Differences in sward structure of ryegrass cultivars and impact on milk production of grazing dairy cows

Animal trials cannot be used in routine in a forage grass- breeding programme or in a cultivar testing procedure. An alternative to these animal trials is to identify plant traits associated with a high animal production under grazing that could be used as selection criteria. For this, the effect of sward structure on milk production was studied, comparing four late- heading diploid perennial ryegrass cultivars ( Lolium perenne L.) under grazing. They were rotationally strip grazed with forty- eight autumn calving dairy cows during three periods in spring 1999. Herbage allowance was 18 kg DM . cow(-1) . day(-1) and the rotation interval was 3 weeks. Differences in milk production were small but significant. Cows that grazed cultivar 1 exhibited an extra milk production of 0.4 kg . cow(-1) . day(-1) than those that grazed cultivar 4. Total biomass, digestibility and nitrogen contents were similar irrespective of the cultivar. Cultivar 4 exhibited a higher biomass per tiller but a lower tiller density, bulk density and green leaf mass in the upper layers of the sward compared to cultivar 1. Cows likely ingested more pseudostems formed by the sheath tubes and the stem growing inside grazing cultivar 4 than the others ( 14 vs. 8 cm). The intermediate layers of the sward, containing pseudostems, were 6.7 points less digestible than the upper layers. Therefore, the ingested herbage of cultivar 4 was likely to be less digestible and less palatable than the others.