A Snapshot in Time of Fatty Acids Composition of Grass Herbage as Affected by Time of Day

Polyunsaturated fatty acid contents in the products of pasture-fed ruminants depend on the amount and composition of fatty acid ingested and rumen biohydrogenation. The total nonstructural carbohydrates (TNC) and DM concentrations of herbage increase during the day; however, it is not known if fatty acids follow the same pattern. This study aimed to quantify the diurnal variation of herbage fatty acid concentrations. Vegetative micro-swards of orchardgrass (Dactylis glomerata L.) and meadow fescue (Festuca pratensis Hud.) were sampled in July and September, respectively, over 2 consecutive days. Sampling occurred 4 times/d: sunrise (0537 or 0650 h), morning (1037 or 1110 h), afternoon (1537 or 1530 h), and sunset (2037 or 1930 h) for orchardgrass and meadow fescue, respectively. Cut herbage was analyzed for DM, CP, TNC, NDF, ADF, and palmitic, oleic, linoleic and α-linolenic acids. Diurnal variation of temperature, relative humidity, and photosynthetic radiation were recorded every 5 min. Time of day affected (P < 0.01) herbage chemical composition. From sunrise to sunset, DM and TNC increased, whereas CP, NDF, and ADF decreased. Time of day did not affect (P > 0.01) concentrations of palmitic, linoleic, and α-linolenic acids of herbage. Oleic acid increased (P < 0.01) 22 and 12.7% from sunrise to sunset in orchardgrass and meadow fescue, respectively. However, this incremental increase in oleic acid did not affect (P > 0.01) time of day effect on total fatty acids. Concentrations of PUFA in grass herbage remain stable during the day, whereas structural and nonstructural carbohydrates, as well as CP, do not.