Growth analysis of timothy grown with varying N nutrition

Nitrogen fertilization increases yield of timothy (Phleum pratense L.) but little knowledge exists on the effect of N fertilization on the growth processes of field-grown timothy. The objective of this study was to analyze the growth of timothy fertilized with increasing rates of N in spring and summer and, in particular, to determine the relative importance of radiation interception and radiation-use efficiency in explaining the differences in shoot growth caused by N deficiency. In two separate experiments conducted in spring 1993 and summer 1994, shoot growth, leaf area development, radiation interception and radiation-use efficiency of timothy grown with four N rates (0, 70, 140 and 210 kg N ha(-1) in 1993; 0, 60, 120 and 180 kg N ha(-1) in 1994) were determined using weekly sampling. Maximum leaf photosynthesis was also measured. The maximum shoot growth and LAI were reached with 140 kg N ha(-1) in the spring of 1993 and 120 kg N ha(-1) in the summer of 1994. The rates of N fertilization required to produce these maximum values of shoot growth were those that resulted in shoot N concentration equal to or greater than the optimal N concentration. The concept of an optimum N concentration was therefore useful in identifying situations of N deficiencies. When no N was applied, the growth rates were 32;and 33% of the maximum growth rates in spring and summer, respectively. The impact of the N deficiency was greater on RUE than on cumulative intercepted PAR. When no N was applied, the RUE and cumulative intercepted PAR were respectively 45 and 61% of their maximum values in spring, and 52% and 80% of their maximum values in summer. A general and synthetic expression of the effect of crop N concentration on shoot growth was developed using the concept of an optimal N concentration.