Cover crops are getting a good deal of attention this season. Frequently mentioned benefits include improved soil quality (such as increased soil organic matter, improved water infiltration, reduced soil erosion), reduced need for external inputs such as nitrogen fertilizer and herbicides, and grazing material for livestock.
This last benefit does not strictly fit with the definition of cover crops in the Resource Conservation Glossary of the Soil Conservation Society of America (1982). It defines cover crops as "a close-growing crop grown primarily for the purpose of protecting and improving soil between periods of regular crop production or between trees and vines in orchards and vineyards."
Forage crops are grown for grazing or hay and may be managed to provide some of the same benefits as a cover crop; for example, by grazing less intensely or swathing at a greater height above the ground.
To the extent that extra biomass is left in the field for ground cover, economic returns from forage production will be reduced. To paraphrase an old economics axiom, you can't have your cover crop and eat it, too.
Dryland crop producers in the Nebraska Panhandle should be skeptical about the feasibility of growing cover crops in this semiarid and highly variable climate. With little or no direct economic return from cover crops, the water they use is difficult to justify from an economic perspective.
In certain situations, some forage crops may produce sufficient income to justify their water use and the typically negative effect on the subsequent crop. This usually requires the presence of livestock on the farm or perhaps adjacent farms, to minimize the cost of swathing, baling, and transporting the forage. If you want to eliminate summer fallow, you must grow a crop that can produce enough income to overcome the loss in subsequent crop yield minus the cost of managing summer fallow.
Whether growing a cover crop, forage crop or grain crop, water is the most limiting resource for crop production in rainfed or dryland cropping systems in the semiarid regions of western Nebraska.
Water used by one crop is not available for use by the following crop. Precipitation is seldom sufficient to produce a profitable crop without supplementation of stored soil water.
In several no-till studies conducted at UNL's High Plains Ag Lab near Sidney over the last dozen years, winter wheat yields following a summer fallow replacement crop have been 25 percent to 60 percent less than winter wheat following summer fallow. This has been directly correlated with reduced soil water at planting following a summer fallow replacement crop compared to summer fallow.
If the value of the summer fallow replacement crop is equal to or greater than the value of the yield loss plus expenses associated with summer fallow, then it may be economically viable to replace summer fallow with a summer fallow replacement crop.
In a study conducted from 1999 through 2002 at Sidney, winter wheat following summer fallow was compared to winter wheat following oat/pea forage, spring canola, proso millet, dry bean, and corn.
Over the three years of the study, gravimetric soil water content in the top 4 feet prior to winter wheat seeding averaged 15 percent (summer fallow), 11 percent (oat/pea forage), 10.2 percent (spring canola), 9.9 percent (proso millet), 10.4 percent (dry bean), and 8.9 percent (corn). In a typical silt loam soil with an average bulk density of 1.3, that would be equivalent to about 3.7, 1.2, 0.7, 0.6, 0.9, and 0 inches of plant available water.
Winter wheat yields after various crops averaged over the three years of the study were 29.9 bu/ac (summer fallow), 23.2 bu/ac (oat/pea forage), 17.0 bu/ac (spring canola), 19.6 bu/ac (proso millet), 14.9 bu/ac (dry bean), and 12.5 bu/ac (corn). In this study, oat/pea forage and proso millet were economically competitive with summer fallow.
Research conducted over 15 years by David Nielsen, a research agronomist with USDA Agricultural Research Service at Akron, Colo., suggests that wheat yield increases by approximately 5.2 bu/ac for every inch increase in soil water at planting. This is similar to the Sidney results. Nielsen's studies showed that soil water at planting following a winter wheat-corn-proso millet rotation averaged 4 inches less than after a winter wheat-corn-fallow rotation. This resulted in an average reduction of more than 20 bu/ac after proso millet compared to fallow.
Value and costs
The question arises: How do you value a cover crop? Cover crops use water like forage and grain crops, but they do not have a direct economic return. They might reduce nitrogen needs or possibly the need for some herbicides, but there is a cost for the seed and for planting. Some of the seed is quite expensive; for example, the so called "cocktail mixes" can cost $13 to $19 an acre. Depending on rainfall and other weather conditions, cover crops used over 6 to 10 years may improve soil quality, but can you stay in business long enough to capture these benefits when you give up 25 percent to 60 percent of your wheat yield every year as a result of growing the cover crop?
Precipitation - Many cover crop advocates live in areas with higher average annual precipitation and/or lower evapotranspiration rates than the Nebraska Panhandle. Cover crops are successfully grown in areas of Nebraska with annual precipitation of 20 or more inches. They also are successfully used in areas of North Dakota where annual precipitation is similar to western Nebraska, but evapotranspiration rates are lower, so crops need less water to survive than they do in the Panhandle.
Evapotranspiration rates - The average daily evapotranspiration rate for June at Clay Center, Neb., is 0.261 inch. This is similar to the 0.260 rate for Bowman, N.D., which is due north of Scottsbluff. This average daily evapotranspiration rate is 0.315 inch at Sidney, Neb., and 0.362 inch at Walsh, Colo., a warmer, more evaporative environment than Sidney. This means that over the month of June, to achieve the same amount of growth in a specific crop, a grower at Sidney will need 1.5 more inches of water than a grower at Bowman, N.D. or Clay Center.
Recommendation - A technology or practice that works well in one area of the country may not be successful in another. Consider various factors including your environment, soils, cropping practices, and variety selection when reviewing research results and deciding whether to make a change in your operation.