As the trend of cool, wet weather continues throughout Ontario, many producers are worried that grain corn will not reach physiological maturity (Black Layer) before a killing frost . To add to the concern, many growers have chosen to plant some very long season corn hybrids. The previous two growing seasons have provided above average heat units in Eastern Ontario and the high yield potential of long season corn was difficult to ignore. With later planting dates and below average heat unit accumulation in the 2000 growing season, certain long season hybrids with later silking dates are at a greater risk of not reaching black layer prior to heavy frost.

Silking time is an important stage of development when determining corn maturity. Silking is the beginning of pollination, followed by the linear grain filling period in which 80 per cent of dry matter is accumulated. Silking occurs in the last two weeks of July under normal Eastern Ontario conditions. Of the total heat units required by a variety to reach black layer from planting, 60 per cent are required to reach silking.

The number of days normally required between silking and black layer is hard to determine, although most research suggests 35 - 45 days is a fair estimate.

If corn starts silking on August 5th, then black layer should occur between September 9 - 19th. The risk of a killing frost increases greatly after September 15th. Corn that has not started silking by August 10 - 15th would be considered high risk.

Although black layer is normally used as a critical period for corn maturity, half milk line could be more of an indicator this year. Half milk line is a normal target for harvesting of corn silage. When half milk line occurs, grain moisture is about 40 per cent and 90 per cent of its grain filling potential has been reached. If a killing frost happens after half milk line but prior to black layer, a yield decrease of eight to 15 per cent could be realized in grain corn due to low bushel weight.

Since grain drying adds to the cost of corn production, harvest maturity is the primary feature in assessing hybrid maturity. Although total grain yield and bushel weight are determined at black layer, the dry down period and eventual harvest maturity can vary depending on variety and environmental conditions. Two hybrids may reach black layer on the same day, although dry down characteristics may cause a variation in harvest maturity.

This year may provide the unpleasant experience of discovering which varieties are able to quickly reach harvest maturity.

Bryan Cook is president of Cropland Consulting, Spencerville