Impacts of Changing Post-harvest Agricultural Practices on Abundance of Waste Grain in California’s Central Valley

Agricultural waste grains are significant for providing nutrients for wintering waterfowl in California. Rice and corn comprise 56% of their nutrient needs in the Central Valley and changes to agricultural practices, such as post-harvest treatments, could impact these food resources. Currently, there is limited data on how post-harvest treatments in rice and corn fields affects the abundance of waste grain, yet these data are essential to determine the carrying capacity of agricultural lands for wintering waterfowl. To address this knowledge gap, we estimated the abundance of waste grain (kg/ha) using dry field transects, dry field soil cores, and flooded field (wet) soil cores. In 2016 and 2017 we sampled 84 rice fields and 47 corn fields. Our results indicate that the abundance of waste rice varied significantly among post-harvest treatments. Fields that received no post-harvest treatment (stubble left standing; no incorporation of straw) had the greatest amounts of waste rice, whereas fields that were disced, disced and rolled, or burned provided the least amount of waste rice. The average abundance of waste rice across all post-harvest treatments was 320 kg/ha in dry fields (arithmetic mean; geometric mean = 228 kg/ha; soil core samples) . Estimates of waste rice in flooded fields averaged only 169 kg/ha (geometric mean 98 kg/ha; soil core samples) , significantly lower than in the same fields prior to flooding. Variation in the abundance of waste corn was greater than rice fields. Fields that did not receive any post-harvest incorporation had the greatest abundance of waste corn, 233 kg/ha on average (arithmetic mean; geometric mean = 72 kg/ha) , whereas fields that were incorporated (Disk or Disk and Roll) contained significantly lower abundance of waste corn averaging 50-60 kg/ha (arithmetic mean; geometric mean = 5-10 kg/ha) . The average, across all post-harvest treatments, was 159 kg/ha of waste corn ( geometric mean = 25-34 kg/ha) . Our results suggest that the abundance of waste grain in rice and corn fields is affected by post-harvest practices and, as these practices change, wintering waterfowl could be impacted. Our results also indicate that the method of sampling waste grain can influence estimates of residual grain abundance.