2019 ASHS Annual Conference

Reducing the risk that vegetables grown for baby food (including winter squash) exceed nitrate thresholds at harvest is a current challenge for some processors. Nitrate accumulation in vegetables is influenced considerably by N fertilization, but other factors such as cultivar, timing, weather, soils, and plant stress can also play a role. The objective of this study was to evaluate the effects of N fertilization rate and source (synthetic vs. organic) on winter squash yield and fruit quality, including fruit nitrate concentration, brix, and vitamin A content. An on-farm experiment (RCBD with 4 replications) was conducted in 2017 and 2018 in an irrigated butternut squash (cv. Ultra) field on sandy soils in Michigan. Treatments included three rates of total plant available N (83, 137, and 190 kg N ha^-1) each consisting of 29 kg N ha^-1 applied with the planter (10-34-0 and UAN 28%) and the remainder of the N applied either as urea (46-0-0) at sidedress or as pelletized chicken manure applied shortly after planting. To track the effects of N treatments on fruit quality over time, squash were subsampled 4 and 2 weeks pre-harvest, at harvest, and following 4 and 8 weeks in storage. Equivalent maximum yields were achieved regardless of whether post-planter N was supplied as urea or pelletized chicken manure. In 2017 only, manure yielded less than urea at 83 kg N ha^-1, but produced equivalent maximum yields as urea at 137 and 190 kg N ha^-1 rates. The significantly lower yield for manure versus urea at the 83 kg N ha^-1 rate may be a result of lower than expected N availability from the manure or greater N losses due to earlier application. Higher N rates were correlated with greater nitrate accumulation in squash fruit; however, only the highest rate of N (190 kg N ha^-1) applied as urea exceeded processor thresholds at harvest in one year. Fruit nitrate declined rapidly and brix levels increased in the month leading up to harvest, suggesting delaying harvest could be an effective strategy to deal with high nitrate squash fields. We observed little evidence that N rate or source substantially impacted brix or vitamin A content at harvest or during storage. While we saw no evidence that current year manure applications increase risk of nitrate accumulation relative to urea, N availability from legacy manure applications need to be considered when determining appropriate N fertilization rates.