Developing Suitable Cover Crop Systems for South Texas: Evaluating Different Late-Summer and Winter Cover Crop Species

For the summer cover crops, weather conditions at both locations severely impacted biomass production by the different species, when compared to previous years data of the same species.  Generally dry conditions following the first planting timing caused delayed stand establishment, which resulted in low biomass production that were similar to that of the mid-planting timing. Following the mid-planting timing, rainfall was high, but the temperatures were unusually cold during September. A combination of high rainfall and cold temperatures greatly impacted cover crop growth.  An early frost also contributed to lower biomass, particularly in the late-planted sub-plot.  Buckwheat however, showed promise with late planting due to its rapid growth and quick maturation.  With respect to weed biomass production, significantly lower levels were observed with sorghum-sudangrass (all planting timings), sunn hemp (mid planting), and buckwheat (early and late planting), compared to the weedy fallow treatment. Despite the low biomass production, sorghum-sudangrass exhibited strong weed suppression potential.  One unforeseen obstacle and learning experience was provided with a high density of volunteer corn at the Thrall location as this study was established in a field that followed a corn crop.  When following corn cover crop establishment may not be feasible unless a rain event occurs that would cause a flush of corn and then a disking on herbicide treatment to eliminate the unwanted competition.  Because the summer cover crops at both locations were not able to reach their full biomass potential minimal residue was still present at the timing of corn planting and little to no weed seedling suppression was provided from cover crop residue.  Other studies at Texas A&M with these species did show potential for sorghum-sudangrass to still provide residue that can push back the initial flush of summer weed seedlings giving the cash crop a head start. When integrated with herbicides, summer cover crop species can provide fall weed suppression in addition to offering soil conservation benefits.  In College Station, cover crops had a net negative impact on soil moisture at the time of corn establishment in the spring. Soil moisture content improved due to rainfall events as the season progressed, especially in sorghum-sudangrass, sunn hemp, and weedy check treatments.  In general, sorghum-sudangrass exhibited the greatest negative impact on soil moisture across all planting and data collection timings. Summer cover crop species and planting timing had no impact on corn yield at both the College Station and Thrall locations. This could be because of the low cover crop biomass production relative to their potential, or the result of sufficient time between cover crop termination and corn establishment.

For the winter cover crop treatments the two experimental locations had drastically different environmental and crop growth conditions during the study, and thus location was considered a factor of interest apriori and the results are presented separately for each location. In both locations, cover crop species by termination timing interactions were significant for cover crop biomass (College Station, P = 0.0055; Thrall, P < 0.0001) and weed biomass (College Station, P = 0.0155; Thrall, P < 0.0181); therefore, the interaction effects are presented for the two factors within each location.  At the College Station location, high biomass accumulation was observed for all the four cover crop species evaluated. At the early termination timing, oat produced the greatest biomass (5 t ha^-1 of dry biomass), with the greatest impact on weed biomass overall across all termination timings. In general, the biomass differences between the early- and mid-termination timings were minimal. However, at the late termination timing, there was a large increase in biomass across all species, with triticale producing the greatest biomass (13 t ha^-1), showing a more than 300% increase in total biomass between the mid- and late-termination timings. The second most biomass was produced at the late-termination timing by oats at 9 t ha^-1. Weed biomass generally did not differ (averaging less than 0.2 t ha^-1) among the cover crop treatments and termination timings at this location. In the weedy check treatment, weed biomass at the late termination timing was at 3 t ha^-1, only 23% of the maximum biomass produced by a cover crop treatment. At the Thrall location, planting was delayed due to unconducive field conditions, which had an impact on cover crop growth and biomass production. At the early termination timing, no differences were observed among the cover crop species; the cover crop biomass production at this timing ranged from 0.6 to 1.1 t ha^-1, with corresponding weed biomass ranging from 0 to 0.8 t ha^-1. At the mid- and late-termination timings, triticale (2 t ha^-1 and 3.4 t ha^-1, respectively) and oat (2.4 t ha^-1 and 2.6 t ha^-1, respectively) accumulated the most biomass compared to other cover crop treatments.  Further, weed biomass was generally low across all cover crop treatments, and the greatest weed densities were observed with mustard and winter pea at the mid-termination timing. This is attributed to the peak vegetative growth of the winter annual weed henbit (Lamium amplexicaule L.), the dominant weed at the experimental site, coinciding with mid-termination timing. At the late termination timing, henbit already senesced and began to disintegrate. Soil moisture status at that time of cash crop planting is critical. In general, with a few exceptions for the early and mid-termination timings, all cover crop species showed a negative impact at planting compared to the weed free, but the differences were minimal (<2% v/v). The largest improvements in soil moisture content was observed for the late-terminated cover crops, with triticale still having 100% more available soil moisture late into the season compared to the no cover (weed-free) treatment.  In general, cotton stand and vigor was not impacted by the cover crop or its residue.  The only exception that we observed was in the late-terminated mustard that had substantial stand loss due to insect feeding, with the lowest average stand density of 25,626 plants ha^-1 compared to most other treatments that had close to 40,000 plants ha^-1. This was only observed in the late-terminated mustard that had a high threshold of false chinch bugs that had carried over from the mustard plants, a common host for this species, and carried over into the young cotton stand.  The cotton lint yield obtained at the College Station location was comparable to average yield levels for the region, with late-terminated winter pea producing the best lint yield of 1,214 kg ha-1. The added soil nitrogen from the later-terminated winter pea is attributed to the greater yield. The treatment that had the greatest negative impact on yield was the late-terminated triticale cover crop with only 675 kg ha^-1. The weedy check had similar yields to that of the weed-free, regardless of the timing of termination. At the Thrall location, cotton lint yield was generally much lower compared to that of the College Station location, due primarily to environmental conditions. The weed-free treatment had the tallest cotton plants and resulted in the greatest yield with 458 kg ha^-1. The lowest yielding treatment was the early-terminated mustard with 262 kg ha^-1. Yields obtained in this study were below-average for this region of Texas, yet consistent with average yields at the Stile’s Farm for that season where the study was conducted. The low yields at the Thrall location was likely the result of low rainfall for much of the cotton growth period. Overall, the cover crop treatments tested had no net positive impact on yield compared to the weed-free check.  More investigations at this location under greater rainfall conditions will be valuable.

The experiments were vital in identifying cover crop species that are suitable for the Southeast Texas region in terms of fast growth and biomass production, high weed suppression, and limited impact on soil moisture. Sorghum-sudangrass, sunn hemp, cowpea and buckwheat were the top four summer cover crops, whereas triticale, oat, mustard and Austrian winter pea were the top winter cover crops. In the cash crops, terminated cover crop residues offered significant weed suppression benefits, even in circumstances where the cover crop biomass was low. Though cover crops extract soil moisture, moisture loss might not be a concern in this region due to soil moisture replenishment by spring rains. In late season soil moisture measurements after cash crop establishment, cover crop treatments often high more available soil moisture when compared to the weed free plots. Overall, cash crop emergence and establishment were unaffected by the cover crops. Results also showed that some changes to crop management, such as appropriate fertilization and monitoring for insect/disease infestation on the seedlings, are imperative for achieving high yields when incorporating cover crops in the production system.