Top Crop Manager

Features Agronomy Seeding/Planting
Making the right choice


Unfavourable weather after early corn planting can result in poor plant stands. Every year, corn growers must make decisions on whether it’s more economical to leave the existing stand or replant.

Until recently, replanting recommendations for Ontario corn growers were based on data from Illinois. David Hooker, a researcher at the University of Guelph’s Ridgetown campus, along with Greg Stewart, agronomist with Maizex Seeds, and Ken Janovicek, University of Guelph, set out to create a Corn Replant Decision Tool for Ontario corn growers based on provincial data.

Hooker and his associates planted six full-season hybrids in 2010 and 2011 at sites in Ridgetown and Elora, with final plant populations that simulated 50, 75 and 100 per cent of the target population. The artificially created but realistic variable plant stands were created by planting a mix of glyphosate-tolerant and conventional corn seed, followed by spraying with glyphosate seven days later.

An economic analysis that considered the cost of replanting, differences in predicted yields, and different drying costs was completed. The analysis also included a comparison of the economics of replanting with the original hybrids used in the earlier planting, versus replanting with comparable hybrids rated 150 to 200 fewer Ontario Corn Heat Units (CHU).

The results of this trial showed that the breakeven population, after all costs are considered, varied between 55 and 65 per cent of the target population, depending on location.

“Stand loss needs to be significant (fewer than 18,000 plants per acre) before replanting approaches an economic threshold,” Hooker says. This was much lower than the previous recommendations that were based on American data. “Fewer replants will be triggered using recommendations based on the Ontario dataset compared to the former recommendations based on the American dataset,” Hooker says.

However, late planting at Elora resulted in lower yields than expected, so replanting may be less feasible in short maturity zones even at very low populations. “In shorter season areas (less than 2,700 CHU), corn replants tended not to be as economical,” Hooker says.

In this particular trial, replanting with full-season hybrids resulted in higher yields compared to replanting with shorter season hybrids. Averaged across all hybrids, a full-season hybrid replanted in the first week of June yielded 81 to 84 per cent of the early planted full-season hybrid in 2010, and 88 to 93 per cent in 2011 in Elora. Shorter-season hybrids planted in the first week of June yielded lower than the full-season hybrids.

However, the researchers concede more data is needed to see if this pattern would hold true in other years. During both years of the trial, autumn weather was warmer than normal with later than average first frosts. This allowed the late-planted corn to mature; although full-season corn hybrids planted in early June did have grain moisture levels that were four to six per cent higher than when a shorter season hybrid was planted. Hooker cautions: “Other work has shown that hybrid maturities need to be adjusted.”

The results of this two-year trial at Ridgetown and Elora concur with an earlier four-year trial that took place at Ridgetown, Exeter and Elora (2006-2009). The earlier trials received some criticism because they were uniformly thinned to simulate a stand with a low population. The results of the 2010-2011 trial, based on a more realistically created plant stand, verify the earlier trial results from 2006-2009, Hooker says. However, there was some deviation at populations less than 45,000 plants per hectare, indicating yield losses were under-predicted at lower populations, he adds.

Given the differences in the length of the Ontario growing season compared to Illinois, Hooker was not surprised his results differed from the Illinois recommendations. “The environment in the United States Midwest is much different from the main corn producing areas in Ontario, especially considering differences in the length of the growing season in Essex County compared to North Wellington County.”

The Ontario research is also based on a bigger pool of hybrids, Hooker continues. “The Ontario research used more than 20 popular Ontario-adapted hybrids compared to only a few United States-adapted hybrids in the United States dataset,” he says.

The 2010-2011 trial at Ridgetown and Elora also tested the theory that flex-ear corn hybrids could maintain high yields in thin stands compared to fixed-ear hybrids. “This research busted the myth that a thin stand of flex-ear hybrid wouldn’t justify a replant compared to a thin stand of a fixed-ear hybrid,” Hooker says.

Hooker used the research results to create the Corn Replant Decision Tool. The Microsoft Excel program calculator is available online at and takes into account the likelihood of producing a lower yield on the replant date, the higher production costs associated with reseeding and higher grain corn moisture levels at harvest. The calculator allows input to reflect current market prices for corn, seed, insurance payouts, and drying charges.

When a late frost or cool wet weather follows early planting, as much as five per cent of Ontario corn acres are replanted, Hooker estimates. Now Ontario corn growers have a better tool to help them make decisions on whether or not it makes economic sense to replant.

Hooker is currently further analyzing the results to look for relationships between corn grain yield and weather during the growing season. Most of the hybrids in the Corn Replant Decision Tool research trials yielded best when planted early at high plant populations, but some of them maintained high yields when planted late or when planted early but in low plant populations. “We are investigating the effect of weather at time of silking and grain fill to help understand these hybrid interactions,” Hooker says.  


February 12, 2016  By Helen Lammers-Helps


Stories continue below