Underpinning Causes for Soybean Yield Gaps in the Western US Corn Belt.

We have collected and summarized soybean yield, plus applied irrigation (I) water amount, and nitrogen (N) amount for hundreds of soybean fields as reported by producers to the Nebraska Natural Resources Districts for as many years as the NRDs has required those producer reports.  In addition, we have conduced personal interviews with many producers across the state to also collect yield, water, and N data for specific fields, but in this case we collected much more data on the crop management practices used by the producer in each specific field.  This collective set of data allows us to determine what specific practices the “average” Nebraska rainfed or irrigated producer uses in an “average” field that (aside from the field’s soil type and that year’s weather) might, on average, have been responsible for the gap between the actual yield achieved in that field and the potential yield that was possible for that field based on our soybean crop models.  Major findings are (1) planting date sets an upper limit to farm yields with an observed penalty in yield potential of 1/2 and 2/3 bu/ac per day of delay in planting in south and north NE, respectively, (2) the upper water productivity boundary for soybean (yield per unit of water supply, which includes stored water at planting and rain plus irrigation) is about 3.7 bushels per acre per inch of water, but most fields are well below this boundary limit, (3) other factors influencing yield gaps are tillage, in-season fungicide application, and NP fertilizer inputs, and (4) irrigation amounts were highly variable among fields located within the same region, even in the same year, which highlights the potential for increasing soybean yields and water productivity by better matching of irrigation applications with crop water requirements.