Critical Precipitation Period for Dryland Corn Production.

Grain yields for dryland corn (Zea mays L.) production in the semi-arid Great Plains of the United States can be very unpredictable because of the erratic nature of growing season precipitation. Because inputs costs for corn production can be very high, farmers need to have a tool that will help them assess the risk associated with dryland corn production. The objectives of this work were to determine the critical period for precipitation during the corn growing season and to develop a relationship between critical period precipitation and corn yield to use as a tool to quantify expected yield variability associated with dryland corn production in this region. Corn yield data were collected at Akron, CO from two dryland cropping systems experiments (1984-2009) in which corn was grown in a wheat (Triticum aestivum L)-corn-fallow rotation. Yields were correlated with weekly precipitation amounts from planting to harvest in search of the period of time in which yield was most influenced by precipitation. Soil water contents at planting were measured either by gravimetric sampling or by neutron attenuation. Yields were found to be most closely correlated with precipitation occurring during the six-week period between 16 July and 26 August. The data separated into two linear relationships defined by whether the sum of available soil water at planting and May precipitation was less than or greater than 250 mm. These two linear relationships between precipitation during this critical period and yield were used with long-term precipitation records to determine the probability of obtaining a corn yield of at least 2500 kg ha^-1 at three locations across the central Great Plains precipitation gradient. This analysis quantified the production risk associated with the highly variable corn yields that result from erratic summer precipitation in this region.