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As July comes to a close, it is time to make plans to seed fall cover crops. Cover crop species, seeding method, seeding rate, and timing will all depend on the goals for that cover crop. In broad terms there are three methods: 1) overseeding, 2) drill seeding, and 3) broadcast seeding with incorporation. Each of these methods has pros and cons, which will be discussed in this article.
Overseeding. Overseeding can be done into a standing corn or soybean crop either by broadcasting with an airplane or using high clearance equipment. Overseeding allows the cover crop to be planted earlier, which can lead to greater biomass growth in the fall. However, overseeding does have some drawbacks. For one, distribution of seed can be more variable with aerial seeding. The best distribution comes from uniform seed lots and heavier cereal grain such as cereal rye and winter wheat. Additionally, stand establishment can be lower than with other seeding options due to rodents and birds scavenging seed as well as if dry conditions exist following seeding. To ensure adequate establishment, overseeding should be done late August to early September, assuming adequate soil moisture. Some of the most successful overseedings are done shortly before a rainfall. The seeding rate for over seeding should be 15-25% higher than drill seeding.
Drilling. Drill seeding provides the most uniform seed distribution and excellent seed to soil contact for establishment, which results in a more consistent stand. One of the biggest drawbacks of drill seeding is that corn and soybean harvest can delay cover crop planting beyond ideal planting dates. Delayed planting can result in reduced biomass growth and less nitrogen uptake. Drill seeding should be completed by mid- to late October.
Broadcasting with incorporation. Broadcast seeding with incorporation after corn and soybean harvest is also a viable option to plant cover crops. It can be accomplished as a 1-pass or 2-pass process. One-pass systems typically have an air seeder attached to the combine or broadcast cover crop seed with fertilizer application. Incorporation can be accomplished with vertical tillage or other tillage implements, but care must be taken to ensure that incorporation is not too deep for plant emergence. Broadcast seeding with incorporation can improve overall stand establishment compared to broadcast seeding without incorporation. Like drill seeding, broadcast seeding with incorporation should be completed by mid- to late October.
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Figure 1 shows cover crop biomass production where broadcast interseeded into corn. Rye produced more biomass than vetch, on average 1,500 lb/ac. Rye biomass increased with the greater seeding rates, but the difference between the medium and high seeding rate was not significant. For vetch, increasing the seeding rate did not improve biomass production which was 400 lb/ac. Vetch should be planted earlier for better productivity. Figure 2 shows biomass production of the cover crops broadcast interseeded into soybean. Rye was more productive than vetch (2,100 lb/ac and 500 lb/ac) but increasing the seeding rates did not change biomass production of either rye or vetch.
When it comes to selecting a seeding rate for broadcasting cover crops, there is little research-based information. Broadcast seeds do not have good seed-soil contact which reduces the seeds’ ability to take up water necessary for germination. Thus, stand counts of broadcast cover crops are often lower than those of drilled cover crops. Could increasing the seeding rate overcome low stand counts and improve cover crop productivity?
We tried to answer this question by carrying out field experiments at the Eastern Nebraska Research and Extension Center near Mead and the South-Central Agricultural Laboratory near Clay Center in 2016/2017 and 2017/2018 in corn and soybean fields under no-till management. Our cover crops were cereal rye, variety Elbon, and hairy vetch, variety not stated. The research plots measured 20 by 30 feet, so we broadcast by hand instead of using equipment.
Cover crops tended to produce more biomass when established in soybean than in corn, but this was not statistically tested. Rye in corn stubble showed signs of not receiving enough light, as it was lighter in color, had longer stems, and fewer tillers than rye in soybean stubble (see figure 3). Fall tillering is associated with greater spring biomass and may be the reason why cover crops planted into soybean were more productive than those planted into corn. Rye can also compensate for low stand counts by tillering, which explains the lack of response to increased seeding rates. On the other hand, the greater amount of residue and taller stalk remaining in corn fields may preserve more soil moisture and may protect from wind, thus benefitting cover crops in dry and cold sites (figure 4).
With increasing seeding rates, seed costs of course increase. Table 2 shows the cost of seeding each cover crop at the three rates – while rye is one of the most inexpensive cover crops, hairy vetch is much more expensive. Considering the cost, it is important to give it optimum conditions for success, such as planting it early and inoculating the seed.
On average, in rye plots, 13% of broadcast seed emerged, with stand counts of 6 plants/sq ft. In vetch plots, 30% of seed emerged, and there were about 5 plants/sq ft. Rainfall within a week after broadcasting is critical for good establishment and was greater at the eastern site than at the south-central site. As a result, more seeds emerged at the eastern site. Stand counts increased with increasing seeding rates.
Table 1 contains cover crop management and sampling dates. For rye, seeding rates (pure live seed) were 60 lb/ac, 90 lb/ac, and 120 lb/ac. Hairy vetch seeding rates were 40 lb/ac, 60 lb/ac, and 80 lb/ac. In the fall, we determined cover crop stand counts by counting the number of plants in two 5×1 ft frames in each plot. In the spring, we measured productivity by clipping biomass in two 5×1 ft squares per plot, drying and then weighing biomass (see table 1). We used analysis of variance to find statistically significant differences (α = 0.05).