant yield depression on the following wheat crop from growing a cover crop, even if termination of cover crop growth occurs by mid-June.
- Any beneficial effects (improved soil health, increased nitrogen availability) of growing the cover crop will need to be weighed against the economic consequences associated with the cover crop seeding costs and the potential yield drag in the subsequent wheat crop.
- Single-species plantings performed similarly to multi-species mixtures in this research, thus growers should consider whether the added cost of seed mixtures is a value.
David Nielsen, USDA Research Agronomist, Central Great Plains Research Station at Akron, Colo.
Drew Lyon, Former UNL Cropping Systems Specialist, and now Professor, Crop & Soil Science Department, Washington State University, Pullman, Wash.