Tuesday, March 12, 2019

The Corn Yield Gap in Wisconsin

Corn yields have been increasing in Wisconsin at the rate of 2 bu/A*yr (USDA-NASS) and there is no indication that corn yields are plateauing. The highest recorded state average yield occurred in 2016 at 178 bu/A. In 2012, Jeff Laskowski (Portage county) recorded the highest corn yield in Wisconsin at 327 bu/A. In the Wisconsin NCGA Corn Yield Contest yields above 300 bu/A have been recorded 21 times.

"Yield gaps" are the difference between potential crop yield and actual farmer yield. Potential yield is defined as the yield of a hybrid when grown in environments to which it is adapted; with nutrients and water not limiting; and with pests, diseases, weeds, lodging, and other stresses effectively controlled. Previous corn yield gap estimates from around the world have ranged from 11 to 84%. Lower yield gaps are typically seen under irrigated conditions. It is not clear if potential yield is determined by soil type or if eliminating water and nutrient stresses is more important. Political boundaries and technology availability also affect potential yield.

The challenge in understanding a yield gap is determining potential yield. The gap depends upon the method used to estimate yield potential. Some researchers use crop modeling techniques, others use yield maps from precision farming, or various statistical techniques, or yields from ag research station experiments, etc. Regardless, potential yield is location specific. The larger the geographical scale used to estimate potential yield and farmer yield, the more difficult it is to estimate a yield gap and to identify management practices that reduce or eliminate the yield gap.

The NCGA Corn Yield Contest consists of three categories: rain-fed, irrigated and conservation tillage. Overall winners of the contest over time regardless of category were used to set the potential yield for corn. Although most winners in the NCGA contest are from southern Wisconsin where farmers use longer-season hybrids with greater yield potential, the overall record and 8 of 21 yields above 300 bu/A are from north central Wisconsin. USDA-NASS average corn yields were used for farmer yields.

The regressions in Figure 1 show farmer and potential yield for Wisconsin. USDA-NASS yield (farmer yield) has increased from 96 bu/A in 1983 to 166 bu/A in 2018. The Wisconsin NCGA winners (potential yield) have increased yield from 184 to 320 bu/A. The yield gap in 1983 was 87 bu/A (47.6%), while the yield gap in 2018 was 155 bu/A (48.3%). The yield gap was widest in 2012 at 207 bu/A (63%) and narrowest in 1997 at 87 bu/A (40%). Clearly corn yields are increasing, however, the yield gap in 2018 is relatively the same as the yield gap in 1983 at about 48%. Surprisingly, the yield gap among NCGA categories is not statistically different (data not shown).

Figure 1. Corn yield gap between USDA average yield (farmer yield) and winners of the Wisconsin NCGA yield contest (potential yield). Data derived from USDA-NASS and NCGA corn yield contest winners.

Wisconsin corn production is a highly developed, sophisticated, high-yielding production system making it unlikely that variation exists in the availability of technology. Many farmers use the best technology available, however, some farmers choose not to employ the same level of technology as yield contest winners. At the farm level, yield gaps in many fields can be reduced by relatively simple changes in management practices. Yield maps are one way to identify yield gaps within a field and on your farm.

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