The Illinois State Water Survey weather recording site in Champaign near the South Farms provided the following rainfall totals (in inches) in 2013: May – 4.65; June – 5.33; July – 3.47; August – 0.49; September – 0.50. The dry weather along with high temperatures in late August into September – it reached 98 degrees on August 31 and again on September 10 – had many of us believing that yields would be lowered. I was also hoping that irrigation in a study we conducted here might bring us 300-bushel yields by preventing such a decline.
The irrigation study was conducted by grad student Josh Vonk, with help from others. It was planted following soybeans, with 180 lb of N applied as UAN before final tillage. On May 20, we planted about 40,000 seeds per acre of DeKalb hybrid DKC 62-08. Irrigation was done using dripline placed between row middles, with a row skipped between lines. We started to irrigate in mid-July, and from then to mid- September, irrigated plots received 9.66 inches of water, or about 1.2 inches per week. Rainfall totaled only about 2 inches over this period.
Besides comparing corn with and without irrigation, we applied as treatments 70 more lb of N on July 9 as Agrotain-treated urea, and foliar fungicide at silking. This made a total of eight treatments, ranging from nothing (the control) to all three added inputs – irrigated + extra N + fungicide. The idea was that extra N and fungicide would help yields more in irrigated plots, where water was not limiting.
We were much surprised when the control (nothing extra added) yielded 240 bushels per acre. Less surprising, adding extra N in non-irrigated plots failed to increase yield. Dry weather like we had resulted in little foliar disease, but many have come to expect that strobilurin fungicide will increase yield by helping to relieve stress under conditions like these. Fungicide did not increase yield in this case.
Adding water only – no extra N or fungicide – increased yield by 19 bushels (8 percent), to 259 bushels per acre. Adding 70 lb of N to irrigated plots added another 18 bushels, bringing yield to 277 bushels per acre. This N would easily have paid for itself under irrigation, but not without irrigation. As we saw in non-irrigated plots, adding fungicide failed to increase yields in irrigated corn, with or without extra N.
So in a season with good pollination conditions but a very dry second half, yields were outstanding even with normal input levels, and they were increased by less than 10 percent by water alone, and by about 15 percent by adding both water and extra N.
While 277 bushel per acre is nothing to complain about, it does leave us wondering what limiting factor prevented yields from being even higher. Temperature? Sunlight amount? We don’t expect we’ll ever know the answer for sure at these high yield levels. But we’ll keep looking.
It’s likely that soils have been “wrung dry” in the top 2 or 3 feet as they have had to provide the water that failed to fall from the sky. We’re also starting to see some evidence that soil nitrogen levels might also be low, presumably because N was taken up with water from deeper in the soil.