LAI evolution of a perennial ryegrass crop estimated from the sum of temperatures in spring time

When there is sufficient fertiliser, the development of the canopy (LAI less than or equal to 4) of a perennial ryegrass crop during regrowth after winter or after a cut in spring time, essentially depends on the temperature. The expression of the Leaf Area Index (LAI) according to the sum of average positive temperatures instead of relatively to the number of days highly improves the relationship. This is verified at three sites, where climatic conditions are different, during 2 consecutive years and is written L = 0.0193 Sigma T-0.3107 (Tin degrees C). On average, 52 degrees C/day are required to allow a LAI increase of 1 unit in spring time. The LAI expansion results from the increase of the number of tillers, of leaves per tiller and of the leaf area. The relationship between the sum of temperatures and each of these factors is not significant, except for the tiller density. Adversely, the result obtained by the product of these factors, the LAI, is significantly correlated to the sum of average positive temperatures calculated from the beginning of the growth after winter. (C) 1999 Elsevier Science B.V. All rights reserved