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The content of fatty acids (FA) in herbage is important for forage quality and animal‐source foods, but there is a lack of knowledge on effects of agronomic practices, and on environmental factors related to seasonal variation in FA concentrations and FA composition. This research investigated the effects of N applications and forage cutting intervals in a factorial combination on FA composition of perennial ryegrass (Lolium perenneL.). The research was conducted during 2 yr that evaluated early, intermediate, and late cutting regimes during various seasons. Positive effects of N application and negative effects of extended regrowth interval on FA concentration and profile were found that appeared to be consistent within periods and years. The total concentration of FA was 32 to 53% higher in spring than in summer, and 18 to 25% higher in autumn than in summer. A positive relationship (P< 0.001; r2= 0.84) between concentrations of total FA and N was observed within and across a wide range of samples. Concentrations of total FA and N were on average 30% higher (P< 0.001) in early‐cut highly fertilized herbage than in late‐cut unfertilized herbage. In generative canopies in late spring, the changes in FA concentration were related to differences in leaf blade proportion (P< 0.001; r2= 0.64). In contrast, in vegetative canopies the changes in FA concentration were associated with seasonal changes in solar radiation and temperature. Overall, applying high rates of N and harvesting perennial ryegrass after short regrowth intervals resulted in higher fatty acid concentrations in herbage.

Six endophyte‐free diploid perennial ryegrass (Lolium perenneL.) cultivars (Abergold, Respect, Agri, Herbie, Barezane, and Barnhem) were examined in an experiment to investigate the selection behavior of grazing Holstein Friesian cows in July and September 2003 and May 2004 and to identify factors related to preference. Three groups of dairy cows (Bos taurus) were allowed to select among these six cultivars that were sown in replicates in a randomized block design with 12 plots (36.7 by 2.0 m). Each experiment lasted 4 d, and every day cows were offered a new field with 12 plots. Herbage intake was measured using the sward‐cutting method, measuring herbage yield before and after grazing. The Chesson–Manly index was used to quantify cattle grazing preference. Dairy cows did select among the cultivars in every period. The pattern of selection was consistent over the three experiments, Abergold was selected most, and Agri and Respect were least selected (22 vs. 14%). In July 2003 and May 2004, cultivars differed in morphological characteristics, but this was not related with grazing preference. In all three experiments, distinct differences were found among cultivars in chemical composition [dry matter (DM), ash, neutral detergent fiber, and water soluble carbohydrates (WSC) concentration] and digestibility of organic matter, especially for WSC concentration, which, when averaged over experiments, ranged between 135 g kg−1DM (Barezane) and 165 g kg−1DM (Abergold). Dairy cows preferred the cultivars with low ash and fiber concentration and high WSC concentration and digestibility.