R.L. (Bob) Nielsen
Agronomy Dept., Purdue Univ.
The decision to replant a field of corn because of poor stand establishment or severe damage to an otherwise good looking field of corn is often not an easy one to make. There are many uncertainties and questions involved with the decision, not to mention the emotions that often get in the way of facts and figures. The underlying question when considering a replant decision, of course, is whether or not you will end up with more dollars in your pocket at the end of the year by replanting a crappy™ stand of corn or leaving the field alone. The answer to that question relies on a few facts plus your skill at reading crystal balls.
Assessing Surviving Populations & Effect on Yield
The important facts to ascertain are the extent and severity of the stand loss throughout the entire field, plus an initial assessment of the health of the surviving plants. The goal is to estimate surviving populations of healthy plants.
Assessing the health of the surviving plants can be difficult and, frankly, many folks tend to overestimate the yield impact from the damage to the plants. Young corn plants have an amazing capacity to recover from aboveground damage caused by stresses like frost, hail, insect feeding, leaf burn from anhydrous ammonia or liquid nitrogen fertilizers, and wind (sandblasting). On the other hand, damage from belowground stresses like insect feeding, saturated soils, starter fertilizer burn, and seedling blight may not seem severe at first, but can have devastating effects over a very few number of days. If you are uncertain about the health of surviving plants on your first visit to the field, give the plants a few more days of sunshine and warmth, then evaluate stand health again. Those few days delay in making your decision will give you more confidence in your decision whether or not to replant.
Assessing and documenting plant populations and plant health across entire fields is obviously quite challenging to do efficiently on foot or even with wheels. Consumer drones (Unmanned Aerial Vehicles, aka UAVs) equipped with high quality cameras offer the potential to scout fields quickly from the air and spot / map problem areas. The challenge with UAV imagery, though, is that assessing the population and health of young corn plants (i.e., short and small) requires flying UAVs at quite low altitudes in order to accurately identify the plants. While technically feasible, such low flight altitudes greatly increase the flight time and number of UAV batteries to thoroughly cover an entire field in a systematic autonomous flight pattern. Alternatively, UAVs can simply be flown manually to quickly “spot check” areas of the field to help you decide which areas to check more thoroughly on foot.
Our research on corn yield response to plant population over the past 10 years (83 field scale trials) allows us to predict with some confidence the yield response of corn to low populations (Nielsen et al., 2019). 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, $240 seed corn, the average EOPP for our 83 field trials was only about 25,500 plants per acre at harvest. Marginal return to seed varied +/- $1 per acre between 23,500 and 27,250 plants per acre at harvest
The results of our research with plant populations in corn are most accurate within a range of 23,500 to 40,000 plants per acre at harvest because that was the most common range of populations evaluated in our field trials around the state. Extrapolating the results to populations beyond those is statistically not appropriate, but we can speculate that final populations as low as 20,000 plants per acre at harvest may result in marginal returns to seed roughly $7 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 Damaged versus Replanted Fields
One of the numerous uncertainties with making an economic replant decision involves 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. First of all, most of us tend to overestimate yield loss due to early season stand problems. It is similarly difficult to estimate the yield potential of a replanted field. While it is tempting to follow a rule of thumb for late planting along the lines of “2 bushel decrease per day of delayed planting beyond May 10”, that may not tell you what you need to know. 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, 2019c). The simplest approach to estimating yield differences between replanted and surviving original stands may be to simply base it on the differences in plant population.
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 only difference may be 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 (Nielsen, 2019b) and check 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 that typically begin to develop in late June – early July (e.g., gray leaf spot, northern corn leaf blight).
“Patch In” vs. “Destroy & Replant”
Another difficult replant decision is whether to kill the original stand of corn or replant right through it. My experience evaluating “patching in” versus “destroy and replant” suggests that “patching in” without killing the original stand should not be attempted 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. Another potential consequence of “patching in” is that grain moisture may be quite variable throughout the field at harvest due to earlier maturing original plants versus later maturing replanted ones.
Choosing to kill the original stand of corn before replanting turns out to be a headache, however, 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 published an article that addresses the challenges of killing an existing stand of corn prior to replanting (Ikley et al., 2017).
Potential Replant Costs
In addition to estimating the potential differences in gross income due to estimated differences in yields between replanted versus original stands of corn, the costs of replanting obviously need to be considered. Replant seed is the largest single replant cost, potentially ranging from $90 to $120 per acre. Some seed companies offer free seed for replanting as part of their customer service, others do not. Other potential replant costs to consider include fuel and labor to replant the field, possible additional herbicide, insecticide or fungicide costs, and possible additional grain drying costs after harvest.
Replant decisions for corn are often difficult ones to make. Key factors that play into such decisions include population and health of surviving plants, relative yield potential of replant versus original stands, and replant costs. Try to park your emotions at the gate and focus on estimating the dollar returns or losses to the replant decision itself.