Yield and nutritive value of the spring growth of an ageing timothy sward

The decline in productivity of forage grasses with age since establishment has been recognised for several years, but this yield reduction and its consequences on nutritive value have not been studied in timothy (Phleum pratense L.). Our objective was to characterize the dry matter (DM) yield and nutritive value during spring growth of an ageing timothy sward grown under non-limiting N conditions. A timothy sward, seeded in 1998, was harvested at four developmental stages (stem elongation, early heading, late heading, and early flowering) during spring growth from 1999 to 2005. The average DM yield observed in production years 5, 6, and 7 was 43% of that measured in the first 4 production years when timothy was harvested at stem elongation or early heading, 52% when harvested at late heading, and 74% when harvested at early flowering. The yield potential of timothy during spring growth decreased with ageing at a rate of 0.66 and 0.91 Mg DM ha(-1) yr(-1) with harvests at early and late heading, respectively. The nutritive value of timothy harvested at stem elongation was greater in older (production years 5, 6, and 7) than in younger (first 4 production years) swards as indicated by in vitro true digestibility of DM (+ 48 g kg(-1) DM), in vitro digestibility of NDF (+ 77 g kg(-1) NDF), and neutral detergent fibre (NDF) concentration ( - 62 g kg(-1) DM); this difference, explained mostly by the lower DM yield of older swards, disappeared when timothy was harvested later on in the growth cycle. Rates of change in nutritive value observed during the phase of reproductive development in spring growth were, therefore, greater in older than in younger swards. Our results demonstrate clearly the decline in timothy productivity with age and its consequences on nutritive value.