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by Amanda Lay-Walters and Dr. Amanda McWhirt - September 12, 2022
Cover crops are a cultural production tool that have various benefits for crop production,
including adding organic matter to the soil, contributing to weed suppression, and
leguminous cover crops can fix nitrogen which increases soil nitrogen content. Weed
control and increases in soil organic matter are generally tied to the amount of cover
crop biomass (plant growth) that occurs. For example, a cover crop planting that has
a “good stand” and high amount of biomass will generally suppress weeds better and
potentially add more organic matter to the soil than a “poor stand” with low cover
crop biomass. Grass cover crops generally produce more biomass than legume or broadleaf
For growers interested in planting a winter cover crop to add organic matter and nitrogen
to the soil, while also suppressing winter weeds, a multi-state trial coordinated
by the SCCC was conducted to test planting different legume cover crops at multiple seeding rates
and interplanted with and without rye.
To assess how different seeding rates of legumes interplanted with and without rye
impacts winter weed control and cover crop biomass an experiment was conducted at
the University of Arkansas Vegetable Research Station in Kibler, AR. Five winter legume
cover crops (berseem clover ‘Frosty’, common vetch, crimson clover ‘Dixie’, hairy
vetch ‘Au Merit’, and winter pea ‘Wyo’) were planted at four seeding rates: High (1.5x
rate), Full (1x rate), Low (0.5x rate), and Lowest (0.25x rate). The specific seeding
rate for each legume are presented in Table 1, and each rate was planted with and
without an interplanting of cereal rye. For plots with interplanting of cereal rye,
the rye was planted at a rate of 30 lbs/ac. Seeding took place on October 12, 2021using
a drill seeder. On April 15, 2022, plots were assessed for weed biomass, legume cover
crop biomass, and cereal rye biomass by harvesting the aboveground plant matter in
a 0.5 m2 area of the plot, which was then calculated and reported in kg/ha. Each combination
of treatments had five replicates.
Table 1. Seeding rates of winter legume cover crops.
Hairy vetch had the lowest weed biomass (665.38 kg/ha) (Figure 1) of the legume cover crops we tested. However, winter pea had very similar weed biomass
to hairy vetch and for some growers there is less concern about winter pea’s potential
for setting seed and becoming an escaped weed like there is for hairy vetch. Berseem
clover, common vetch and crimson clover had nearly 2-3x higher weed biomass than Hairy
vetch and Winter pea. However, statistically Winter pea performed similarly to berseem clover, and common
vetch in weed suppression (Figure 1). Note that similar weed suppression was seen
for each legume type regardless of the seeding rate. So lower seeding rates resulted
in similar weed suppression as higher seeding rates.
Overall hairy vetch had the highest cover crop biomass and winter pea was the next
highest biomass producer (Figure 6). However, we saw that seeding rate impacted legume
cover crop biomass, so we will talk about those results in more detail in the 3rd section of this report.
Figure 1. Weed biomass (kg/ha) by type of winter legume cover crop.
An advantage of planting cereal rye in a mix with a winter legume cover crop was 50%
lower weed biomass (Figure 2). Another advantage was that total (legume + rye) cover crop biomass was about 50% higher
when rye was interplanted with a legume (Figure 3). This significantly higher cover
crop biomass would result in more organic matter to be incorporated into the soil
or used as organic mulch.
Figure 2. Winter weed biomass (kg/ha) with and without cereal rye in the planting.
One disadvantage was that legume biomass was about 40% lower when cereal rye was combined
with legumes (Figure 4 & 5). This decrease in legume biomass could mean that the legume + cereal rye cover crop
mixes have a lower amount of total nitrogen in the biomass compared to legume cover
crops planted without cereal rye.
In general, no, the ‘full’ and ‘high’ seeding rates had similar cover crop biomass
for each legume species, so planting a higher rate didn’t result in more cover crop
However, there were some legumes that had higher biomass at the full seeding rates than lower seeding rates. In our experiment
the ‘Full rate’ of crimson clover produced significantly more crimson clover biomass
compared to ‘Lowest rate’ of crimson clover (Figure 6). This indicates that using
a lower seeding rate of crimson clover to save on seed costs, could result in a reduction
in cover crop biomass.
However, for all the other legume cover crops we tested statistically similar legume
biomass was observed between the ‘Lowest’ and ‘High’ seeding rates for each type.
This means that reduced seeding rates could be used for Berseem clover, common vetch,
hairy vetch and winter pea and achieve similar results as the full seeding rate which
would save on seed costs.
We recommend specialty crop growers plant Hairy Vetch at a rate of 5 lbs./ac or Winter
Pea at a rate of 20 lbs./ac rate for growers in Arkansas.
These rates are suitable when planting using a drill. Higher rates may be required
if broadcast planting due to lower germination rates.
Planting cover crop mixes (winter legume + cereal rye) on average resulted in a 50%
increase in cover crop biomass and 50% reduction in weed biomass compared with planting
a legume by itself.
Other research at the University of Arkansas has found good results with Cereal Rye
+ Austrian Pea planted before spring planted watermelon crops (Hotz et al. 2020).
We recommend planting winter cover crop mixes in mid-September in northern Arkansas,
and late September to early October in South Arkansas.