| Summary: | Breeding forages for increased fatty acid (FA) concentrations has the potential to improve the FA profile of ruminant products (meat and milk). Twenty perennial ryegrass genotypes from an “experimental” breeding population and four genotypes from a “benchmark” mapping population were used to assess genotypic variation in FAs across a growing season. Mean total FA (TFA) concentration for cuts one through five were 29.0, 31.7, 31.1, 34.4 and 42.0 g kg<sup>−1</sup> DM, respectively. Six main individual FAs, namely palmitic acid (C16:0), trans-3-hexadecenoic acid (C16:1Δ<sup>t3</sup>), stearic acid (C18:0), oleic acid (C18:1Δ<sup>c9</sup>), linoleic acid (C18:2Δ<sup>c9,12</sup>) and α-linolenic acid (C18:3Δ<sup>c9,12,15</sup>), accounted for between 90% to 96% of TFA. Population means differed (<i>p</i> < 0.001) for TFA and all individual FAs, expect for C18:2Δ<sup>c9,12</sup> (<i>p</i> = 0.106). “Benchmark” mapping population on average had 8–44% higher FA concentrations compared to the “experimental” breeding population, except for C18:0 where the mapping population had lower concentrations. Individual genotypes from each population differed for all individual FAs and TFA (<i>p</i> < 0.05), with differences between the lowest and highest concentrations ranging from 8% to 23% amongst the mapping population genotypes and between 20% and 39% for the breeding population genotypes. Cutting date had a strong effect on population and genotype means (<i>p</i> < 0.001) with an overall trend for FA concentrations to increase through the season. However, several significant population and genotype × cutting date interactions were also found highlighting the need for further investigations to strengthen our knowledge and understanding of how genetics and environment interact for this particular trait. Nevertheless, candidate “high-lipid” genotypes were able to be identified using multivariate analysis which could be taken forward into a breeding program aimed at increasing forage FAs.
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