Italian ryegrass and nitrogen source fertilization in western Oregon in two contrasting climatic years. I. Growth and seed yield1

Abstract

Prescription based nitrogen (N) fertilization of crops in a given local environment begins by understanding factors affecting crop N use and relating these factors to a time scale most directly related to crop growth. For these reasons the following objectives were sought for an economically important grass seed crop in western Oregon, Italian ryegrass (Lolium multiflorum Lam.). One, to determine the influence of N source (NH4‐N and NO3‐N) on crop growth and seed yield. Second, to determine the relationship between accumulated growing degree days (GDD) and plant ontogeny and relate these findings to N‐source effects on growth and seed yield parameters. Field plots were established in 1991 and again in 1992. Five N‐source treatments were applied: calcium nitrate (CN), ammonium nitrate (AN), ammonium sulfate (AS), ammonium chloride (AC), and urea‐dicyandiamide (DCD). Nitrogen‐source treatments had no effect on root and shoot dry mass accumulation in either year. Ammonium‐N fertilization increased tiller number per unit area compared to all NO3‐N fertilization in 1991 only. Nitrogen source did not affect plant dry mass either year. Between year differences in dry mass accumulation across N treatments were concluded to be the result of climatic and lodging factors. Across N treatments, 1992 seed yield was approximately 2.5‐fold greater than that of 1991. Early crop lodging in 1991 contributed to reduced seed yield. In the drier, warmer year of 1992, seed yield was greatest with AN and urea‐DCD fertilizers. Temperature was the major regulator of growth and development under conditions of equally applied N. Even with contrasting annual climatic conditions, growth data comparisons on a GDD basis was a very effective means of comparing plant growth between years. Grass seed growers could potentially increase N‐use efficiency and economic seed yield by more effectively matching timing and amount of N applied to crop demand by using a GDD time scale. By using this scale, greater consistency of on‐farm management practices (e.g., fertilization) form year‐to‐year could result. Data support findings that ryegrass seed production is best under enhanced NH4‐N nutrition.