R.L. (Bob) NielsenAgronomy Dept., Purdue Univ.
The recent spate of rainy weather and chilly temperatures does not bode well for some corn fields planted prior to the onset of the nasty weather. Problems with germination, emergence, or survival of emerged seedlings are likely to occur in fields that received truly excessive rainfall and / or are poorly drained and susceptible to ponding or soil saturation for days on end. Some fields are damaged or destroyed by outright flooding of creeks and rivers. Some of the damage will be caused by the smothering effects of surface residues drifted about by ponding or flooding of fields. Significant surface soil crusting will likely develop in some conventionally-tilled fields and restrict emergence of the corn plants. There may well be some imbibitional chilling injury to seed in fields planted just ahead of the cold, wet spell. Other fields planted a bit earlier may exhibit corkscrewed elongation of mesocotyls and underground leafing out in response to cold temperature shock during emergence (Nielsen, 2015). The potential for frost damage to emerged crops was real in parts of Indiana in recent days (Nielsen, 2017a). Seedling blight may yet develop in earlier planted fields once fungicidal seed treatments break down 14 to 21 days after planting.
In short, there is a significant risk for a “boat load” of crappy-looking fields of corn within the 45% of the statewide acreage planted through the end of April.
Assessing Surviving Populations & Effect on Yield
As soils slowly dry and weather slowly turns warm, some growers will face the difficult decision whether to replant damaged fields. Replant decisions are always based on a combination of known facts, uncertain outcomes, and emotions. The important facts to ascertain include the extent and severity of the stand loss throughout a field, plus an initial assessment of the health of the surviving plants.
As questionable fields dry to the point where they can be easily scouted, stand counts should be made throughout the field to estimate surviving populations of healthy plants. If you are uncertain about the health of surviving plants on your first visit to the field, give it a few days of sunshine and warmth, then evaluate stands again.
Our recent research on corn yield response to plant population allows us to predict with some confidence the yield response of corn to low populations. The good news is that modern hybrids are fairly tolerant to populations in general. What this means is that not only will they tolerate high plant populations without dramatic decreases in yield, but will also tolerate low plant populations without dramatic yield decreases. Because the yield response to plant population is fairly flat, the economically optimum plant population (EOPP) at harvest is already lower than you probably thought. Based on $3.50 market price for corn and $240 seed corn, the EOPP for much of Indiana is about 26,250 plants per acre at harvest. Final stands as low as 24,500 to as high as 28,000 plants per acre at harvest translate to marginal dollar returns to seed only $1 per acre lower than the EOPP of 26,250 plants per acre.
The results of our research with plant populations in corn are most accurate within a range of about 23,500 to about 39,500 plants per acre at harvest because that has been the most common range of populations evaluated in our field trials around the state. Extrapolating the results to populations beyond those is a bit risky, but we speculate that final populations as low as 20,000 plants per acre at harvest may result in marginal returns to seed only about $12 per acre lower than that at the EOPP. That at least gives you an estimate to work with if you are considering replanting and your estimates of surviving populations are no lower than 20,000 plants per acre.
Yield Potential of Replanted Field
One of the many uncertainties involved with making an economic replant decision revolves around the difficulty in predicting yields of an untouched, but damaged, original field of corn versus that of a field replanted at some date in the future. While it is tempting to follow a rule of thumb for late plantings along the lines of “2 bushel decrease per day of delayed planting beyond May 10”, that may not turn out to be an accurate estimate. The reason is that planting date itself is not an accurate predictor of absolute number of bushels per acre. Planting date is only one of about a gazillion factors that influence yield (Nielsen, 2017c). The simplest way to approach estimating yield differences of replanted versus original stands may simply be to base it on differences in population, as previously discussed.
Seeding Rates for Replanting or Late Planting in General
The target EOPP for fields replanted in mid- to late May is essentially unchanged from that targeted with late April plantings. The difference is that the success rate for germination / emergence with later planting is typically greater than early plantings because of typically warmer soils in late plantings. Instead of using seeding rates 5 to 10% higher than the targeted EOPP, late planting of corn can probably be done using seeding rates much closer to the targeted final population.
Hybrid Maturities for Replanting or Late Planting in General
Replanting a damaged field of corn in mid- to late May might require the use of a shorter-season corn hybrid than the one originally planted in the field. Consult my article about hybrid maturities for delayed planting and start checking with your seed dealer about availability of earlier maturity hybrids that also have good disease resistance characteristics. The latter is important because late-planted corn, relative to earlier-planted corn, is vulnerable to infection at relatively younger growth stages by foliar diseases (e.g., gray leaf spot, northern corn leaf blight) that typically begin to develop in late June – early July.
“Patch In” vs. “Destroy & Replant”
One of the difficult decisions to make when considering replanting is whether to kill the original stand of corn or replant right through it. My limited experience evaluating “patching in” versus “destroy and replant” suggests that “patching in” without killing the original stand should not be done unless surviving stands are roughly 25% or less of the original population. The risk with “patching in” surviving stands with populations higher than that is the original survivors will provide too much competition for the newly emerging replant population. There is also the tendency to “patch in” at the same original seeding rate, assuming that the planter will destroy quite a bit of the original stand, and then ending up with a final stand that is 1.5 times or more what you intended because a lot of the original stand survived.
Choosing to kill the original stand of corn before replanting turns out to be a headache because of the preponderance of herbicide-tolerant traits in modern hybrids (e.g., tolerance to glyphosate, glufosinate). Fewer herbicide options exist to terminate fields of damaged corn planted to such hybrids. My colleagues in Weed Science recently published an article that addresses the challenges of killing an existing stand of corn prior to replanting.