How Late Can We Plant Corn and Soybeans?

The latest report from NASS indicates that 96 percent of the corn and 62 percent of the soybean crop in Illinois had been planted as of June 9. Some of the corn is struggling, however, with 13 percent of the crop rated as poor or very poor. Much of this is due to heavy rainfall, which has caused problems with stands, including areas in fields where the crop has drowned out from standing water. In other cases, the crop has emerged poorly in fields or areas in fields due to crusting and some low temperatures.

It’s very late to plant or to replant corn, and for many with unplanted fields or with poor stands that may need to be replanted, the choice is whether it makes sense to plant corn this late, and if not, whether the best option is prevented-planting insurance or replacement with soybeans, as crop insurance provisions allow.

Table 2.3 in Chapter 2 of the Illinois Agronomy Handbook http://extension.cropsci.illinois.edu/handbook/  indicates that we can expect a corn crop to yield about two-thirds of its expected (early-planted) maximum yield if the crop is planted on June 8 and has a full stand. That is a projection, since most of our corn planting date studies include planting only through the end of May. Based on this, however, we have projected that corn reaches the point where we can expect 50 percent of maximum yield if it is planted sometime between June 15 and June 20.

I reanalyzed our more recent planting date data, and if – this is a big “if” – we can accept projections of yield that go well past the last planting date, we would move the planting date from which we’d expect half a crop a little later, closer to the end of June. But we know that corn planted during or after the middle of June will produce fair to good yields in some years and very little yield in other years, depending on unpredictable weather that follows. Hence it makes sense to consider June 15 to 20 to be that last “practical” date on which to plant corn if we want to produce grain.

If we do plant corn in mid- to late June, planting a very early hybrid, having the option of harvesting the crop as silage if grain production looks unlikely, and getting good rainfall throughout the rest of the season will all improve the chances of ending up with a profitable crop. The chances of having enough frost-free days to grow a crop are higher in central and southern Illinois than farther north, but higher water loss rates and lower water-holding capacity of soils can cancel this advantage. It may also be difficult to get seed of very early hybrids, and because early hybrids are not developed for the central and southern Corn Belt, there is no guarantee that they will do well under late planting.

If it’s too late to plant corn and we don’t expect enough yield to make a profit (or at least to make more than crop insurance would pay to plant nothing), does it make sense to plant soybeans instead? We have run our soybean planting date studies into the first or second week of June, but we still have to project expected yields past the last date we actually planted.

Going through the same exercise as we did for corn, we would expect soybeans planted at the end of June or early in July to yield half what they would if planted early. This is about two weeks later than the normal doublecrop planting date in southern Illinois. Doublecrop soybeans have averaged 72 percent of full-season soybean yields over the past 10 years at Brownstown, so using early July as the 50-percent-of-maximum-yield planting date seems reasonable.

We know that doublecrop (or very late-planted) soybean yields can range from zero to good, and there’s no way to predict when they are planted which end of this range they’ll be on. As many found out in 2012, planting into bone-dry soils is not usually conducive to high doublecrop soybean yields. And in northern and central Illinois, doublecrop soybeans or soybean planted (or replanted) in late June or early July have had a considerably lower rate of success than doublecrop soybeans in southern Illinois.

There is one important difference between doublecrop soybeans and soybeans that are planted late but that don’t follow wheat harvest. Wheat removes a substantial amount of water from the soil as it matures, and in years with average June rainfall, the soybean crop that follows wheat has much less soil water available to it than does the crop that follows only the crop from the previous year. As is always the case, good rainfall through the rest of the season can cancel out this advantage, but it won’t eliminate it averaged over years.

Soybean planted in mid- to late June needn’t be managed much differently than early-planted soybeans. Our recent research indicates that narrow rows tend to yield more regardless of planting date, and raising seeding rates seldom produces an advantage when planting late. Unless a late-maturing variety (for example, a Group 4 variety in central Illinois) was the first choice, there is no advantage to changing to an earlier variety for late planting.