•Summer planting is not a desirable strategy for introducing clover cover crops in the High Plains region.
•Cowpea is a promising forage crop that can be successfully planted late in spring to early summer in the High Plains if residue cover is limited and adequate amount of soil moisture is available.
•Yellow clover is the most promising winter legume; however, further studies are needed to identify the best planting time.
Texas County in the Oklahoma Panhandle is the largest agricultural producing area in the state and one of the nation’s agricultural leaders, with farm receipts exceeding $1.0 billion annually. The downside to this tremendous agriculture activity is that the predominant grain-cattle production systems are far from sustainable. Crop production relies heavily on external inputs and the county is by large a net importer of animal feed from neighboring states.
Early in the summer, after wheat harvest, the soil is typically left fallow (unplanted) until the following spring when corn is planted. This fallow strategy, which is commonly used throughout the High Plains, prevents farmers from utilizing summer precipitation efficiently and reduces the opportunity to produce additional forage. In addition, the lack of biological activity and increased soil degradation associated with intense herbicide applications and tillage make the fallow period an undesirable management strategy for the Southern High Plains region.
We are proposing the introduction of legume cover crops to help mitigate the negative impact of the fallow period after wheat harvest and to enhance the sustainability of grain-cattle operations. Cover crops have been rarely studied in the Oklahoma Panhandle; thus, we plan to evaluate four potentially adaptable legume species: sweet clover, berseem clover, crimson clover, and cowpea. Successful establishment of legume cover crops will allow farmers to extend their grazing season. Cattle normally graze on young wheat from December to March. Interseeding cover crop into wheat stubble immediately after harvest would make grazing possible during late summer and early fall. The four legumes were selected for their potential to perform well under high temperature conditions, their ability to produce large quantities of high quality biomass, livestock preference and low bloating potential, and their ability to tolerate drought conditions.
Our main objective was to identify legume cover crops that can be successfully introduced and managed in the Oklahoma High Plains. Success was measured in terms of biomass quantity and quality, winter survival and expansion of the grazing period, agronomic viability, and soil quality improvement.
For practical and demonstration purposes, this research was established in half of an irrigated circle (approximately 24 ha) in 2003 and in a full small irrigated circle (approximately 34 ha) in 2004. The participating farmers insisted that research of this magnitude will allow proper management on the scale to which they are accustom to working. In addition, this size creates a more credible experiment and results for other farmers in the area. The research design was four randomized replicated entries for each cover crop and the control treatment. Yellow sweetclover (Melilotus officinalis), berseem clover (Trifolium alexandrinum), crimson clover (Trifolium incarnatum), and cowpea (Vigna unguiculata) were planted in 2003 and 2004 while sunhemp (Crotalaria juncea) was planted only in 2003 due to difficulties in seed availability.
The production viability of the cover crops in response to climate and agronomic practices was measured according to various performance parameters including final plant populations, biomass yield, and weed suppression abilities. Above ground biomass samples were taken just prior to grazing. During the grazing period, forage yield were periodically measured from sampling areas protected with metal enclosures. Biomass samples were analyzed for moisture and quality including protein and nitrate content. Stockers’ performance were measured based on the length of the grazing period. The response variables were weight gains and possible livestock health related problems influenced by the type of diet (legume forage); for example, disorders associated with bloating.
Soil samples were taken in all treatments at various depths to measure selected soil biological and chemical indicators of quality, including total organic C and N, labile N pool size, and soil fertility parameters. Laboratory incubations using soil samples collected early in the following spring (at the 0-10 cm depth) were implemented to determine changes in the labile N pool size due to the presence or absence of a cover crop. Erosion potential was determined annually using USDA-NRCS formulas based on the amount of crop residue left in the soil measured by the line transect method.
Early in July of 2003 and 2004, the legumes were no-till planted after wheat harvest. Wheat residue cover prior to planting averaged 95% in 2003 and 100% in 2004. The excessive amount of wheat residue coupled with high summer heat resulted in minimal germination and final plant population (Table 1).
Table 1. Final plant populations of legume cover crops.
Legume Plant population
Berseem clover 8
Crimson clover 6
Yellow sweetclover 5
In 2003, 7.6 cm of irrigation water was applied to all the legumes while it was not needed in 2004 due to higher precipitation and lower temperatures in July and August (Table 2). In 2003, cowpea and sun hemp germinated by August 15 and the clovers a month later. In 2004, the cowpea and the clovers germinated earlier (late July) than the previous year; due to wetter and milder temperature conditions. In both years, the clovers reached their maximum fall growth by mid October. Cover crop biomass in the fall was small (Table 3); and in 2004 biomass was even smaller than in 2003 due to the higher amount of wheat residue that negatively affected germination. Protein content in the legumes ranged from 17% in cowpea, 18% in yellow and crimson clovers and sunhemp, and 19% in berseem clover; compared to 16% in the volunteer wheat.
Table 2. Monthly precipitation and average temperature near the experimental site.
Legume Precipitacion (mm) Temperature (oC)
2003 2004 2003 2004
January 0 1 2 1
February 5 3 1 2
March 29 45 7 10
April 12 30 14 12
May 52 2 19 21
June 132 105 21 22
July 44 70 28 24
August 22 73 26 22
September 38 79 19 21
October 4 16 15 14
November 15 83 6 5
December 5 18 3 3
Table 3. Biomass of legume cover crops and the control treatment
Legume Biomass (kg ha-1)
Berseem clover 524
Crimson clover 590
Yellow sweet clover 533
Control (wheat + weeds) 2,234
In a nearby related study at the Oklahoma Panhandle Research and Extension Center, cowpea (1,535 and 2,557 kg ha-1 in 2003 and 2004 respectively) and sun hemp (2,140 kg ha-1 in 2003) biomass production was considerably higher using similar seeding rates and under dryland conditions. The soils in this case have had considerably less residue cover and germination was near optimum.
Early in November 2003, ninety six head of cattle were released into the entire experimental area. Since more than 70% of the biomass was volunteer wheat, we decided that the limited legume growth did not justify fencing and grazing management by individual legume. The cattle were removed early in February after 82 days of grazing. Overall weight gain was 0.82 kg per head per day and there was no indication of any health related problem associated with grazing.
All the legumes except yellow clover were winter killed. Yellow clover produced a significant amount of additional biomass (1,434 kg ha-1) during the following spring prior to corn planting; indicating that this specie can be successfully introduced into the area’s predominantly wheat/cattle-corn cropping system. We contend that frost seeding of yellow clover into wheat may be a more desirable strategy to avoid the high amount of residue after wheat harvest and the high summer temperature during germination. Successful establishment of legume cover crops is likely to improve the soil’s capacity to produce plant available inorganic N. The labile N pool, measured at 70 days of incubation, was reduced from 18.1 mg kg-1 at the start to 6.1 mg kg-1 at end of the experiment. This indicates the inclusion of a productive legume (low C:N ratio) is needed to counterbalance the effect of the large amount of residue with high C:N ratio entering the soil (from wheat and corn) that may be immobilizing inorganic N into organic forms.