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 les s than or equal to 4) of a perennial ryegrass crop during regrowth after wi nter 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 averag e 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 = 0.0 193 Sigma T-0.3107 (Tin degrees C). On average, 52 degrees C/day are requir ed to allow a LAI increase of 1 unit in spring time. The LAI expansion results from the increase of the number of tillers, of le aves per tiller and of the leaf area. The relationship between the sum of t emperatures and each of these factors is not significant, except for the ti ller density. Adversely, the result obtained by the product of these factor s, the LAI, is significantly correlated to the sum of average positive temp eratures calculated from the beginning of the growth after winter. (C) 1999 Elsevier Science B.V. All rights reserved.