Overseeding Annual Ryegrass and Cereal Rye into Soybean as Part of a Multifunctional Cropping System: I. Grain Crop Yields, Winter Annual Weed Cover, and Residue After Planting

Luke B. Smith, Graduate Research Assistant, and Robert L. Kallenbach, Associate Professor, Division of Plant Sciences, University of Missouri, Columbia 65211

Abstract

Annual ryegrass (Lolium multiflorum Lam.) and cereal rye (Secale cereale L.) are two forages that fit well into mixed row-crop/livestock operations. They can be used both as a cover crop and as a source of winter pasture. However, few studies have investigated the potential to integrate these forages into a soybean [Glycine max (L.) Merrill]-winter pasture-corn (Zea mays L.) rotation. This experiment was conducted with each of these cover crop/forage species overseeded into soybean at different stages of development, and subsequently harvested as forage throughout the winter. Corn was grown as a subsequent crop. Soybean yield was not altered by overseeding annual ryegrass or cereal rye. The overseeded treatments had at least 60% more residue cover and at least 70% less weed cover compared to the control plots. In Year 2, corn yield was lower when annual ryegrass was overseeded regardless of soybean developmental stage and when cereal rye was overseeded at soybean development stage R 6.5. An extended period of low rainfall during the summer of 2005 strongly influenced Year 2 corn yields. Our results demonstrate that mixed row-crop/livestock operations in the lower Midwest could use cereal rye or annual ryegrass as a cover crop but subsequent corn yield can be reduced in years where soil moisture is limiting.

Introduction

Row-crop producers and mixed row-crop/livestock operations in the lower Midwest are considering annual ryegrass and cereal rye as winter cover crops in a soybean-corn rotation. The establishment of annual ryegrass or cereal rye prior to or immediately after soybean harvest has three potential benefits. First, annual ryegrass and cereal rye can serve as cover crops to reduce soil erosion (3,8). Second, a winter cover crop could suppress winter annual weeds (9,13), which are becoming more of a problem in Roundup-Ready soybean (2). Finally, annual ryegrass or cereal rye could provide a winter forage source for beef cattle and stocker calves as discussed by Kallenbach et al. (5) and Moyer and Coffey (7). Although this last benefit seems like a natural option for mixed row-crop/livestock operations, it has largely been ignored in cover-crop research. These advantages could increase the profitability for both beef and row-crop operations, but research regarding these potential benefits in an integrated system is lacking.

Many producers are concerned that insertion of cover crops in a two-year soybean/corn rotation will reduce grain yields. In many studies with annual ryegrass and cereal rye used as cover crops, subsequent grain yields were reduced, but in almost all of these studies, the cover crop was not harvested as forage. Leaving this forage could have a large impact on subsequent corn yields, as Kessavalou and Walters (6) found that cover crop residue at planting often is allelopathic. However, in an integrated system where forage is removed during winter, the allelopathic and competitive effects of a cover crop might be reduced or easier to manage.

Our objectives were to: (i) determine if overseeding annual ryegrass and cereal rye into soybean effects concurrent soybean yield and subsequent corn yield in rotation; (ii) determine how annual ryegrass and cereal rye seeding dates impact winter annual weed cover in fallow soybean fields; and (iii) determine the effect of annual ryegrass and cereal rye on residue cover after planting corn. In a companion paper (14), we described the forage yield and nutritive value of annual ryegrass and cereal rye when used as part of a soybean-winter pasture-corn rotation.

Overseeding annual ryegrass or cereal rye

This experiment was conducted at the Bradford Research and Extension Center, near Columbia, MO (38°57’N, 92°20’W). The soil type at this location was a Mexico silt loam (fine, smectitic, mesic Aeric Vertic Epiaqualf). Two "cycles" of this experiment were conducted on different sites at the same location, with each cycle taking approximately 18 months as shown in Fig. 1. Hereafter, the first cycle will be referred to as Year 1, and the second cycle will be referred to as Year 2. Within each cycle, soybean was planted, overseeded with annual ryegrass or cereal rye at various stages, and then the soybean harvested for grain. The overseeded forage was harvested throughout winter. Corn was planted upon removal of the overseeded forage the following spring.

Fig. 1. Timeline of a single cycle of overseeding annual ryegrass and cereal rye into soybean, forage harvests, and subsequent corn crop.

Roundup Ready ‘Pioneer 92B75’ (maturity group 2.7) soybean was planted on 24 May in Year 1 and on 17 May in Year 2. The field was chisel plowed in autumn and disked in the spring prior to planting. Glyphosate was used to control weeds during the growing season. Soybean was drilled in 7.5-inch-wide rows at 175,000 seeds/acre. Soil P and K was maintained at the levels recommended by the University of Missouri Soil Testing Laboratory.

This experiment had seven treatments, consisting of three different overseeding dates for both annual ryegrass and cereal rye into soybean and an unseeded control (Table 1). Annual ryegrass and cereal rye were overseeded at soybean reproductive stages R 5.5, R 6.5, and R 8 (11). Individual plots were 35 ft × 30 ft and replicated five times. At the stages and dates listed in Table 1, either ‘Saddle Pro’ annual ryegrass or ‘Wintergrazer 70’ cereal rye was overseeded into soybean. The seeding rates for the annual ryegrass and cereal rye were 35 and 125 lb/acre of pure live seed, respectively. The annual ryegrass and cereal rye were overseeded with a drop type seeder that had seed openings on 5.1-inch centers. The drop seeder was modified with 5-ft-diameter tires (Fig. 2) that provided 2.5 ft of ground clearance, which was sufficient to clear the soybean crop.

Fig. 2. Modified drop seeder used for overseeding annual ryegrass and cereal rye into soybean.

Table 1. Annual ryegrass and cereal rye overseeding treatments based on soybean developmental stage and Year 1 and Year 2 overseeding dates. Year 1 was for the 2003-2004 cycle and Year 2 was for the 2004-2005 cycle.

Species	Reproductive stage of soybean	Description of treatment	Year 1 seeding dates	Year 2 seeding dates
Annual ryegrass	R 5.5	2 weeks before leaf drop	18 August	5 August
Cereal rye	R 5.5	2 weeks before leaf drop	18 August	5 August
Annual ryegrass	R 6.5	At leaf drop	4 September	19 August
Cereal rye	R 6.5	At leaf drop	4 September	19 August
Annual ryegrass	R 8	At harvest	18 September	13 September
Cereal rye	R 8	At harvest	18 September	13 September
Control	—	—	—	—

Soybean yield was measured in all seven treatments by direct combining three, 5-ft × 35-ft strips from each plot. In Year 1, soybean was harvested on 17 and 18 September and in Year 2 soybean was harvested on 13 September. Sub-samples from each plot were retained for moisture determination. After soybean harvest, 60 lb N per acre (as ammonium nitrate) were broadcast to stimulate annual ryegrass and cereal rye growth. In early March, an additional 60 lb N per acre were applied to maximize spring growth. Nitrogen was not applied to the control plots as it would not normally be applied to a fallow soybean field. Forage growth was evaluated weekly with a rising plate meter and harvested as described in a companion paper by Smith and Kallenbach (14). In addition, winter annual weed suppression was measured by visually rating plots for percent weed cover on 9 October and again on 17 March for Year 1, and 15 October and 14 March for Year 2.

After harvesting forage growth throughout early spring, the annual ryegrass and cereal rye were terminated on 27 April of Year 1 and Year 2 with glyphosate at 1.3 lb a.i./acre. Corn planting was delayed two weeks to eliminate the potential allelopathic effects of cereal rye (10). After this two-week period, ‘Pioneer 33P67’ corn was no-tilled into plots with a 6-row corn planter. The corn hybrid was planted on 30-inch rows at 29,000 seeds/acre. In Year 1, due to poor stands caused by soil crusting, the corn was replanted on 2 June. In Year 2, the annual ryegrass was not sufficiently terminated with the glyphosate, so all plots were sprayed with an additional 2.0 lb atrazine per acre, 0.39 oz nicosulfuron per acre, and 0.19 oz of rimsulfuron per acre on 25 May 2005. One hundred sixty lb of N per acre were applied at planting to the corn each year. Other soil fertility, weed, and insect control applications followed the management practices recommended for corn by the University of Missouri. Directly after planting corn, surface residue cover was estimated using a 100-point line transect (12). Five counts were taken in each plot with a 20-point line transect and the cumulative residue count was expressed as a percentage (6).

On 12 November for Year 1, and 13 October for Year 2, corn yield was measured in all treatments the year following the annual ryegrass and cereal rye seeding. Corn yield was measured by hand harvesting four, 15-ft rows from each plot. The corn was shelled using a stationary sheller. Sub-samples from each plot were analyzed for moisture content.

Statistical Analysis

The seven treatments were replicated five times in a completely randomized design [35 total plots (5 replications × 7 treatments)]. Analysis of variance was conducted on forage type by seeding date combinations (main plots in a 2 × 3 factorial arrangement), years (sub-plots), and all possible interactions using the model outlined by Steel and Torrie (15). Statistical Analysis Systems software (version 8.2) was used to analyze the data (SAS Institute Inc., Cary, NC). Main effects and all interactions were considered significant when P < 0.05.

Soybean Yield after Overseeding

Soybean yield differed by years due to dry conditions in the late summer of 2003 (Table 2). Mean yield for Year 1 was 16 bu/acre, compared to 64 bu/acre for Year 2 (Table 3). Soybean yield was not affected by any of the overseeding treatments in either year (Table 3). It appears that neither annual ryegrass nor cereal rye seedlings compete with maturing soybean for nutrients or water. These results were similar to those of Hively and Cox (4) who also found that overseeded annual ryegrass and cereal rye did not affect soybean yield. Initially, we were concerned that standing forage might interfere with soybean harvest; however, we found that the forage material fed through the combine with ease. Our research shows that overseeding cover crops into soybean can be successful at the R 5.5 stage or later without affecting soybean yield or interfering with soybean harvest.

Table 2. Monthly precipitation and average air temperature at Columbia, MO during 2003, 2004, and 2005. Historic averages represent 36 years of data.

Month	Monthly total precipitation (inches)				Average temperature (°F)
Month	2003	2004	2005	Historic average	2003	2004	2005	Historic average
January	—	1.7	5.2	1.5	—	25	30	27
February	—	0.5	1.9	1.9	—	32	39	32
March	—	5.9	0.9	3.2	—	46	43	45
April	—	2.6	4.3	3.8	—	57	57	55
May	5.4	4.7	2.1	5.0	63	66	65	64
June	6.6	1.7	3.6	4.3	70	70	79	72
July	1.7	4.4	0.5	3.7	77	73	79	77
August	2.8	5.1	8.8	3.3	79	70	77	75
September	9.5	0.9	4.5	3.9	64	68	72	68
October	2.3	3.1	2.3	3.2	57	57	56	57
November	2.7	4.8	—	2.9	46	46	—	45
December	3.0	0.9	—	2.5	36	36	—	32

Table 3. Soybean yield for Year 1 and Year 2 with annual ryegrass and
cereal rye overseeded at different soybean reproductive stages. Year 1
was for the 2003-2004 cycle and Year 2 was for the 2004-2005 cycle.

Species	Reproductive stage of soybean	Year 1	Year 2
Species	Reproductive stage of soybean	Yield (bu/acre)
Annual ryegrass	R 5.5	17	64
Cereal rye	R 5.5	15	67
Annual ryegrass	R 6.5	17	64
Cereal rye	R 6.5	14	63
Annual ryegrass	R 8	17	63
Cereal rye	R 8	18	62
Control	—	15	65
LSD (0.05)	—	NS	NS

Weed Cover

Weed cover for Year 1 was about 2.5 times greater than for Year 2. In autumn, there were significant differences between treatments in weed cover both years but they did not follow a consistent trend. In spring, weed cover remained low for all the overseeded treatments, but in the unseeded control plots, weed cover was 4 to 30 times greater in the control than in the annual ryegrass or cereal rye plots (Fig. 3). Our results were in agreement with Shrestha et al. (13) who found lower weed pressure in plots with cover crops such as cereal rye compared to plots without cover crops. It is clear that seeding annual ryegrass or cereal rye into soybean fields reduces the prevalence of winter annual weeds.

Fig. 3. Weed cover in the autumn and spring of Year 1 and Year 2 of annual ryegrass and cereal rye overseeded at different soybean reproductive stages. Year 1 was for the 2003-2004 cycle and Year 2 was for the 2004-2005 cycle. Plots were rated visually on a 0 to 100 scale. Bars with different letters within a season and year are significantly different (P < 0.05) using Fisher's protected LSD.

Residue Cover

In the spring of both years, the annual ryegrass and cereal rye treatments had 2.5 to 4 times more residue cover than the control treatment (Fig. 4). Comparisons among overseeded treatments gave inconsistent trends between years. In the overseeded treatments the residue was almost entirely from the annual ryegrass or cereal rye. In the control treatment, the residue was mainly from winter annual weeds. Overseeded treatments ranged from 52 to 68% residue cover, and while at times there were some significant differences between these treatments, it is doubtful that these differences represent any biological significance in terms of soil erosion or runoff potential. However, using the model of Dickey et al. (1), the control treatment had an erosion potential 1.7 times greater than any of the overseeded treatments. The values for residue cover were lower than those found by Kessavalou and Walters (6), but this was expected as their cover crops were not harvested for forage. Nonetheless, the overseeded treatments had significantly more residue cover than the control, and this would help prevent soil erosion.

Fig. 4. Percent residue cover after corn planting in Year 1 and Year 2 of treatments that had annual ryegrass and cereal rye overseeded at different soybean reproductive stages. Year 1 was for the 2003-2004 cycle and Year 2 was for the 2004-2005 cycle. Bars with different letters within a year are significantly different (P < 0.05) using Fisher's protected LSD.

Corn Yield in the Subsequent Year

Corn yields in Year 1 averaged 142 bu/acre (Table 4) and no significant differences existed between overseeding treatments. Corn yields in Year 2 averaged only 61 bu/acre, and were strongly influenced by an extended dry period during May, June, and July of 2005 (Table 2). Corn planted into the annual ryegrass treatments for Year 2 yielded 33 to 39% less than the control (Table 4). This impact was likely due to the difficulty in terminating the annual ryegrass in spring, coupled with the dry growing conditions in mid-summer. Cereal rye had less impact on subsequent corn yields, with cereal rye overseeded at the R 6.5 stage being the only treatment yielding less (24%) than the control. Our results were similar to those of Hively and Cox (4), Vyn et al. (16), and Kessavalou and Walters (6) who found that annual ryegrass and cereal rye cover crops could negatively affect corn yields. From our results, there is relatively little risk of reducing subsequent crop yields in years where monthly rainfall is near normal or irrigation is a possibility. In years where moisture is a limiting factor, planting corn after annual ryegrass or cereal rye overseeded at the R 6.5 stage would likely reduce grain yields. Additionally, annual ryegrass is more difficult to terminate in spring than cereal rye; care must be taken to be sure it is fully dead before corn is planted or yields may be reduced.

Table 4. Corn yield following annual ryegrass and cereal rye that had
been overseeded at different soybean reproductive stages and a control treatment that was not overseeded. Year 1 was for the 2003-2004 cycle
and Year 2 was for the 2004-2005 cycle.

Species corn was seeded after	Reproductive stage of soybean	Year 1	Year 2
Species corn was seeded after	Reproductive stage of soybean	Yield (bu/acre)
Annual ryegrass	R 5.5	147	50
Cereal rye	R 5.5	152	70
Annual ryegrass	R 6.5	139	48
Cereal rye	R 6.5	132	60
Annual ryegrass	R 8	142	53
Cereal rye	R 8	138	69
Control	—	147	79
LSD (0.05)	—	NS	12

Conclusions

Overseeding annual ryegrass and cereal rye into soybean in late summer did not affect soybean yields. Corn yields the following year were unaffected by overseeding annual ryegrass or cereal rye for Year 1. However, Year 2 corn yields were lowered by an average of 36% when annual ryegrass was overseeded into soybean regardless of soybean maturity stage, and by 24% when cereal rye was overseeded at soybean maturity stage R 6.5. The discrepancy in corn yield between years was related to a summer drought in Year 2. Terminating the annual ryegrass in the spring of Year 2 was difficult and took an additional herbicide treatment. Our data show that overseeding annual ryegrass or cereal rye reduces winter annual weeds and improves soil conservation by providing residue cover that otherwise would not be present in a typical soybean-corn rotation. However, the extra expense to establish the cover crop and the potential to lower corn yields the following year, make it a risky choice for many producers. Producers would be more likely to overseed annual ryegrass or cereal rye if direct income were available from the cover crop. In our view, the best way to provide this income would be to use annual ryegrass or cereal rye as a source of winter pasture for livestock.

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