Plant Population and Sowing Date Effects on Organic Spring Oat Production.

Spring oats (Avena sativa L.) are an essential part of organic row-crop systems in the Corn Belt region of the United States. Their functionality in weed suppression and legume establishment, in addition to grain and straw production, make them important to multi-year rotations. Unfortunately, oats are often the financial weak link in these systems due to a combination of average to poor yields and substandard grain quality (test weight), an important metric in accessing more profitable food-grade milling markets. As a result, oats receive little agronomic attention from farmers and researchers alike. Because of a paucity of regionally-specific information on economically optimal crop plant populations, we investigated the effects of a delayed sowing date and the degree to which, or if,  populations should be increased as a result of a delay in sowing. Small plot research was performed on certified organic land at the Marsden Research Farm in Boone, IA in 2015. Using a split-split plot design oats were sown on three different dates (whole plots) at an interval of 11 days beginning on April 6. The crop was planted at four different rates (subplots) on each date, reflecting desired plant populations of 161, 237, 312 and 387 plants m^-2. Alfalfa was sown at each date at a set rate of approximately 12 kg ha^-1. A further treatment, in which areas were hand-weeded until the early milk stage of development, was applied to each sub-plot (sub-subplots). Of interest were the effects of sowing date, population, weed control and any possible interaction between or among these three factors on grain yield, test weight, straw yield, alfalfa biomass and weed biomass. Grain yield followed a negative quadratic response with respect to oat population. The highest yield (1869 kg ha^-1) occurred at the second lowest population (237 plants m^-2) with an interpolated daily grain yield loss of approximately 35 kg ha^-1 day^-1 over the 22-day period. Weeded sub-subplots yielded higher than control (non-weeded) areas (1784 kg ha^-1 and 1680 kg ha^-1, respectively). Only sowing date had a statistically significant effect on test weight with dates 1 and 2 (445 kg m^-3 and 425 kg m^-3) having heavier test weights than date 3 (304 kg m^-3). Date, population and weed control did not have any individual or two-way effects on straw yield but a three-way interaction among them was significant (p = 0.03). Alfalfa biomass displayed a negative response to delayed sowing date with greater biomass quantity occurring at dates 1 and 2 with a precipitous drop in production at date 3 (474 kg ha^-1, 487 kg ha^-1 and 307 kg ha^-1 respectively). A significant two-way interaction between sowing date and weed control was observed between weeded and un-weeded controls at date 2. Main effects of date and population had no significant effect on weed biomass. These data suggest that an economically optimal seeding rate may occur on the lower range of populations tested, meaning that a lower cost of production may be most profitable without negative consequences in weed suppression and alfalfa establishment. It does not appear that populations need adjustment to compensate for a delayed sowing. These data also point to the necessity of an early planting date for favorable yield, test weight and alfalfa biomass.