Amy Shober, Professor and Extension Specialist, Nutrient Management and Environmental Quality, ashober@udel.edu; Jarrod Miller, Assistant Professor and Extension Specialist, Agronomy, jarrod@udel.edu; Jason Challandes, Regional SARE Educator, Delaware State University
Over the past eight years, we have collected more than 12 site-years of cover crop data as part of Delaware NRCS funded studies designed to assess the effects of species type, planting date, seeding rate, seeding method, and soil N status on the establishment of fall planted cover crops. When it comes to soil coverage and biomass production, one factor seems to stand out above the rest — planting date.
In our experience, cover crops that are planted in mid- to late-September fare much better over winter and in spring than cover crops planted later in the fall. Even delaying planting into early October can have a significant effect on fall soil coverage and total biomass production at termination. For example, we reported significantly higher fall coverage for early planted (9-Sep 2015) cover crops on a cooperator field in Laurel, DE in 2015 when compared to cover crops planted in early- to mid-October. We also evaluated three seeding rates based on standard NRCS Code 340 seeding rates (high), ½ standard rates (low), and an intermediate rate (medium) and found that the increased seeding rate did not improve coverage for later planted crops.
Figure 1. Effect of planting date and seeding rate on fall soil coverage provided by cover crops planted in fall 2015 at a cooperator field in Laurel, DE.
In fact, only the early planted plots at the Laurel site produced enough harvestable biomass during the 2015-2016 trial year. As such, we were unable to make direct comparisons of planting date or seeding rates on biomass production. However, results from a parallel study planted in fall 2015 at DSU’s Smyrna location showed the significant effect of planting date on biomass production (Figure 2). The DSU plots were planted 30-Sept, 13-Oct, and 30-Oct with similar seeding rates as used in the UD trial. At DSU, we saw a trend for higher biomass production with earlier planting, with the most biomass production at harvest for the low seeding rate planted in September. In most cases, biomass production did not decline significantly for the plots planted in mid-October. However, delaying planting until late October had a significant negative effect on total biomass production. Again, increasing the seeding rate could not overcome the challenges of establishing a cover crop in late fall.
Figure 2. Effect of planting date and seeding rate on total aboveground biomass produced by cover crops planted in fall 2015 at the DSU Smyrna site.
The effect of planting date is also clearly shown with a rye cover crop that was broadcast planted in fall 2018 at our cooperator site in Georgetown, DE (Figure 3). Seeding rate and species were all held constant; the only variable was planting date. The soil coverage for the crop planted on 1-Oct was 70% compared to only 17% coverage when planted on 23-Oct. The dominance of weeds in the late planted plot is also noticeable. We see similar trends when we look at data from the other cover crop trials we planted between 2015 and 2019. Establishment, soil coverage, and biomass production were best for plots planted before mid-October. Notably, several of our late planted trials resulted in no harvestable spring biomass.
Figure 3. Comparison of spring soil coverage by a rye cover crop planted in early- (left) and late- (right) October 2018 at a cooperator field site in Georgetown, DE. Note the significant reduction in cover and increase in weed pressure with the later planted plot (right).
Evidence suggests that weather is the significant factor affecting successful cover crop establishment. It makes sense that earlier plantings are likely to result in better germination and establishment, as soil temperatures and growing degree units tend to drop off after mid-October. This will be particularly true for small grain cover crops, which need additional warm weather to build biomass through tillers. It is difficult to find reliable information about soil temperatures needed to support germination and growth of common cover crop species, but most available resources indicate that winter cereals and legumes (like red clover) require soil temperatures between 37 and 41°F to germinate. Rye is a bit hardier and can germinate at 34°F, although the SARE publication, Managing Cover Crops Profitably, indicates that vegetative growth of rye will not occur below 38°F. The most common cover crops prefer soil temperatures in the mid- to upper- 60s for germination and growth.
Average soil temperature on 1-Oct 2018 (early Georgetown planting) was 68°F and average soil temperatures ranged from 62-75°F during the two weeks after planting. By 23-Oct (late Georgetown planting), the average soil temperature was at 56°F and it hovered in the 50s during the two weeks following the late planting. The early planted cover crops (such as the rye in Figure 3 left) accumulated 616 growing degree days (GDD; base 40°F) in the first 30 days after planting compared to the late planted crop (e.g., rye crop Figure 3 right), which accumulated 301 GDD . Based on accumulated GDD, we can estimate the growth stage of the rye crops in Figure 3 to be at Feekes 5 for the early (1-Oct) planted cover crop vs. Feekes 3 for the later (23-Oct) planted crop. Clearly, the early planted crop had the advantage of warmer soil and air temperatures, which allowed for better fall establishment and growth and ultimately resulted in more spring biomass. Rainfall appeared to be less of a factor in 2018, as the early planted crop received 4.22 inches and late planted received 6.39 inches; however, lack of moisture can be an issue for germination. Other factors, such as species, seeding rate, and planting method (broadcast vs. disked) had little to no effect on soil coverage and biomass in our trials.
Of course there is no guarantee that early planted cover crops will establish successfully and thrive as the crop performance will always be tied to weather conditions and soil moisture status, but the odds are definitely better that the crop will experience favorable soil and air temperatures if you can get them seeded before mid-October. Our advice, plant early!