Holly Newkirk Wins ACPA Research Conference Undergraduate Student Competition

Newkirk won a first place award with a presentation titled Development of Defoliation Thresholds in Hybrid and Pureline Rice Cultivars. Armyworms are commonly found in rice fields across Arkansas and have the capability to cause severe defoliation to rice plants which can lead to significant yield losses. Recent studies have established a damage threshold in pure-line and hybrid rice cultivars for armyworms. However, this threshold is based on a single defoliation event. The current threshold needs to be evaluated against a “continuous” defoliation more similar to what is observed when armyworms infest a field. To address this issue, a study was conducted in 2025 at the Rice Research and Extension Center in Stuttgart, AR to compare the yield loss caused by a one-time defoliation event compared to continuous defoliation. A hybrid (RT 7421 FP) and pureline (Ozark) were planted in April, May, and June to determine plant response and yield loss at the varying defoliation schemes across the rice planting window. The one-time defoliation treatments were mechanically defoliated to 100% foliage destruction. Continuous defoliation plots were mechanically defoliated to 25% at the initiation timing and subsequently defoliated to 25% four and eight days after initial defoliation. Defoliation events were initiated at the two-three leaf, mid-tiller, and greenring growth stages.  A difference in yield was observed between the defoliation schemes at the 2-3 leaf growth stage for April planted Ozark. There were no differences between continuous and single defoliation events for the mid-tiller and greenring defoliation timings however, all treatments reduced yield when compared to the untreated check. There were no differences in yield loss between single and multiple defoliation events at any growth stage for RT 7421 FP planted at the early planting date. In the second planting date, the single defoliation event reduced yield to a greater degree than the prolonged defoliation on both the Ozark and the RT 7421 FP cultivars. At the third planting date, there were no differences in yield at any defoliation timing in the Ozark cultivar. Yield differences were observed between the two defoliation forms in the RT 7421 FP cultivar in the third planting date. Continuous defoliation caused greater yield loss in 2-3 leaf rice while greenring rice experienced greater yield loss from the single defoliation event. The results from this study suggest there may be minor differences between the defoliation schemes and that the current thresholds are correct. These studies will continue to be repeated to ensure that our defoliation thresholds in rice are correct to help protect growers from economic losses caused by defoliation events.