Automatic drip irrigation of corn and soybean.

Irrigation scheduling goals may include maximizing yield and maximizing water use efficiency - two goals that usually conflict. Which goal is chosen depends on many factors. But, clearly, an automatic irrigation system that allowed either goal to be chosen would be a useful farming tool. We tested a system that uses species-specific threshold temperatures and region-specific threshold times, compared with daily canopy temperature time above the threshold, to automatically schedule and apply irrigations. Corn (Zea mays L., hybrid Pioneer 3162) was grown in 1997 and 1999; and soybean ( Glycine Max (L.), var. Pioneer 9481) was grown in 1996 and 1998, all with surface and subsurface drip irrigation. Four automatic treatments were implemented each season. The default threshold temperatures are 27°C and 28°C for soybean and corn, respectively, as determined from the peak photosynthetic enzyme activity measured as a function of temperature. We applied treatments using the default threshold temperatures and thresholds 2°C higher; and for each threshold we used two threshold times. The times were the daily mean time that well-watered crops of each species were above the threshold temperatures during the irrigation season in previous studies. If canopy temperature, measured every minute with an infrared thermometer, was above the threshold temperature for more than the threshold time in any day, then an irrigation equal to peak consumptive use (10 mm) was applied that night. Each year, the four automatic treatments were compared with a manual weekly irrigation regime that was 100% replenishment of water to field capacity as measured by neutron probe. Treatments of 67% and 33% of the 100% amount were also applied manually to provide data for curves of yield vs. water use. Treatments were triply replicated. Most or all of the automatic treatments out yielded the 100% treatment in each year. Corn yields were 0.94 to 1.21 kg m -2 for automatic treatments and 0.65 to 1.15 kg m -2 for the 100% treatment. Soybean yields were 0.35 to 0.43 kg m -2 for automatic treatments and 0.36 to 0.40 kg m -2 for the 100% treatment. In general, increasing threshold temperature or time shifted treatments away from maximum yields and closer to maximum water use efficiencies. In mid-season, the mean corn canopy temperature for the 28 and 30°C treatments (three replicates) was separated by about 2°C through most of the daylight hours. As with corn yield, total water use (mm) and water use efficiency, WUE (kg m -3 ), were stable over the 1997 and 1999 years for the automatic irrigation treatments (means of 752 mm and 1.48 kg m -3 , respectively). Water use and WUE varied widely over the two years for the manual treatments (569 and 756 mm, and 0.65 and 1.52 kg m -3 for 1997 and 1999, respectively, for the 100% treatment). For soybean, yields and WUE were not more stable for all automatic treatments. It appears that manipulation of temperature and time thresholds will allow a choice between maximum yields and maximum water use efficiencies to be achieved for corn, but not for soybean.