Source: Bill Cox and Phil Atkins, Department of Crop & Soil Sciences, Cornell University

Corn planting dates have moved up earlier in the spring across the USA because of warmer spring temperatures, improved cold tolerance of modern corn hybrids, and improved seed protection from early-season soil-borne insects and pathogens via seed treatments. For example, at the Aurora Research Farm, the average number of growing degree days (GDD, 86/50 0F system) have increased by 22 GDD in April and 31 GDD in May during the 1981-2010 period compared with the 1951-1980 period (Table 1). Also, corn hybrids now begin the germination process at about 46 0F compared with 50 0F for the hybrids of yesteryear. Finally, seed-applied insecticides that protect corn kernels in cool soils from soil-borne insects and pathogens were commercialized during this past decade. Consequently, there is much less risk involved with early planting of corn compared to 30 years ago. Nevertheless, only 55 percent of the corn is planted by 15 May on average in New York (New York Ag Statistics). We initiated a study at the Aurora Research Farm (Cayuga Co.) and Sparta Farms (Livingston Co.) to evaluate the response of six corn hybrids when planted from April 20-25 compared with planting from May 15-20.

A Broad Optimum Planting Date for Corn?

April and May had above-average growing degree days in 2009 and 2010 (Table 1) so April-planted corn had minimum overall cold stress during emergence in both years of the study. Consequently, emergence averaged 92 percent for the April-planted corn and 95 percent for the May-planted corn in both growing season, negating any effect of stand establishment on yields in this study. Temperatures did go down to 28 degrees, however at the Sparta Farm site on 12 May in 2010, which thoroughly singed off all corn leaves at the 3-leaf (V3) stage of development. All hybrids, however, recovered quickly and new leaves emerged from the below-ground growing point a few days after the frost event.

Grain yield had a strong year by location by planting date interaction so we will present data for each year and site. When averaged across hybrids, the late April-planted corn had a 3.7 percent yield advantage over the mid-May planted corn at the Aurora Research Farm in 2009 (Table 2), a cool and moist growing season that was stress-free throughout. Of equal importance, especially in the 2009 growing season when dry-down was quite slow because of cool conditions, grain moisture at harvest averaged 2.2 percentage units lower, which would greatly decrease drying costs (Table 3). At Sparta Farms, however, corn yield averaged the same between the two planting dates when averaged across hybrids (Table 2). As at Aurora, grain moisture at harvest averaged 2.6 percentage units dryer at the April-planting date, which would make that the more profitable planting date at Sparta Farms in 2009 (Table 3). Because the 2009 growing season was stress-free and frost did not occur until mid-October, we believe that the 2009 results clearly show the planting date effect with no confounding weather interactions.

A Broad Optimum Planting Date for Corn?

When averaged across hybrids, the April-planted corn yielded 7.3 percent lower at Aurora and a stunning 19.6 percent lower at Sparta Farms when compared with mid-May planted corn in 2010 (Table 2). The April-planted hybrids at Aurora were at the silking stage from 9-11 July, when temperatures averaged 93 0F during the 7-day period preceding silking. In contrast, the May-planted hybrids were at the silking stage from 18-20 July when temperatures averaged about 82 degrees for the 10 day period before and after silking. Corn is most vulnerable to drought or heat stress in the 10-day period before and after silking. We believe that the lower yield at Aurora for the April-planted corn was associated with the timing of heat stress, which is a random event, rather than a planting date effect. Grain moisture at harvest averaged 2.1 percentage units lower at harvest for the April compared with the mid-May planted corn, mitigating somewhat the loss of profit with the early planting date at this site.

A Broad Optimum Planting Date for Corn?

Explanation of the 19.6 percent yield decrease for the April-planted corn at Sparta Farms is more problematic. The 12 May frost delayed development so silking did not occur for the April-planted corn until 14-15 July compared with 20-21 July for the May-planted corn. Consequently, there was only a 1.1 instead of the typical 2 percentage unit difference in grain moisture at harvest between planting dates because of the delayed development for April-planted corn after the frost. Temperatures averaged 89 degrees F in the 10-day period before silking for the April-planted corn compared with 85 degrees F for the 10-day before and after silking for the May-planted corn. The temperature difference should not have resulted in a 19.6 percent yield difference because there was no moisture stress at this site. We believe that the frost stress followed by moderate heat stress resulted in multiple stresses for the April-planted corn, which contributed to the significant yield hit. There is a much greater probability that a significant frost event will occur after corn emergence for April-planted compared to mid-May planted corn. Consequently, we believe that the results at Sparta Farms in 2010 are indicative of the risk of planting corn in April for valley locations, such as the Sparta Farms site.

Conclusion
Spring conditions have warmed considerably over the last 30 years and corn growers in the USA have taken advantage by planting corn earlier. In New York, especially on dairy farms where manure is spread in the spring, corn planting frequently does not occur until May so only 55 percent of the corn is planted in New York by mid-May. An April-planting date for locations near the Lakes, such as Aurora, will probably result in a small but significant yield advantage because there is limited danger of killing frost (<28 0F) during the month of May. Of equal importance is that grain moisture at harvest will average 2 percentage units lower for April compared with mid-May planted corn, which will greatly reduce drying costs, especially in years with high LPG prices. In locations where there is a danger of killing frost in mid-May, April-planted corn may be vulnerable to some yield damage, especially in years when it is warm in late April and the first half of May. The growing point of corn will still be below the ground so the stand will probably recover but development could be delayed and yield may be impacted. Nevertheless, we recommend planting corn anytime after 20 April in central/western NY if soil conditions are dry enough in all locations because weather conditions are random and heat and drought stress can occur at silking for corn planted on any date. If soil conditions are wet in late April or early May, there is no need to mud it in because there is very limited yield loss, if any, for corn planted from 15-20 May compared with corn planted from 20-25 April. Grain moisture will be 2 percentage units higher at harvest, however, so planting earlier hybrids is a consideration if planting drags on until late May or early June.