By Angie Peltier
Each year personnel at the Northwestern Illinois Ag Research & Demonstration Center (NWIARDC) in Monmouth establish a corn planting date trial. In 2016 the same 110-day corn hybrid was planted at the same seeding rate (35,000 seeds/A) and planting depth (1.75 inches) at one of four planting dates in plots 8 rows wide and 100 feet long (Table). Each plot was split in half length-wise and either the front or back 50 feet of the plot was randomly assigned a fungicide treatment (Table). Each treatment combination was replicated four times.
Table. Planting date and fungicide treatments and application timing in a corn planting date x fungicide trial at the NWIARDC in 2016
Storm-damaged corn in this trial. If you'll recall this very trial was damaged by a severe wind and hail event on June 22. The corn plants in the photos featured in the June 23 Bulletin article were in this trial. While neither green-snap nor root lodging occurred in this trial, the April 5-planted corn remained goosenecked (the lower stalk is bent over while the stalk tissue higher up is vertical) for the rest of the growing season while the other planting date treatments recovered to mostly vertical.
If we had to take an educated guess….. We might expect the April 15 or May 5-planted corn to have the highest yields. Only very rarely in these planting date trials would corn in the early-April or Mid-May planting dates out-yield corn planted mid-April or early-May. The earliest-planted corn tends to remain below-ground for a long time, accumulating a similar number of growing degree day units as later planted corn, but over a longer period of time. Additionally, it is this earliest-planted corn that is most likely to encounter frost. Indeed, the first two planting dates reached tasseling at the same time (Table). Corn planted in mid-May tends to put more resources into vegetative growth, developing longer internodes and taller plants and encountering higher disease pressure from some of the later-season diseases. Rust fungi do not survive the Illinois winter and are moved northward with weather systems during the growing season. While these fungi may cause largely superficial symptoms and signs on the earlier planted corn, repeated cycles of disease may result in more severe symptoms on the latest-planted corn and at more yield-critical growth stages. This may be why we have sometimes observed a yield increase with a fungicide application on the latest-planted corn.
What we actually saw. Yield differences were detected among the planting dates (P=0.09), with the May 5-planted corn yielding higher that both the April 5- and April 15-planted corn (Figure). As might have been expected, grain moisture was higher for corn from the latter two planting dates than the earlier two dates (P<0.01). Percent moisture was 16.7, 17.0, 20.1, and 20.3 for the April 5, April 15, May 5 and May 19-planted corn, respectively. Only one difference was detected between fungicide-treated and untreated corn (P=0.05): for the May 19-planted corn, untreated corn yielded significantly more (222 bu/A) than fungicide-treated corn (211 bu/A).
From an integrated disease management standpoint, the low disease pressure in this trial would not have warranted fungicide treatment, even with the additional stress of hail damage. The results of research trials in Iowa and Illinois suggest that factors other than hail damage should determine whether a foliar fungicide is warranted. The 10 bushel difference between the fungicide-treated and untreated May 19-planted corn may be striking, but is not unheard of. While it may be difficult to attribute a cause to these results, fungicide trials conducted by land-grant university researchers as part of a larger working group have sometimes resulted in negative yield-responses with one or more foliar fungicide applications (when compared to untreated checks).