Developing Organic Cropping Systems for Grain Production in Texas

Cover Crop Biomass Yield

Cover crop biomass varied by species and year. Cover crop biomass declined each year of the study period. Mean cover crop biomass was 4.3 Mg ha^-1 in 2018 and decreased to 0.6 Mg ha^-1 in 2020. Variation in cover crop biomass was affected by competition with winter weeds, which increased in coverage during the study period. Variation in agronomic practices (planting date, fertility, etc.) did not affect cover crop productivity. Establishment was good each year and environmental conditions were sufficient for cover crop growth. Cover crop biomass was not estimated in 2019 due to contamination of samples with winter weeds, primarily Italian ryegrass. It was estimated that ryegrass represented >50% of total biomass in all plots. Ryegrass was separated from cover crop biomass during clipping to estimate cover crop biomass in 2020.

Despite declining biomass production over time, variation in biomass production was observed among species evaluated. No difference was observed due to later termination dates associated with soybeans compared to corn termination timing. Therefore, mean cover crop dry matter is presented by year (except 2019 where weed contamination prevented accurate estimation of cover crop biomass). Spring barley, spring wheat, and oats consistently outperformed (p<0.05) elbon rye and rye/vetch mixtures (Figure 1). Mean dry matter yields for spring type cereals and oats were 2.2-fold greater than elbon-rye and rye/vetch mixtures. When growth is heat unit dependent and does not require vernalizing, it enables greater biomass production during the Fall and warm periods encountered during the Winter in central and south Texas. This confirms that selecting spring types will enable greater biomass production ahead of planting dates used in Texas for corn and soybean. This is in contrast to other regions of the U.S. where rye and similar cereals have performed well in organic and conventional systems.

Figure 1. Cover crop dry matter production (Mg ha^-1) by year, presented as means of corn and soybean cover crops. Error bars represents standard error of the mean. The same letter above a bar indicates means are not significantly different (p>0.05) using L.S.D.

Cover Crop Termination Efficacy

Termination efficacy was evaluated on a 0-9 scale using visual ratings for kill, planter row cleaning, and laydown. A score of zero rating no kill, unclean planter row, or cover crop that was completely erect 2 weeks after planting.

Efficiency of cover crop termination was dependent on cover crop species and level of biomass present. Ratings for cover crop kill declined in year 2 and 3 as cover crop biomass declined and amount of ryegrass weed biomass increased. There was no difference in kill rating across species or timing in 2019 or 2020. In 2018, there was a significant difference (p <0.05) between termination timing (corn or soybean schedule) and cover crop species. Similar to biomass production, Spring barley and Spring wheat resulted in better kill following roller crimping compared to elbon rye or rye/vetch mixture. Unexpectedly, cover crops terminated better ahead of corn planting (earlier) than soybeans. In addition, despite similar levels of biomass production oats to spring cereal types, oats were not successfully terminated by roller crimping.

Figure 2. Efficacy of termination of cover crops ahead of corn or soybean planting by roller crimping. Score of 0 indicates no kill, 9 indicates complete kill. Bars with the same letter are not significantly different (L.S.D. p = 0.05).

Figure 3. Rating on laydown for contrasting cover crops in corn and soybean cropping systems. All ratings in 2019 and 2020 were 0.

Cover crops were rated for laydown at two weeks after planting with 0 indicating erect plants and 9 indicating complete laydown. This was measured at 3 to 4 weeks following mechanical termination using a roller crimping device. As with termination (kill) ratings, the amount of biomass and maturity was expected affected the cover crops ability to recover and remain erect following termination.  However, no significant difference was observed between cover crop species or termination timing. Cover crop in 2018 (with greater biomass) did lay down and stay down better than the following 2 years. Poor biomass production in 2019 and 2020, combined with ryegrass contamination produced short plants that lacked the height required for sucessful mechanical termination. This is evident by mean laydown ratings in 2019 and 2020 less than 1 compared to 5.1 in 2018. The lack of good laydown ratings in any year for any species indicates that the dense layer that is expected to serve as a weed barrier during grain production never materialized. Visual differences in laydown were observed in 2018 for individual plots, yet, variation precluded statistical differences (Figure 4).

Figure 4. Image of cover crop after planting. Variation in kill, laydown, and planter rows following mechanic termination are shown.

Creating a clean planted row is expected to improve seedling establishment for corn and soybean. The planter used was fitted with Yetter row cleaner and coulters for no-till planting into heavy residue (Figure 4). Variation in planter row scores was observed every year and across cover crop species (Figure 5). Planter rows appeared clean at the time of planting. However, living and even dead residue eventually covered planted rows, creating a competitive environment for establishing crops (Figure 6). Notice the plant outside of the cover crop in Figure 6. Presence of ryegrass weeds which persisted for almost two months into the season (corn and soybean) resulted in severe limitations to crop growth and diminished efforts at planting to provide optimum environments for corn and soybean growth.

Figure 5. Rating for how clean planter rows were about two weeks after planting. 9 = clean rows, 0 = complete covering of planted rows.

Figure 6. Corn and soybean stands within contrasting cover crop species. The plant outside of the cover crop area illustrates the impact weed and cover crop residue had on growth of corn and soybeans.

Weed Control

Weed control ratings for contrasting cover crops were determined prior to planned harvest of corn or soybean.   Ratings provided are visual estimates of percent control. No difference was observed between corn or soybean, ratings represent the mean of grain crops for each cover crop. Grass weed control ranged from 32.9 to 35.3 % in 2018 and 2019, declining to 6.8% in 2020.  In 2018, Spring wheat and Spring Barley provided a small improvement of grass weed control compared to all other cover crops. Regardless of variation in grass weed control, weed control was not adequate and declined over time in a no-till cover crop system.

Figure 7. Impact of cover crops on grass control. Mean of weed ratings for corn and soybean are provided. Bars with the same letter above are to significantly different (L.S.D, p=0.05).

Similar to grass weed control by cover crops, small differences were observed in 2018 with Spring wheat and Spring barley showing better control than other cover crop species (Figure 8). Mean % control of broadleaf weeds was 37.5% in 2018 and declined rapidly to 0% control in 2019 and 2020. We speculate that winter weeds (ryegrass), combined with cover crop residue contributed to an environment that enable broadleaf weed infestation in grain crops, especially late-season. Lack of sufficient residue to develop dense mats of residue during roller crimping that lay down and remain down to provide shading of soil between rows was evident. As cover crop productivity declined with time, lack of weed control was exacerbated.  

Figure 8. Impact of cover crop species on % control (visual rating) of broadleaf weeds. Values represent the mean of grain crops for each cover crop. Bars with the same letter are not significantly different (L.S.D. p=0.05).

Crop Yields

There was no harvestable yield from either corn or sorghum in 2018. In 2019 there was no harvestable corn yield, however soybeans were harvested on September 6^th, 2019 by hand from two 10 m lengths from the center two rows. There was no difference in soybean yield between any of the cover crop species (Figure 9). No harvestable yield was produced in 2020 for corn or soybean.

Soybean yield

Figure 9. Yield for soybean in 2019 adjusted to 13% moisture. Yields were not significantly different (p=0.05).

While variation in cover crop biomass, termination efficacy, and weed control were observed for contrasting cover crop species at planting, weeds and/or living cover crop biomass severely limited crop growth and development. Efforts to provide clean planted rows did not persists through early growth stages. Stands were generally adequate, however, weed and cover crop biomass quickly recovered, shading establishing corn and soybean plants. Competition for light, water, and nutrients resulted in poor grain crop performance (Figure 6). Contamination of cover crops with winter weeds, primarily Italian ryegrass, likely had greater impact than summer weeds. Ryegrass weeds aggressively competed with cover crops reducing cover crop performance (dry matter yield) and aggressively competed with establishing corn and soybean plants. Ryegrass persistence was progressively worse throughout the study period suggesting cover crops had little influence on ryegrass populations. Alternative management practices should be explored when ryegrass is present in organic grain fields. This may include strategic tillage to control ryegrass seedlings and to reduce weed seed banks.      

Cover Crop Decomposition

Cover crop biomass used for litterbag studies under field conditions revealed that cover crop species affected decomposition rates for residue placed on the soil surface. Rates at 0.5 and 2 X decomposed at rates similar to 1 x rates (representing 2018 cover crop biomass yields). This indicates that variation in biomass yields annually will not affect percent of residue amounts that remain on the soil surface during grain production. Cover crop species or mixes did decompose at different rates. Following 120 days on the soil surface, mean (combined rates) amount of residue remaining ranged from 27 to 69% (Figure 10-12)). In 2018 and 2019, Spring wheat, Spring Barley, and Winter wheat retained greater (p < 0.05) amounts of residue compared to other cover crops. Greater C:N ration of residue contributes to slower decomposition rates (Figure 13). Greater biomass and lower decomposition rates of Spring type cover crops likely contributed greater shading of soil surface and increased weed control in 2018. This benefit declined as cover crop biomass declined and winter weed biomass increased over time.

Figure 10. Percent of remaining cover crop residue by weight in the litter bags in 2018. Error bars in the line graph are the standard errors of the mean.

Figure 11. Percent of remaining cover crop residue by weight in the litter bags in 2019. Error bars in the line graph are the standard errors of the mean.

Figure 12. Percent remaining biomass weight of cover crops residue at harvest on litter bag experiment during 2018 and 2019.

Figure 13. Tissue composition of cover crop residue at harvest in 2018 and 2019. *Means followed by the same letters in the column are not significantly different.