Nitrogen - moddus interaction in perennial ryegrass seed production.

Abstract

Six nitrogen (N) rates were compared with and without trinexapac-ethyl (Moddus®) plant growth regulator in adjacent dryland and irrigated blocks of perennial ryegrass. There was a large positive response to Moddus with an average seed yield increase of 430 and 710 kg seed ha for dryland and irrigated crops respectively. Moddus increased the optimum N rate for a biological response from 50 to 150 kg N (applied + mineral 0-30cm) ha for irrigated and from 150 to 200 kg N ha for dryland respectively using a linear split line regression model. The predicted optimum using a polynomial model was higher. An economic optimum that accounted for both the cost of N and application cost was ≥ 30 kg N ha lower than the biological optimum. Moddus delayed the onset of lodging and this probably had the greatest influence on the seed yield response. Crops that did not lodge had higher photosynthetically active radiation (PAR) at the flag leaf level in the canopy. There was a minimal irrigation response as only one irrigation application was applied in early spring. Late spring rains were adequate and twice the average rain fell in December. Seed germination in the lowest and highest N treatments was similar. Additional keywords: Lolium perenne, plant growth regulator, seed yield Introduction Trinexapac-ethyl (Moddus) plant growth regulator is a stem shortener which blocks gibberellic acid (GA) biosynthesis (Rademacher, 2000) and increases seed yields (Chastain et al., 2003). Surveys in New Zealand indicate that Moddus is used on 99-100 % of all perennial ryegrass (Lolium perenne L.) seed crops (Rolston et al., 2004). Trials on N rate responses in the mid-late 1990’s by Rowarth et al. (1998) were undertaken prior to the introduction of Moddus. There has been speculation that the seed yield responses to N, reported from the 1990’s trials may differ when crops are grown with Moddus. The effect of moisture stress on response to Moddus has also been the subject of speculation. Trials were established to compare N rate responses in adjacent dryland versus dryland versus irrigated blocks with and without Moddus at each N rate. This study reports on a series of trials started in 2003 with the aim of predicting the spring N requirement for perennial ryegrass seed crops based on combined soil N and applied N responses. Earlier data in this series has been included in joint Danish-US-NZ research (Gislum et al., 2007; Hart et al., 2007). Seed yield N responses are commonly polynomial, with an optimum followed by a decline at higher N rates (Rowarth et al., 1998; Gislum et al., 2007).