Terminating cover crops ahead of corn

While the cool, wet conditions across most of Iowa have helped alleviate drought, they have interfered with cover crop (CC) termination ahead of planting. We normally recommend terminating CCs with a herbicide application at least 10 days before planting corn – because corn is a “princess” and there is more risk and potential issues when planted green (i.e. cover crop terminated at or after corn planting).

Planting green is associated with reduced corn yield.

We recently completely a multi-state project comparing “planting green” (winter cereal rye (WCR) CC terminated with glyphosate 0-3 days after planting corn) with “planting brown” (WCR CC terminated with glyphosate at least two weeks before planting corn) with the Precision Sustainable Ag team. In 2021, 2022 and 2023, the relative yield of corn planted green was lower than the yield of corn planted brown at 10 of 13, 11 of 14, and 10 of 12 locations (Fig. 1), respectively. To date, the mechanisms responsible for yield reduction in corn following WCR are unclear. Nitrogen immobilization, allelopathy, seedling disease, and planting errors have been hypothesized. In this blog we will focus on seedling disease.

Figure 1. Relative yield of corn planted into winter cereal rye (WCR) terminated with glyphosate at >14 days before planting (planting brown), 3-7 days before planting (planting yellow-brown) and 0-3 days after planting corn (planting green). Relative yield (%) = 100 – [(yield of corn planted after WCR/ yield of corn planted after fallow) X 100]

Corn seedling disease

Seedling disease of corn following a WCR CC is most often caused by Pythium spp. WCR is a host of Pythium spp. and when WCR is terminated, Pythium spp. are released from the dying roots. If no host, like corn, is present, the population decreases to endemic levels within 7 to 10 days. However, if corn is planted within 10 days of applying herbicide to  WCR to terminate the cover crop, it is at greater risk for seedling disease because more Pythium spp. are present to infect the seedling (Table 1). Seed treatments reduce risk of infection, but they do not provide complete protection.

Table 1. Radicle and seminal root severity of corn planted after a WCR cover crop terminated (sprayed with glyphosate) at different times before and after corn planting in 2019 and 2020.

Seedling disease is more likely when emergence is slow due to cold, wet conditions. Germinating seeds leak exudates that attract Pythium spp. Symptoms of seedling disease may include death of seedlings, but, more often, infected seedlings survive and are less vigorous than their non-infected neighbors (Fig. 2). If stunted, yellowed seedlings are carefully dug up, you may notice the radicle is rotted, and/or there is rot on the seminal roots or mesocotyl (Fig. 3). The rot impedes water and nutrient uptake by the seedling and consequently the seedling is less vigorous than its neighbors. While the seedling often survives, it does not become a productive plant – often remaining barren (no ear) or producing a very small ear. Consequently, yield may be affected. “What’s the yield effect of uneven corn heights” summarizes various studies that looked at how uneven plant heights affected yield of corn.

Figure 2. In corn planted after a winter cereal rye cover crop, occasional stunted seedlings with yellow, wilted leaves may indicate seedling disease is prevalent.

Figure 3. A rotted radicle and seminal roots are symptoms of corn seedling disease. 

Recommendations

We recommended terminating a CC at least 10 days before planting corn.  However, if this cannot be done, planting into warm soils (>60°F) will encourage rapid emergence and reduce the risk of infection by seedling pathogens. Fields should be scouted around V2 to check for problems with planting and emergence.  In future years, consider precision planting or strip-tilling small grain cover crops such that the future corn row is free of the cover crop. Recent research funded by the Iowa Nutrient Research Center has shown keeping the future corn row free of WCR by precision planting or strip-tilling reduces seedling disease and can mitigate reduced corn yields.