Multifunctionality of Cover Crops in South Texas: Looking at multiple benefits of cover cropping on small farms in a subtropical climate

Final report for GS15-148

Project Type: Graduate Student
Funds awarded in 2015: $8,953.00
Projected End Date: 09/30/2017
Grant Recipient: University of Texas-Rio Grande Valley
Region: Southern
State: Texas
Graduate Student:
Major Professor:
Dr. Alexis Racelis
University of Texas - Rio Grande Valley
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Project Information



Situated in deep South Texas, the Lower Rio Grande Valley (LRGV) is considered one of the most productive agricultural regions in the southern US. With the highest concentration of organic farms in the state (Hidalgo county), the LRGV has a strong potential to be a leader in sustainable agriculture. However, not much research in sustainable agroecosystem management has been conducted in this subtropical region, where pests and weed pressures are immense and year-round, and, as such, farmers are looking for viable strategies to deal with these barriers. Finding management practices that comply with organic certification and increase the health of the agroecosytem and the farmers working the land are increasingly pertinent. Cover cropping, or the intentional planting of non-cash crop vegetation, can serve multiple functions in an agroecosystem by decreasing environmental pollutants that originate from the agroecosystem, reducing inputs needed for crop production, and potentially decreasing on-farm costs for farmers—overall increasing the sustainability of the farm. To date, there is no research on appropriate cover-cropping strategies in this important agricultural region. The aim of this research was to generate baseline information on the potential of different cover crops species to increase soil organic matter, nitrogen content, impact on beneficial soil microbes (mycorrhizal fungi), and weed suppression. The cover crops included in this study were: tillage radish (Raphanus sativus), winter rye (Secale cereal), lablab (Lablab purpureus), sunn hemp (Crotalaria juncea), sudangrass (Sorghum drummondii), and pearl millet (Pennisetum glaucum). Results from this study indicate that these cover crops have the potential to enhance ecosystem services on agricultural lands in the LRGV by increasing soil organic matter, nitrogen concentration, weed suppression and the mycorrhizal fungi population in the soil.




Project Objectives:

Project Objectives

  1. Identify summer cover crops suitable for South Texas and the rest of planting zone 9b,
  2. Determine which cover crops provide the most multifunctional benefits for farmers in South Texas by assessing: (a) weed suppression; (b) soil fertility; and (c) beneficial soil microbes,
  3. Assess the economic viability for small growers in South Texas to use cover crops in their land management plan,
  4. Improve access to information on cover crops so that agricultural professionals can make better recommendations and producers can make better management decisions.




Click linked name(s) to expand
  • Dr. Alexis Racelis (Educator and Researcher)
  • Dr. Mike Morris (Educator)


Materials and methods:

Study Site

The final study was conducted at a 14-acre certified organic working farm in Harlingen, Texas (26o 09’20.89″ N 97o 42’19.74″ W). As for much of the region, this site is considered subtropical, with annual average rainfall of 69.9 cm and an annual average temperature of 23.3°C. July and August are the warmest months, with average daily high temperatures of 35°C and 35.6°C respectively. Most of the rainfall falls between May and September, with September having the highest average precipitation of 13.4 cm. In general, soils at the site are characterized as deep calcareous soils that are level to nearly level (heavy-textured Harlingen clay, average pH 7.6). Based on weather data from the regional airports (National Centers for Environmental Information, 2016), the monthly average precipitation during this study did not differ significantly from twenty year averages. Although total rainfall during the study period was lower than the interannual average, there was considerable (above average) rainfall in the month of May 2015 preceding the planting date, which is typically when RGV region plants their summer crops. Higher than average rainfall in June 2015, typically ideal time for cover cropping in this region, delayed the planting of cover crops for this study until July 2015.

Cover Crops

Four different cover crops (2 legumes and 2 grasses) were selected for this study: lablab (Lablab purpureus), sunn hemp (Crotalaria juncea), pearl millet (Pennisetum glaucum), and sudan grass (Sorghum drummondii). The cover crops were planted on a 2-acre field divided into 16 plots, allowing for two replicates of each cover crop and two control plots measuring 7.3×30.5 m. Three of the four cover crops: lablab, sunn hemp, and sudangrass were planted 1.5 times and 3 times the recommended seeding rate. After three planting attempts, pearl millet did not grow to provide a good cover crop stand and was determined to be unsuccessful if planted during the summer months in the RGV. The seed source, seeding rates and temperature tolerance for each cover crop is given in Table 1. The cover crops were planted on 7/9/16 due to excessively wet soils that did not allow field access to plant. Half of the field was planted using a hand broadcast seeder and the other half was planted with a drop seeder attached to a walk behind tractor (BCS, city ST), and immediately tilled in by the farmer using a disk implement attached to a tractor. The cover crops were flood irrigated two times during the study period: two days after planting (DAP) and 28 DAP. After 8 weeks the plants were tilled on September 3rd 2015, and the cover crop biomass was measured.

Table 1: The Seed Source, Seeding Rates, & Temperature Tolerance for Each Cover Crop

 Cover Crop

Seed Source

Recommended Seeding Rate lb/acre


Optimum Temperatures

Sudan Grass

Johnny’s Selected Seed Co.



Soil temp at least 18.3°C

Pearl Millet

Johnny’s Selected Seed Co.



Soil temp at least 18.3°C


Hancock Seed Co.



13-30°C Can tolerate light frosts

Sunn Hemp

Hancock Seed Co.



Warm temperatures with no frost dates

Soil sample collection and analysis

Four weeks after tilling the cover crops soil samples were collected from the plots and analyzed for mycorrhizal fungi spores and soil organic matter status. 3 soil samples were collected from each plot with a soil corer (diameter = 2.5 cm). The 3 soil samples from each plot were mixed thoroughly to create a composite sample for each cover crop treatment. A portion of soil samples were then stored in a 4oC refrigerator until analysis. Soil samples for soil chemical analysis were air dried and ground in a mortar pestle and shipped in air tight containers to the Soil and Plant Tissue Testing Laboratory, University of Massachusetts, Amherst, MA. Soil organic matter was estimated by loss on ignition method.

Mycorrhiza Spore Extraction

Mycorrhizal spores were extracted from soil by using a modified wet sieving and decanting technique. 10 g of soil was added to 500 ml of water and mixed vigorously to separate the spores from soil aggregates. The soil mixture was then passed through a series of sieves and washed until the water flowing through the sieves was clear. The sievate retained on the sieves was washed and centrifuged with water to remove the organic debris. The pellet in the bottom was resuspended in a 50% sucrose solution, and centrifuged for one minute at 1600 RPM to separate the spores from denser soil components. Immediately after centrifugation, spores in the sucrose supernatant were washed into petri dishes for counting. The spores were counted under ZEISS Discovery V5 stereomicroscope. Spores collected from each treatment were grouped based on the color, size and shape. They were then observed under 100x magnification and identified to genus level following the International Culture Collection of VA Mycorrhizal Fungi (INVAM) based on the spore morphology.

Cover Crop Height and Biomass Measurement

To estimate the standing biomass of cover crops, height of the cover crops was measured every two weeks. Height of the cover corps was measured in 0.25m2 in five randomly selected quadrats in each cover crop treatment. At the end of the eight-week trial period above ground biomass from 5 random quadrants was collected for each cover crop treatment. These biomass samples were transported to the lab and weed and cover crops were sorted and dried in an oven at 55o C for 72 hours. The dry cover crop and weed weight in each cover crop treatment was measured.

Light readings

Light readings, measuring the photosynthetic photon flux density (PPFD), were collected every two weeks after planting the cover crops. A Licor Quantom Line Sensor (LI-COR, INC. Lincoln, NE USA) and data logger (LI-1400, LI-COR, Lincoln, NE, USA) were used to measure PPFD at the soil surface below the cover crop canopy and at the top of the cover crop canopy at clear conditions at 12:00pm-1:00pm.

Data analysis

All the data was stored in an excel spreadsheet and analyzed with Statistical Analysis System (SAS). Data that did not meet assumptions of normality were log-transformed. Difference in the soil nutrient status due to cover crops was calculated by subtracting the soil nutrient concentration at the beginning of the experiment from the soil nutrient concentration at the end of the experiment. Analysis of variance (ANOVA) was done to compare the height of the above ground biomass of the different cover crops, and an ANOVA was done to compare the amount of light penetration through the cover crop canopy that reached the ground in each cover crop treatment. Pearson correlation analysis was used to look at the relationship between cover crop height and the amount of cover crop biomass, biomass of the cover crop and the PPFD at the soil surface, and soil spore density and soil chemical and physical properties.

Research results and discussion:

Soil nutrient status

Aim of this study was to assess the effects of 4 different summer cover crops and fallow on the spore density and diversity of mycorrhizal fungi, soil organic carbon and nutrient status in an organic vegetable farm. The study site had high levels of calcium concentration ranging from 18913 ppm to 22511 ppm, high soil pH and very low organic matter content, an environment not conducive for the microbial activity. The soil texture was silty loam with very poor drainage, which is a common characteristic of the Lower Rio Grande Valley soils. The nutrient distribution along our field varied considerably, this could be because of the variation in the water drainage pattern in the field. 

The difference in the soil nutrient status after cover crops are given in Table 2. Our results show that cover crops had mixed results in the soil nutrient status. This is an expected result as all cover crops do not provide the same benefits and farmers should select the cover crops based on their needs and what fits their farms conditions.

Table 2: Change in the soil nutrient status after each cover crop (in ppm). Numbers are calculated by subtracting the pre cover crop soil nutrient concentration from the post cover crop nutrient concentration.


Pearl Millet


Sunn hemp

















































































Mycorrhizal fungi spores

Overall the cover crops had a positive effect on the mycorrhizal spore density compared to the control. Total spore density varied significantly among the different cover crop treatments, ranging from 52 to 187 spores per 10 grams of dry soil (Figure 1).  Our results indicate that the mycorrhizal spore density was influenced by the cover crop identity. Highest numbers of spores were found under sunn hemp followed by sudangrass and lablab while the lowest numbers of spores were found under pearl millet followed by control. Among the four cover crops pearl millet had the lowest number of spore count. Low spore density under pearl millet could be because of the high weed density which could have suppressed the mycorrhizal fungi growth.

There was also a difference in the spore size among the different cover crops. Sunn hemp and sudangrass had generally bigger spores while the control and pearl millet generally had smaller (50 spores) with few large spores. 14 different species of mycorrhizal fungi were identified in the study from three different genera: Glomus, Aculospora and Gigaspora. The distribution of these spores varied among the different cover crops. Glomus was most dominant in the sunn hemp and lablab and a high density of Gigaspora occurred in the sudangrass treatment. This could be the result of different soil magnesium concentration under different cover crops. Similar results of higher density of Glomus in soils with high magnesium concentration and high Gigaspora density in low soil magnesium concentration has been reported in previous studies (Schenck & Siqueira 1987; Gryndler et al 1992). The overall diversity of mycorrhizal fungi, however, was similar for the all of the different cover crops and was lower in the control treatment.

Correlation analysis of the number of spores and soil physical and chemical properties provided mixed results. There was a strong positive correlation between the number of spores and soil magnesium (r = 0.96, p=0.0095) and boron concentration (r = 0.92, p=0.025). A strong negative correlation was seen between the number of spores and soil moisture (r = -0.93, p=0.021) indicating that the number of mycorrhizal spores were higher in dry soils compared to wetter soils. There was no significant correlation between the total number of spores and soil organic matter, nitrate or phosphorus concentration.

Soil organic matter is the backbone of sustainable agriculture and when growing vegetables and specialty crops, a soil high in organic matter is very desirable. Cover crops are known to increase soil organic matter and enhance the natural productivity and fertility of soil. In our study all the cover crops increased the soil organic matter compared to the control treatment, fallow. In the control treatment, the total soil organic matter at the end of the experiment was lower compared to the beginning of the experiment. In general grass cover crops are known to contribute more soil carbon than legumes (Hoorman 2009). In our study, sudangrass produced the highest above ground biomass compared to the other cover crops but surprisingly it contributed less organic matter compared to sunn hemp when measured after 4 weeks of incorporating cover crops into the soil. A possible reason for this could be the sudangrass had lignified when tilled and may not have fully decomposed in the soil.

As expected, sunn hemp resulted in highest nitrate concentration in the soil; however contrary to our expectation nitrate lablab caused a decline in the soil nitrate. A possible explanation could be the high density of weeds in the lablab plots. Similarly, sunn hemp followed by lablab and sudangrass had the highest soil phosphorus concentration while the control plot had the lowest phosphorus concentration in soil. While the total calcium concentration in the soil remained fairly stable in all the treatments, cover crops along with the associated mycorrhizal fungi do have the potential to offset the negative effect of high pH on nutrients availability.

Cover crop growth and weed suppresion

Seeding method did not have any significant difference on cover crop performance (data not shown). However, there was a significant difference in the growth and weed suppression potential among the different cover crops in the growth of varying degrees of cover crop and weed growth among the different cover crop treatments. (Table 1). Sudangrass planted 1.5 times the recommended rate produced the highest amount of biomass (2910 kg/ha) followed by sudangrass planted at 3 times the recommended rate, while lablab planted at both rates 1.5 times and 3 times produced the lowest cover crop biomass (94.4kg/ha and 107 kg/ha respectively). As expected, the biomass cover crops was negatively correlated with the weed biomass (r(14) = -0.53, p = .05). The weed to cover crop biomass in the different cover crop plots ranged from 434 kg/ha of weeds in a plot of lablab 1.5 times recommended with a biomass of 944 kg/ha 20 to 220 kg/ha of weeds in a plot of sudangrass 3 times recommended rate with a biomass 1950 kg/ha. The control treatment of 570 kg/ha had the highest above ground biomass of weeds when compared to all of the cover crop treatments at all seeding rates. Weed to cover crop biomass ratio was lowest in the sudangrass treatments at all seeding ratings. The correlation between plant height and the PPFD of the different cover crops show that sudangrass grew vigorously and blocked the most of the light from reaching the soil surface. While there is a possibility that PPFD could have been affected by the weeds our results show a strong negative correlation between the cover crops’ height and the PPFD at the soil surface. A two-way ANOVA of cover crop type and seeding rate on height of the biomass was conducted to compare the effect the cover crop type and seeding rate had on the height of the above ground biomass in lablab, sudangrass, and sunn hemp. A significant main effect of cover crop type on above ground biomass height was found, F(2,56) = 175.071 p=< 0.001. The main effect of seed rate on biomass height was not significant. The cover crop type and seeding rate interaction was significant F(2,56) = 7.084 p=0.002. Of the four different cover crops, sudangrass at both the seeding rates had the highest heights followed by pearl millet. The 1.5 times the seeding rate in sudangrass grew taller than the 3 times the seeding rate sudangrass plots. On the other hand, sunn hemp at 1.5 times the seeding rate had the lowest height growth. Results from this study show that light penetration to the soil surface decreased significantly over time. Among the seven treatments, sudangrass with seeding rate 1.5 times the recommended rate and 3 times the recommended rate resulted in the lowest light penetration along with pearl millet 2 times the recommended rate. (F(3,66) = 3.12 p=0.0319).

Cover crops are a fundamental component of organic farm management. Cover crops have the potential to produce large amounts of biomass and contribute to the maintenance and/or improvement of the physical, chemical and biological characteristics of the soil, including adaptation of effective soil depth through their roots which help in weed suppression and promote soil quality in short period of time. This is the first study documenting the potential for cover crops to suppress weeds during the summer season in the Lower Rio Grande Valley. Without the option to use chemical herbicides, weed management is a challenge for organic growers, and they need to use several biological, chemical, and cultural practices (Liebman and Davis 2000). Our results indicate that, in addition to other soil health benefits (Soti et al 2016), cover crops could serve as a successful weed management tool in organic farms. Cover crops emergence and ground coverage (represented by the light reading) varied significantly among the cover crops selected and their seeding rates. This could be the result of the seed temperature tolerance, field conditions etc. Sunn hemp performs best on well-drained soils with a pH from 5.0-7.5 (USDA NRCS). The soil at this location had a pH of 8.00 and had very high clay content. This may account for the poor performance of sunn hemp. Sudangrass followed by pearl millet had faster growth and biomass accumulation these results are similar to previous studies (Bicksler & Masiunas, 2009; Creamer & Baldwin, 2000; Teasdale, 1993; Ong & Monteith, 1985; Maiti & Bidinger, 1981).

The weed species growing in the control plots parthenium (Parthenium hysterophorus), pigweed (Amaranthus palmeri), and johnsongrass (Sorghum halepense) represent the major problematic weeds in the region. Parthenium is an annual weed that causes major problems in rangelands and cropping systems throughout the southern US and other subtropical regions. It can cost up to $22 million dollars a year in reduced crop production and increased management costs (CRC Weed Management, 2003). Pigweed, an annual weed which causes serious problems for farmers throughout the Southern US, can cost $60-$80 per acre for farmers to manage in their fields (Thompson, 2016). Johnson grass is a perennial weed and is very difficult to control with a single cultural methods or herbicide application (Johnson et al 1997). Johnson grass is one of the most costly weeds that farmers encounter and costs millions of dollars a year in lost crops, poor quality grain, and lower crop yield (Shawnee Co. Weed Department). Our study indicated that the grass species of cover crop, sudangrass and pearl millet, with faster growth and higher biomass accumulation, were more successful in weed suppression and growth compared to the legume species used, lablab and sunn hemp. Potential of grass cover crops to suppress weeds is well documented (Teasdale, 1996; Teasdale et al., 2007; Burgos & Talbert, 1996; Yenish, J. P., Worsham & York 1996.) Cover crops with faster growth rate and above and below ground biomass accumulation rates can suppress weeds by competition for resources such as light, water, and nutrients. Additionally, the reduction of weed biomass could be in part due to the allelopathic effects. Sudangrass is known to have allelopathic compounds and exudes sorgoleone which is very active at very low concentrations and has been shown to suppress weeds and may have been a factor in the low amount of weeds (Scott, 1991). With the growing market demand for organic produce, there is also a growing demand to find practices that reduce the amount of weeds in crop fields in an environmental and economically sound manner.

Participation Summary
2 Farmers participating in research

Educational & Outreach Activities

1 On-farm demonstrations
2 Online trainings
1 Published press articles, newsletters
3 Webinars / talks / presentations
1 Workshop field days

Participation Summary

12 Farmers
12 Ag professionals participated
Education/outreach description:

Results of this study were presented to the organic and organic-transitioning farmers in south Texas region in the annual Subtropical Organic Agriculture Research (SOAR) Symposium and Field Day on November 2016. It was also presented at regional meetings of The Texas Organic Farmers and Gardeners Association (TOFGA) (2/16 in San Antonio TX), and the Subtropical Agriculture and Environment Society Meeting in Weslaco TX (2/16), and the Southern Cover Crop Conference in North Carolina (summer 2016). 

Audio Video Materials

This work was featured in two you tube videos produced by the National Center for Appropriate Technology, published as videos #4 and 5 here:

This research also helped inform the ATTRA publication Cover Crop Options for Hot and Humid Areas (April 2017)

Peer reviewed publication

Soti, P. G., Rugg, S., & Racelis, A. (2016). Potential of Cover Crops in Promoting Mycorrhizal Diversity and Soil Quality in Organic Farms. Journal of Agricultural Science8(8), 42.

Rugg, S., C. Moreno, P. Soti, A. Racelis (2016). Multifunctionality of Cover Crops in South Texas: Looking at multiple benefits of cover cropping on small farms in a subtropical climate. Subtropical Agriculture and Environments Society Conference Abstracts 67-2016. Weslaco, TX.  Feburary 5.  (Peer reviewed abstract)

Project Outcomes

4 Farmers changed or adopted a practice
1 Grant received that built upon this project
Project outcomes:

Assessment of Project Approach and Areas of Further Study:

This study was a first study on the overall benefits of using cover corps in the Lower Rio Grande Valley. The major challenge faced during this project was the exceptionally high amount of rainfall during the study period, which combined with the high clay content in the soil created impossible conditions for operation of farm equipment and project execution on time. The outcomes of the study could be different if the cover crops were grown for a longer time (June-September). Thus further long term studies with additional cover crop species is needed to better understand the overall benefits of cover crops. Additionally, this study indicated that cover crops promote mycorrhizal fungi but a closer look at the benefits to the soil biology of the populations of mycorrhizae spores in the soil would also be necessary to further understand what types of mycorrhizae are being enhanced and what that means in terms of plant health

One aspect that was not addressed in this report is the beneficial insect biodiversity provided by the cover crop habitat. Many beneficial insects such as lady beetles, lacewings, parasitic wasps, and syrphid flies can maintain populations in flowering cover crop strips and provide control of pests in surrounding crop fields (Chandler et al., 1998; Altieri, 1999). Research is needed to understand if certain cover crops provide more habitat for beneficial insects than others, or to see if any cover crops are harboring crop pests. This is important because if one of the cover crops hosts similar pests to the subsequent cash crop it would not be a good chose for the farmer.

Any opinions, findings, conclusions, or recommendations expressed in this publication are those of the author(s) and do not necessarily reflect the view of the U.S. Department of Agriculture or SARE.