Integrating Perennial Living Mulches into Irrigated Cropping Systems

Integrating Perennial Living Mulches into Irrigated Cropping Systems

Summary

One of the challenges associated with this system is how to adequately suppress the living mulch without killing it. We found that burndown herbicides such as paraquat and glufosinate allowed for rapid recovery of legume living mulches, which resulted in reduced yields of the annual cash crop due to competition. The combination of strip-tilling and use of glyphosate herbicide appears to be the most effective strategy for suppressing the living mulch. Of the legumes tested (alfalfa, birdsfoot trefoil, and red, white, and kura clover), it appears that white clover has the greatest ability to recover from increasing rates of glyphosate.

Objectives/Performance Targets

1. Determine methods of establishing various perennial plant species potentially adapted for use as living mulches under irrigation.

2. Evaluate methods of suppressing living mulches to avoid reduced yields of associated crops.

3. Quantify the environmental and economic benefits of using living mulch systems under irrigation.

4. Demonstrate the benefits of using living mulch systems for crop production under irrigation to producers through on-farm trials.

Accomplishments/Milestones

Two sites near Fort Collins, Colorado were evaluated for legume establishment in the spring of 2010. Each of these sites included birdsfoot trefoil, white clover, and a mix of red, white, and kura clover, which were seeded with corn and oats in 2009. The goal of this study was to determine whether these legumes could be co-established with the annual crops for use as a living mulch in future years. This would eliminate the need for taking cropland out of production during the establishment year. Our results indicated that all of these legumes could be established without any adverse effects on annual crop yield or quality when compared to a control with no living mulch. The spring evaluation consisted of both biomass sampling and visual ratings. Legumes grown under oats tended to establish better, likely because the oat hay was harvested significantly earlier than the corn silage, allowing a longer competition-free growing period. In terms of biomass, legumes established under corn averaged 276 kg/ha while legumes established under oats cut at the boot stage and soft dough stage yielded 951 and 611 kg/ha, respectively (Fig. 1). Between the legume treatments, the clover mix yielded the highest, averaging 869 kg/ha across annual crops, followed by birdsfoot trefoil and white clover yielding 542 and 427 kg/ha, respectively. The only drawback to the clover mix was that its high biomass production after oat harvest, due largely to the red clover, made an ideal cover and food source for voles, which destroyed parts of some plots. This could likely be prevented by late summer mowing.

In continuation of the co-establishment study conducted at Fruita in 2009, a legume mix of birdsfoot trefoil, red clover and white clover was seeded with corn using a Buffalo brand no-till corn planter. This co-establishment trial was unique in that the planter was modified to seed the corn, broadcast the legume seed and apply a granular herbicide in a single pass. The herbicide, Harness, was applied in a thin strip down the middle of the bed for weed and legume control in the corn row. By eliminating the need for a separate planting operation, growers can reduce the cost of establishing the mulch to the cost of legume seed. Later in the season, strips within the field were treated with glyphosate for weed control and side-dressed with nitrogen, while others were left with no additional herbicide or fertilization. Grain yield and residue data were collected for each treatment. Overall, there was a decrease in fall legume biomass in comparison to the previous year when legumes were planted in a separate operation prior to corn planting. However, additional modifications could be made to increase legume seed to soil contact and possibly increase legume establishment using the corn planter. Treatments of glyphosate without side-dress N, glyphosate with side-dress N and side-dress N without glyphosate yielded 78, 41 and 22 kg/ha of legume biomass, respectively. Legume biomass was greatest when neither side-dress nitrogen nor post-emergence glyphosate were applied averaging 163 kg/ha. However, weed pressure was intense under treatments with no glyphosate application, resulting in unacceptable competition with the corn.

Variable rates of glyphosate were applied to established living mulches as a means of spring suppression prior to corn planting at two sites near Fort Collins, Colorado. This was done to help determine an appropriate sub-lethal rate capable of adequately delaying legume regrowth (minimizing early competition with corn) without completely killing the cover crop. Glyphosate was chosen due to its potential for post-emergence weed or mulch control in glyphosate-resistant corn. Legume species tested included white clover, birdsfoot trefoil, and a mix of red, white, and kura clovers. Rates of 1.0, 1.5 and 2.0 kg a.e./ha of glyphosate were applied to each legume species. Paraquat (0.7 kg a.i./ha) was also tested at one site. Sites were strip-tilled and planted three days after spray applications. Glyphosate was applied to all plots in early July at a rate of 1.0 kg a.e./ha. At this point, all initial glyphosate treatments were under intense weed pressure, and the paraquat treatment had not adequately suppressed any of the legumes.

All corn was harvested for grain. Residue samples were collected, and corn stover components as well as legume biomass were separated and weighed. The contribution of these legumes is important because of their potential to increase the forage quality of grazed crop aftermath by supplementing protein. Yield and residue data will be analyzed to determine the effects of spray rate on both legume recovery and corn yield.

A similar suppression study was conducted at the Western Colorado Research Center (WCRC) at Fruita. In this case, a mix of red, white and kura clovers was tested with five different suppression treatments. These included glyphosate applied at 1.5, 2.0 and 2.5 kg a.e./ha, as well as paraquat and glufosinate at 0.7 kg a.i./ha and 0.5 kg a.i./ha, respectively. A light broadcast application of glyphosate for weed control was also deemed necessary at the end of June. Based on observations in early September, the burndown herbicide treatments (paraquat and glufosinate) appeared to have greater legume growth and slightly more weed pressure than the glyphosate treatments. Yield and residue data were collected as in the aforementioned study.

In addition to these suppression studies, alfalfa, birdsfoot trefoil, red clover, white clover and kura clover were tested for glyphosate tolerance and recovery time in both greenhouse and field trials. Plants were visually evaluated every seven days after glyphosate application at rates of 1.0, 1.5, 2.0 and 2.5 kg a.e./ha. Total above ground biomass, and root mass in the case of the greenhouse trial, was recorded and compared between treatments after eight weeks. An additional above ground biomass sample was taken after 16 weeks in the field study. The clovers proved to be much more resilient than alfalfa and birdsfoot trefoil. After eight weeks, all showed decreasing biomass as spray rate increased (Fig. 2). However, after 16 weeks, there were no differences among white clover treatments (avg. 3730 kg/ha), including the untreated control, indicating a full recovery from even the high rate of glyphosate. The other clovers did not recover completely, but even the highest spray rate yielded 1510 and 1150 kg/ha for kura clover and red clover, respectively. Alfalfa showed the weakest recovery, with the three highest rates yielding less than 50 kg/ha. Trends were similar in the greenhouse trial with the clovers recovering faster than the other species. However, in comparison to the untreated controls, recovery was much less than in the field study. This was probably due to the root growth limitation imposed by the pots. Also, there was a more dramatic drop in biomass yields between the 1.0 and 1.5 kg a.e./ha rates after which level of recovery seemed to stabilize. Some mortality of birdsfoot trefoil and alfalfa occurred at the 1.5 and 2.0 kg a.e./ha rates. The highest rate (2.5 kg a.e./ha) was not tested in the greenhouse. The knowledge gained from these experiments will improve understanding of how these species recover from different rates of glyphosate. This can be used to determine differences in how they should be managed as living mulches.

At Matsuda Farms (our cooperating organic producer), triticale, which had been planted the previous fall, was harvested for hay in mid-June. The triticale proved to be the most successful annual grass tried at the site in terms of its competition with the living mulch (a mix of red, white and kura clover). Its early-spring growth highlights the potential of cool-season annuals, particularly in organic systems where suppression options are limited and establishment of the annual is a challenge. In addition to the high triticale yield, the hay still included a fair amount of clover, improving its quality. The field was then strip-tilled and seeded with sudangrass in early July. However, much like the foxtail millet attempted the previous year, it was unable to compete with the clover. Based on these observations, it appears that even with an aggressive summer annual, mowing and strip-tilling alone are insufficient as a means of living mulch suppression.

Our main goal in 2010 was to explore different spring suppression options for the living mulches. Trials in previous years have indicated that a combination of strip-tilling and herbicide application is the most effective control strategy. However, finding the appropriate herbicide and rate has proved difficult. Our hope is that upon analyzing this year’s yield and residue data across different study sites and legume species, we will be able to identify some of the more promising chemical-suppression regimes. From our observations during the growing season, it appears that the burndown herbicides tested allowed for rapid recovery of the legumes, which resulted in competition between the annual crop and the mulch. While the efficacy of glyphosate can be influenced by a number of environmental and physiological factors, we hope to determine an effective range of application rates and rule out those which are too high or low, resulting in complete kill or inadequate suppression.

Further, field experiences this year confirmed the necessity of an herbicide-resistant annual crop. A mid-summer glyphosate application for weed suppression was deemed necessary at all study sites. Without this option, yield losses through competition with the mulch, weeds or both can be substantial.

• Spring biomass of birdsfoot trefoil (BFT), white clover (WC), or mix of red, white, and kura clovers following co-establishment the year before with early- or late-cut oats or corn harvested for silage.
• Recovery of 5 legume species 8 and 16 weeks after spraying with glyphosate herbicide at increasing rates.
• Planting corn into legume living mulch plots following application of herbicides and strip tilling.
• Small plots of alfalfa, birdsfoot trefoil, red clover, white clover, and kura clover 4 weeks after treatment with variable rates of glyphosate.
• White clover living mulch under corn (Sept. 23, 2010).
• Triticale in a mixed clover (red, white, and kura) living mulch.
• White clover living mulch under corn (Oct. 27, 2010).

Impacts and Contributions/Outcomes

Our results from this year provide some important knowledge and guidelines for producers considering implementation of a living mulch system. The legume species we tested can all be co-established with annual crops, including oats and corn, which reduces cost of establishment. Once fully analyzed, the results of the suppression and herbicide tolerance studies will provide guidelines for chemical suppression, which is the main obstacle for this system. Ultimately, optimum rates will be unique to individual producers based on their production goals. Results were presented in poster form at the combined American Society of Agronomy (ASA), Crop Science Society of America (CSSA) and Soil Science Society of America (SSSA) International Annual Meetings in Long Beach, California. Another poster was displayed at a media day at the Western Colorado Research Center at Fruita, at which local journalists were invited to learn about current agricultural research in western Colorado. A seminar was also presented through the Soil and Crop Sciences Department at Colorado State University.

Collaborators: