By Greg Stewart, OMAFRA Corn Specialist

2001 corn yields were surprisingly good or depressingly low. In the face of very low rainfall, how much water your soil could supply to the crop made all the difference.

In years where rainfall is in scarce supply, the ability to store water in the soil and then have it available to the rapidly growing corn crop generally dictates yield levels. This ‘available’ water is the water that exists in the soil between the extremes of the wilting point and the field capacity. Wilting point is when the soil is dry to the point that water is held so tightly by the soil that corn roots can’t pull it out. Field capacity is the amount of water left in the soil after it has been saturated and then left to drain freely for 48 hours. Sands and clays have the least available water to the plant; sands because they can’t hold much water (low field capacity) and clays because they hold too large a portion of their water too tightly (high wilting point). After 2001, those growers working on coarse textured sands or fine textured clays probably don’t need to be reminded of this fact as their yields were generally well below average.

Good Root Growth
One bright spot in the growing season this year was the fact that corn was, for the most part, planted early and got off to an excellent start. Early planting generally allows roots to explore slowly drying soils, and thus results in steady downward root growth. We also benefited this year from rainfall in the April to June period that was very close to normal (see figure 1). This prevented roots from slamming into overly dry soils as is the case when dry spells hit early in the season. The lack of excessive rain prevented roots from getting stalled out by elevated water tables. The net result of this moderate April-to-June rainfall pattern was exceptionally well-developed corn roots that went deep into the soil profile by early July. These roots extracted water to the point that pollination and grain-filling proceeded well on the medium textured soils, even when rainfall dropped significantly below normal in the July-August period.

Weed Control Critical
Tight water supply in 2001 also accentuated the need for weed control that was excellent and timely. Those corn plants that competed with weeds well into the critical weed-free period ran the risk of drastically reduced yields. The critical weed-free period for corn is from the 3rd leaf stage through to the 8th leaf stage. Corn plants that do not have to compete with weeds during this period develop better root systems and are more efficient at supplying adequate water to maintain yields.

Reduced Tillage Impacts
Often reports from other jurisdictions (i.e., southern Corn Belt) report greater levels of success with no-till corn in drier years than they get with average or wetter years. At first glance, this would seem obvious if one could store more soil water in the undisturbed, residue-covered soil. My experience in Ontario is that this is not always the case. In years when soils dry quickly due to low rainfall, the no-till has often been too hard for the roots to explore for vital soil moisture. 1988, another dry year, was a good example when the dry weather came much earlier in the season than in 2001, corn suffered from a lack of root exploration, and yields were hurt worse particularly in no-till.

So What?
Some reminders while looking back at 2001.

1. Sands and clays that inherently have low water availability can be improved by improving structure through crop rotations that include cereals and forages.

2. Planting corn early allows for root expansion while soils are often drying more slowly, thus promoting deeper roots.

3. Tile drainage only removes the soil water that is above the field capacity; it does not remove water that would be used by the crop. Tile drainage promotes deeper rooting and is perhaps the most important consideration when considering techniques to make your corn crop more drought-proof.

4. If delayed weed control cost you yield in 2001, you may want to consider changes that will allow you to keep the corn crop weed-free throughout the critical period.

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