Effects of perennial ryegrass variety and ploidy on voluntary dry matter intake and in vivo digestibility in sheep

Two tetraploid varieties, Astonenergy and Delphin, and two diploid varieties of perennial ryegrass (Lolium perenne), Tyrella and Glenroyal, were evaluated for voluntary dry matter intake (VDMI) and in vivo organic matter digestibility (OMD) using Texel wether sheep housed in digestibility stalls. The structural profile and leaf, stem, dead proportions of the sward were also measured. The experiment was repeated on three occasions (Time Stage (TS)): May/June 2013 (TS1), August/September 2013 (TS2) and September/October 2014 (TS3). Data from both years were analysed separately. There was no variety effect on VDMI during TS1 (1481 ± 44.4 g/day), TS2 (1621 ± 41.7 g/day) or TS3 (1223 ± 86 g/day). There was a significant interaction between TS and variety for OMD (P<0.05) in 2013. OMD declined from TS1 to TS2 for all varieties (P<0.001). This was associated with an increased content of dead material in the sward in TS2 (P<0.001). There was a significant interaction (P<0.05) between TS and ploidy for VDMI as a percentage of bodyweight (VDMI % BW) and OMD in TS1 and TS2, as diploid varieties had higher values in TS1 and lower values than in TS2. In TS3, tetraploid varieties had higher VDMI (% BW) (P<0.05) and a tendency towards higher OMD (P=0.07) when compared to diploid varieties. These results indicate that there is a season by variety (and ploidy) interaction for VDMI and OMD. Future work should use an increased number of varieties to investigate the causes and timing of this interaction.