COMPARISON OF LIMITED IRRIGATED VS DRYLAND CROPPING SYSTEMS IN THE USGREAT-PLAINS

Irrigated hectares in the Great Plains are being converted to dryland because of the declining water table and increasing energy costs for i rrigation. A study was conducted in the central Great Plains near Gard en City, KS, from 1989 through 1991 to compare limited-irrigated cropp ing systems for use on land that would otherwise be converted to dryla nd. Irrigated continuous winter wheat (Triticum aestivum L.) and grain sorghum [Sorghum bicolor (L.) Moench] were compared with the wheat-so rghum-fallow rotation where either one, both, or neither crop was irri gated. Irrigated wheat yields in the rotations averaged 26% more than dryland. Rainfall eliminated any benefits of irrigation on the 1989 an d 1990 grain sorghum, but yields in the rotations were increased an av erage of 168% over dryland in 1991. Average irrigated continuous wheat and sorghum yields were 19 and 8% less than irrigated rotated wheat a nd sorghum yields, respectively, in 1990 and 1991, but when yields wer e annualized continuous wheat: and sorghum averaged 20 and 59% more th an the irrigated rotations. Annualized irrigated rotation yields were similar to dryland in 1990, but 68% more in 1991. Irrigating either cr op in the rotation produced similar annualized yields, but irrigating both crops increased yield an additional 13%. Irrigation water use eff iciencies were highest for the rotated crops, and lowest for the conti nuous crops. More grain can be produced with very limited irrigation t han with dryland systems, but no single system is best for all conditi ons. Producers with less water should probably irrigate fallow systems , while those with more water can crop more intensely.