The Response of Spring Barley Varieties Contrasting in Lodging Resistance Scores to N Application and Plant Growth Regulator Program.

Excessive nitrogen applications can result in significant losses of potential profit margins for spring barley growers due to increased lodging risks and low yield responses. Varieties characterised by strong resistance to lodging may allow for a reduction in lodging risk at higher levels of nitrogen application and the need for plant growth regulator applications. The aim of this study was to determine whether the economic optimum application rate of nitrogen and the yield response to PGR application differed between spring barley varieties that contrasted in lodging resistance.  Three spring barley varieties, classified as either resistant, moderately resistant or susceptible to lodging, were sown  at two sites (Carlow and Belfast) during 2014 and 2015 seasons, and treated with either 0, 60, 120, 180 or 240 kg N/ha. In addition, at the Carlow site, crops of each variety grown with 180 kg N/ha during 2014 or 240 kg N/ha during 2015 were treated with one of three PGR treatments: untreated (0PGR), chlormequat chloride and trinexapac-ethyl at GS30 (1PGR), or 1PGR in addition to mepiquat-chloride plus 2- chloreoethylphosphonic acid at GS37 (2PGR). Data for lodging, yield and yield components were subjected to a two-way ANOVA, while economic optimum application rates of N were estimated using a parallel curve approach. A greater degree of lodging was observed in crops of the susceptible variety treated with 240 kg N/ha when compared to the resistant variety at all sites evaluated. Despite this, the susceptible variety incurred a significantly lower yield, and a significantly lower optimum rate of N (138 vs 160 kg N/ha), at only one of the four site-seasons (Belfast 2014). The yield of crops treated with either 1PGR or 2PGR did not differ from untreated crops during either season.