Metabolisable energy concentration in perennial ryegrass pastures: multi-site analysis of effects of cultivar, nitrogen fertiliser and white clover content

Nutritive value of perennial ryegrass cultivars will be included in the DairyNZ Forage Value Index (FVI) using metabolisable energy (ME) content as the measured trait. There is little published information on genetic variation in ME among NZ perennial ryegrass cultivars, and no information on interactions between cultivars and environmental or management factors that may affect relative cultivar rankings in the FVI. These data are essential for a) calculating FVI performance values for individual cultivars, and b) ensuring that interactions between cultivar and environmental or management factors are accounted for in evaluation systems. Hypothesis: there is genetic variation in ME in New Zealand perennial ryegrass cultivars but there are no interactions between cultivar/functional type and environment, N fertiliser level, or presence/absence of white clover. A common experimental design, comprising all combinations of two levels of N fertiliser (‘low’ and ‘high’), two levels of white clover (pastures sown with or without white clover), and eight perennial ryegrass cultivars was established in 2012 at four sites (Waikato, Manawatu, Canterbury and Southland). Whole-pasture herbage was sampled pre-grazing in spring, summer and autumn in each of three years for estimation (by near infra-red spectroscopy) of ME content. Data from all sites were combined and analysed as a mixed model fitted using REML in GenStat. The main effect of cultivar was significant (P<0.001) in every season of every year. The site x cultivar interaction was also significant (P<0.01 or =0.001) in most seasons but overall cultivar rankings were very consistent across sites (tetraploids > late-season heading diploids > mid-season heading diploids). Main effects of N fertiliser and clover presence/absence were significant in most seasons. However, only one significant cultivar x clover interaction (summer of year 1 after sowing, P=0.02) and two significant cultivar x N interactions (both P = 0.024) were observed (out of a total of 9 season x year combinations in each case). These were all scaling interactions: there was no re-ranking among cultivars. The hypothesis was supported, with the exception of the cultivar x environment interactions, which were observed in most seasons. The relative ranking of perennial ryegrass cultivars does not differ between ryegrass monocultures (‘without clover’ treatment) and grass/clover mixtures, indicating that field evaluation systems based on monocultures are reliable for the purposes of the FVI. However, data for absolute ME content are required from multiple sites to account for the cultivar x environment interactions observed here.