Irrigation and Plant Growth Regulator Effects on White Clover Seed Crops.

White clover seed crops are a vital part of cool-season seed production systems around the world. While white clover seed crop acreage has increased over the past decade in Oregon, seed yields have shown smaller increases over that period. This 2-year study was undertaken to determine whether plant growth regulators (PGRs) and irrigation can be utilized to increase white clover seed yield. One objective was to determine whether PGRs could be used to manipulate the development of stolons in irrigated stands by reducing the internode length, thus providing a better environment for flowering. Fields plots were established with ladino white clover near Corvallis, Oregon in the fall of 2014 and followed for two seed harvests. The experimental design was a randomized complete block with a split-plot arrangement of treatments with four replications. Main plots were irrigation treatments, and subplots were PGR products and rates. Irrigation treatments included 1) no irrigation and 2) 100 mm of irrigation applied at early flowering. PGR subplots included trinexapac-ethyl (TE) rates of 300 (2016), 600 (2015 and 2016), and 800 (2015) g ai ha^-1 and paclobutrazol (PB) at a rate of 1,000 g ai ha-¹ applied during stem elongation. Irrigation had no effect on seed yield, but increased seed weight in both years. Irrigation had no effect on dry weight, inflorescence m^-2, florets/inflorescence, or harvest index. In 2015, TE decreased seed yield and seed weight and when rates were reduced in 2016 there was no effect on seed yield, although seed weights still decreased. PGRs had no effect on dry weight but TE had mixed effects on inflorescence m^-2, florets/inflorescence, and harvest index. PB had no effect on seed yield, seed weight, or any yield components in either year. Results indicate that irrigation and PGRs do not increase seed yield of white clover seed crops.