By: Katja Koehler-Cole

The middle to end of September is a good time to establish cover crops by broadcasting seeds into corn or soybean before harvest. Broadcast interseeding before harvest allows cover crops to capture more sunshine, growing degree days and rainfall than drilling after harvest. Previous studies have shown that cover crops in eastern Nebraska usually have greater productivity when they are established before harvest than drill-planted after harvest. Cover crops with more biomass will better reduce erosion and run-off and return more organic matter to the soil. In legume cover crops, the amount of N fixed increases the more biomass is produced.

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 seed’s 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) — a highly productive winter annual cover crop — and hairy vetch (variety not stated), a cool-season legume for N fixation. The research plots measured 20-by-30 feet, so we broadcast seed by hand instead of using equipment.

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-by-1-foot frames in each plot. In the spring, we measured biomass by clipping plants in two 5-by-1-foot squares per plot, drying and then weighing biomass (see Table 1). We used analysis of variance to find statistically significant differences (α = 0.05).

Table 1. Crop management and sampling schedule.
Activities Clay Center (south-central site) Mead (eastern site)
Corn Soybean Corn Soybean
Year 1
Plant main crop 12 May, 2016 13 May, 2016 3 June, 2016 9 June, 2016
Plant cover crop 20 Sep. 23 Sep. 21 Sep. 21 Sep.
Harvest main crop 18 Oct. 19 Sep. 1 Nov. 7 Nov.
Count cover crop 1 Dec. 1 Dec. na† na
Biomass cover crop 4 May, 2017 4 May, 2017 8 May, 2017 8 May, 2017
Year 2
Plant main crop 8 May, 2017 12 May, 2017 12 May, 2017 31 May, 2017
Plant cover crop 22 Sep. 14 - 19 Sep. 19 Sep. 21 Sep.
Harvest main crop Late Oct. Mid-Oct. Late Oct. 6 Nov.
Count cover crop 20 Nov. 20 Nov. 30 Nov. 30 Nov.
Biomass cover crop 4 May, 2018 8 May, 2018 9 May, 2018 8 May, 2018
†na — data not taken

On average, in rye plots, 13% of broadcast seed emerged, with stand counts of six plants per square foot. In vetch plots, 30% of seed emerged, and there were about five plants per square foot. Stand counts increased with increasing seeding rates. 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.

Figure 1 shows cover crop spring 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 versus 500 lb/ac) but increasing the seeding rates did not change biomass production of either rye or vetch.

Graph of Biomass production
Figure 1. Biomass production (in lb/ac) of cover crops broadcast interseeded into corn in September. Cover crops were either cereal rye (blue bar) or hairy vetch (orange bar), broadcast at three seeding rates. Low seeding rate for rye was 60 lb/ac, medium was 90 lb/ac, high was 120 lb/ac. For vetch, low seeding rate was 40 lb/ac, medium seeding rate was 60 lb/ac, high seeding rate was 80 lb/ac.
Graph of Biomass production
Figure 2. Biomass production (in lb/ac) of cover crops broadcast interseeded into soybean in September. Cover crops were either cereal rye (blue bar) or hairy vetch (orange bar), broadcast at three seeding rates. Low seeding rate for rye was 60 lb/ac, medium was 90 lb/ac, high was 120 lb/ac. For vetch, low seeding rate was 40 lb/ac, medium seeding rate was 60 lb/ac, high seeding rate was 80 lb/ac.

Of course, seed costs increase with increasing seeding rates. 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, rather than increasing the seeding rate.

Table 2. Cover crop seed costs at each of the three seeding rates. Seed costs are the average calculated from the cost of seed per lb obtained from three regional seed dealers.
Cereal rye seed cost ($0.37/lb) Hairy vetch seed cost ($2.45/lb)
Seeding rate 60 lb/ac 90 lb/ac 120 lb/ac 40 lb/ac 60 lb/ac 80 lb/ac
Seed cost in $/ac $22.2/ac $33.3/ac $44.4/ac $98/ac $147/ac $196/ac

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. 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 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 offer protection from wind, thus benefitting cover crops in dry and cold sites.

Take-home Message

Despite low emergence, cereal rye was a productive cover crop when established by late-season broadcast interseeding. When broadcast interseeding cereal rye into corn, we found that a seeding rate of 90 lb/ac produced the most biomass in the spring. When broadcast interseeding into soybean, a seeding rate of 60 lb/ac produced the same amount of biomass as higher seeding rates. Vetch biomass production was low and was the same at the seeding rate of 40 lb/ac than at the higher seeding rates. Vetch needs to be established earlier than the mid- to late September seeding dates in our study.