Final report for GS18-179
Problem addressed: Cover crop systems are widely promoted for their potential benefits on weed suppression, water quality, nutrient cycling, among others. While cover crop systems have witnessed widespread adoption in the Northern parts of the country, adoption in places such as South Texas has been very limited. Some of the major limitations include lack of tools and knowledge to facilitate cover crop selection, insufficient biomass production prior to cash crop planting in spring and perceived soil moisture loss caused by cover crop growth. Addressing these limitations is critical to promote cover crop adoption in this region.
Research approach: A total of 26 summer and winter cover crop species were evaluated for their suitability for the Southeast Texas region. The summer covers were established during late August, following the harvest of corn/grain sorghum in the region, whereas the winter covers were established during mid-October, to fit production scenarios following a cotton crop. The experiments were conducted in a randomized complete block design with four replications. Cover crop seeding rates were determined based on production guides. Cover crop growth, biomass, decomposition rates, weed suppression and soil moisture dynamics were documented. Four best species selected from the initial experiment were then established at different planting timings (for summer covers) or termination timings (winter covers) prior to corn or cotton to evaluate the impact of cover crops on cash crop growth, weed suppression, soil moisture and crop yield.
Research conclusions: 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. 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.
Adoption actions: Findings from this study were broadly communicated to farmers, crop consultants and scientific community. The knowledge generated from this experiment is expected to facilitate cover crop adoption in the region.
1. Evaluate the influence of warm-season cover crop growth on summer annual weed suppression and soil moisture demand;
2. Evaluate the impact of the late-summer cover crop planting timing on weed suppression, soil moisture dynamics and yield in corn;
3. Evaluate the influence of cool-season cover crop and biomass production on winter annual weed suppression and soil moisture demand;
4. Evaluate the impact of the fall cover crop termination timing on weed suppression, soil moisture dynamics and yield in cotton.
A total of 26 summer and winter cover crop species were evaluated for their suitability for the Southeast Texas region. The summer covers were established during late August, following the harvest of corn/grain sorghum in the region, whereas the winter covers were established during mid-October, to fit production scenarios following a cotton crop. The experiment was conducted in a randomized complete block design with four replications. Cover crop seeding rates were determined based on production guides. Cover crop growth, biomass, weed suppression, and soil moisture dynamics were documented.
After year one of the 26 species evaluation, 8 species (four summer and four winter cover crops) were selected based on soil moisture management, biomass, and weed suppression potential, and then established at different planting timings (for summer covers) or termination timings (winter covers) prior to corn or cotton to evaluate their season long impacts. These cover crops were established to observe and quantify their impact on cash crop growth, weed suppression, soil moisture, crop yield, and cover crop decomposition rates. The experiment was conducted in a split-plot design with cover crop species (Summer: sorghum-sudangrass, buckwheat, cowpea, and sunn hemp at a 28 kg ha-1 seeding rate; Winter: winter pea 67 kg ha-1, shield mustard 11 kg ha-1, oat 67 kg ha-1, and triticale 100 kg ha-1) being the main-plot factor (8 x 73 m) and planting or termination timing being the sub-plot factor (8 x 24 m). A weed-free (conventional) and weedy check were also established as main-plot factors to compare to the cover crop treatments. Seeding rates were determined based on recommendations from seed suppliers and the cover crop establishment guide (Clark 2008). Both objectives were conducted at two site locations at the Texas A&M University Research Farm, College Station, Texas and at the Stiles Farm Foundation, Thrall, TX (2018-2019).
For the summer cover crop establishment, variation in planting timing was done by having an early- (late-August), mid- (early-September), and late- (late-September) planting timings. Cover crops were then rain fed and biomass (tonnes/ha) was collected at termination. Termination was allowed to naturally occur when nighttime temperatures reached below zero degrees Celsius. Within this region of Texas, the first frost historically occurs in mid-December, but an early frost came in 2018 and the summer cover crops were terminated November 13, and November 15 in Thrall and College Station, respectively. Prior to corn planting biomass was again collected to observed differences in cover crop residue decomposition between species. Corn was planted on April 9 and March 7, 2019 in College Station and Thrall, respectively.
For the winter cover crops establishment, all species were planted at the same timing with establishment on October 30, and November 27 at the College Station and Thrall locations, respectively. The Thrall location was planted substantially later due to heavy soil texture and frequent and heavy rain events that occurred leading into the study establishment. Cover crops were terminated at 6-weeks (early), 4-weeks (mid), and 2-weeks (late) at each respective location prior to cotton planting using a lethal dose of glyphosate at 1,261 g ae ha-1. Cotton was planted on May 24 and May 22, 2019 in College Station and Thrall, respectively.
Soil moisture data was collected using a Dynamax®/Delta-T® PR2 profile probe from four independent depths (10, 20, 30, and 40 cm), at a two- to- three-week interval depending on weather conditions. Within each plot and immediately after corn planting, two 1 m2 quadrats were permanently established. Within each quadrat, weed seedling emergence was counted at about biweekly intervals, until emergence completely ceased. Corn was harvested in both locations using a three-row research plot combine from the center three rows for the entire length of the plot (24 m). At the College Station location, seed and lint yield from each plot was measured by the yield monitor unit attached to a cotton stripper. A sub-sample was collected from each plot sample, weighed, de-burred, weighed, ginned, and weighed again to determine the total lint yield. The lint to burr and seed weight ratio from each sub-sample was used to calculate total lint yield (kg ha-1). At the Thrall location, the cotton was handpicked (1/1000 of an acre) , and the samples were de-burred and weighed, then ginned and weighed to estimate the plot lint weight.
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.
Educational & Outreach Activities
Results from this study were communicated to the broader stakeholders through various outreach and educational activities. A total of 8 talks and presentations were delivered in venues such as the Texas Plant Protection Association (TPPA) Annual Meeting, the Southern Weed Science Society (SWSS) meeting, and the Weed Science Society of America (WSSA) annual meeting. The TPPA meeting is widely attended by farmers, crop consultants as well as other agriculture professionals in the region. A total of over 100 farmers and 150 ag professionals were reached with these presentations. Additionally, 14 consultations were provided to farmers/crop consultants who had specific questions regarding cover crop choice and establishment. One on-farm demonstration was carried out, which was attended by 23 farmers and crop consultants. An outreach video was produced and distributed via the www.integratedweedmanagement.org website.
Conference Proceedings (SARE’s contribution was acknowledged in the presentations):
- L. Samuelson, M.V. Bagavathiannan. 2020. Impact of four winter cover crop species and termination timing on weed suppression, soil moisture dynamics, and yield in cotton. Proc. Weed Sci. Soc. 60:473.
- L. Samuelson, M.V. Bagavathiannan. 2020. Effect of four summer cover crop species and planting timing on weed suppression, soil moisture dynamics, and yield in corn. Proc. Weed Sci. Soc. 60:215.
- L. Samuelson, M Bagavathiannan. 2020. Effect of four winter cover crop species and termination timing on weed suppression, soil moisture, and yield in cotton. Proc. Southern Weed Sci. Soc. 73:193.
- L. Samuelson, M.V. Bagavathiannan. 2020. Impact of four summer cover crop species and planting timing on weed suppression, soil moisture, and yield in corn. Proc. Southern Weed Sci. Soc. 73:21.
- L. Samuelson, J.E. Mowrer and M.V. Bagavathiannan. 2019. Developing suitable cover crop systems for South Texas: Evaluating different winter cover crop species. Proc. ASA-CSSA-SSSA International Meeting 399-10.
- L. Samuelson, and M.V. Bagavathiannan. 2019. Developing suitable cover crop systems for South Texas: Evaluating different late-summer cover crop species. National Sustainability Summit.
- L. Samuelson, J. Hernandez, and M.V. Bagavathiannan. 2019. Evaluation of the suitability of 13-summer cover crop species for Southeast Texas. Proc. Weed Sci. Soc. 59:255.
- L. Samuelson, J. Hernandez, and M.V. Bagavathiannan. 2019. Evaluation of the suitability of 13-winter cover crop species for Southeast Texas. Proc. Southern Weed Sci. Soc. 72:165.
- L. Samuelson, B. Young, M. Bastos Martins, and M.V. Bagavathiannan. 2018. Impact of 13 summer cover crops on weed density and soil moisture in southeastern Texas. Proc. Texas Plant Protection Association Conference 29:23.
Communications pending: three manuscripts are being prepared for submission to peer-reviewed journals
The information generated in this project is unique and is expected to offer valuable foundational knowledge for growers considering the integration of cover crops in their production systems. Thus, the project has the potential to offer significant economic, environmental and social benefits to the farmers of Southeast Texas and beyond.
I and my team have gained a lot of practical knowledge on cover crops and their value in sustainable agriculture. We also gained a lot more enthusiasm and commitment towards sustainable agriculture research and development after witnessing first-hand the multitude of benefits offered by cover crops. Our team is excited to continue and expand this line of research to benefit the farming community.
In the current study, we evaluated the suitability of individual cover crop species. We recommend that future studies look into cover crop mixes.