Yield Gaps and Yield Relationships in Central US Soybean Production Systems

The magnitude of yield gaps (YG) (potential yield-farmer yield) provides some indication of the prospects for increasing crop yield. Quantile regression analysis was applied to county soybean [Glycine max (L.) Merr.] yields (1972-2011) from Kentucky, Iowa, and Nebraska (irrigated) to estimate the attainable potential yield (APY) (yield in the most favorable environments in the 40-yr record). The YG for each year was the difference between the APY and the county yield. There was substantial variation in the 40-yr mean county yields (186-335 g m(-2)) within and among states. The mean county APY within each state increased linearly (P < 0.0001) in conjunction with mean county yields. The mean relative YGs varied from 9 to 24% of APY and were largest in the lowest yielding counties and they decreased linearly (P < 0.0001) as the mean county yield increased for each state. The large YGs were partially related to very low yields that occurred in some years during the 40-yr period and these YGs may partially reflect the ability of the soil to supply water to the crop. The difference between the maximum APY in each state and each county APY defined a second YG. The largest values ranged from 12 to 19% of the maximum APY. Irrigation should partially eliminate the first YG, but the second may be more intractable to the degree it is related to soil characteristics. These results suggest that soil conditions may play an important role in determining the size of YGs.