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Five cows were used in a 5 x 5 Latin square to evaluate the effects of forage type and stage of herbage maturity on milk fatty acid (FA) composition. Four silages were prepared from primary growths of timothy-meadow fescue and red clover swards at an early and late stage of growth. Treatments consisted of these silages and an additional treatment comprised of an equal mixture of late-cut grass silage and early-cut red clover silage. Forages were fed ad libitum and supplemented with 9 kg/day of a standard concentrate. Compared with grass silage, red clover silage diets enhanced milk fat monounsaturated FA (MUFA) and polyunsaturated FA (PUFA) content, 18:3 n-3 in particular, responses that were associated with concomitant reductions in 10:0 to 16:0 concentrations. Decreases in 16:0 and increases in PUFA concentrations were more pronounced for early than late growths of red clover. Even though red clover resulted in marginal increases in total milk fat trans 18:1 content, milk trans-11 18:1 concentrations were lower for red clover than for grass silage diets. It is concluded that replacing grass with red clover silage prepared at an early stage of maturity represents a sustainable nutritional strategy for enhancing the MUFA and PUFA content of milk.