Effects of two mixtures of perennial ryegrass cultivars with contrasting heading dates, and differing in spring-grazing frequency and silage harvest date, on characteristics of silage from first-cut swards

This experiment quantified the effects of: (i) heading date of perennial ryegrass, (ii) grazing frequency in spring and (iii) date of silage harvest, on the ensilability of herbages harvested for silage, and on the conservation and estimated nutritive value of the resultant silages. Replicated field plots with two perennial ryegrass mixtures (intermediate- and late-heading cultivars) were subjected to three spring-grazing regimes (no grazing, grazing in late March and grazing in both late March and late April) and were harvested on four first-cut harvest dates between 20 May and 21 June. Herbage from each of the four replicates of these 24 treatments was precision-chopped and ensiled unwilted and with no additive in laboratory silos. Herbage from the sward with the intermediate-heading cultivar had a higher (P < 0·001) dry-matter (DM) content and buffering capacity than that from the late-heading cultivar, whereas water-soluble carbohydrate concentrations increased (P < 0·001) with more frequent grazing in spring. Later harvesting enhanced herbage ensilability through an increased (P < 0·001) DM content and reduced (P < 0·001) buffering capacity and pH. Fermentation profiles of the silage were not markedly influenced by the cultivar mixture used but grazing in both late March and late April resulted in a more extensive fermentation with the acids produced increasingly dominated by lactic acid. The concentrations of acetic acid, and to a lesser extent, ethanol declined as silage harvest date was delayed. Overall, the relative effects of grass cultivar mixture were smaller than those of spring-grazing treatment or silage-harvesting date although on any given harvest date the herbage from the intermediate-heading cultivar mixture was easier to preserve as silage than herbage from the late-heading cultivar mixture. Delaying the harvesting of the late-heading swards by 8 d removed the differences related to growth stage in buffering capacity, pH and DM content.