Grazing increases the unsaturated fatty acid concentration of milk from grass‐fed cows: A review of the contributing factors, challenges and future perspectives

Grazing animals walk outdoors and harvest fresh herbage from a three‐dimensional canopy. Botanical composition and grassland management in various farming systems affect herbage intake and milk fatty acid (FA) composition. Grazing animals encounter a vertical gradient while grazing down a sward; responses in milk FA composition are discussed. Grazing animals have diets rich in poly‐unsaturated fatty acids (PUFA), particularly α‐linolenic acid (C18:3n‐3). Concentrates and starch‐rich fodders affect rumen retention time of the feed, increase linoleic acid (C18:2n‐6) intake, and alter biohydrogenation pathways toward less n‐3 PUFA and less rumenic acid (conjugated linoleic acid (CLA) isomer C18:2 c 9 t 11), leading to lower contents of these compounds in the milk. Milk from grazing‐based production systems including organic systems, therefore, has less saturated and more n‐3PUFA and CLA c 9 t 11 which is considered beneficial for health. The occurrence of natural trans FA ( t 11) in milk predominates with grazing whereas intensive feeding practices enhance t 10‐FA, with possible negative health effects. FA intake – output relations can differ among plant species due to rapid rumen passage rate or compounds protecting PUFA, resulting in more PUFA in milk. Methodological constraints for FA measurement in herbages are discussed. Trends in grazing and dairy farming systems and implications for milk FA composition are addressed. Practical applications: Grazing is important for fresh forage intake, animal health and natural behaviour, landscape values and grassland biodiversity. Grazing is a political, societal and marketing issue in The Netherlands where retailers since 2011 sell dairy products labelled as “pasture‐milk” and dairies pay a premium to farmers whose cows get access to pasture. Grazing affects green leaf intake because animals can select. Pasture allowance enhances α‐linolenic acid and CLA c 9 t 11 (rumenic acid) concentrations in milk fat. FA need to be designated by unique names; CLA is a generic term and isomers give rise to different biological responses. As some forbs reduce biohydrogenation, grassland species diversity may improve PUFA transfer efficiency from feed to milk. Trends in animal genetics and increased herd size, TMR feeding and automated milking systems cause reduction of grazing in Europe, which can increase saturated and trans ‐10 milk FA concentrations and reduce naturally occurring fat‐soluble vitamins and antioxidants in dairy products. Grazing (pasture‐feeding versus indoor with conserved forage and concentrates) increases n‐3 PUFA and trans ‐11 over trans‐10 FA in cow milk when cows can select, as leaves have high n‐3 PUFA concentrations; biodiverse grasslands enhance this effect as some forbs show less PUFA rumen biohydrogenation than grasses, thus increasing efficiency of PUFA transfer from forage to milk.