Fatty acid composition of milk from dairy cows fed fresh alfalfa based diets

Objectives were to study effects of seasonal variation and feed additives on milk fatty acid (FA) composition of dairy cows fed fresh alfalfa based diets. To meet the first objective, a survey of eight selected alfalfa-grazing dairy farms was conducted in which the farms were visited four times in the four seasons of the year to obtain information on diet composition, herd management, days in milk (DIM) and milk yield. The proportion of alfalfa pasture in the total diet averaged 680 ± 167 g/kg of dry matter (DM), and estimated DM intake and milk yield averaged 15.0 and 15.1 kg/cow/d, respectively. Mean concentrations of conjugated linoleic acid (CLA) (13.6 g/kg fat) and trans 18:1 (48 g/kg fat) were positively associated with the proportion of alfalfa in the diets, while short chain FA ( i.e., C4–C14) proportions of milk FA were lower in spring high alfalfa diets versus winter low alfalfa diets with intermediate values in summer and autumn. The opposite trend occurred for most of the medium and long-chain FA ( i.e., >C14). Seasonal variations of milk FA composition were associated with the proportion of alfalfa pasture in the diet, particularly for CLA and trans 18:1. To study effects of feed additives on milk FA composition of milk from dairy cows fed fresh alfalfa based diets, impacts on milk FA composition of different feed additives used to control rumen fermentation was evaluated. The treatments were a control diet (Control), composed of ad libitum freshly cut pre-bloom alfalfa pasture, and three feed additives: Control +200 g/cow/d of dietary buffers (DB; calcium carbonate 400 g/kg; sodium bicarbonate 300 g/kg; magnesium oxide 150 g/kg; bentonite 150 g/kg), Control + antibiotics (MV; 300 mg monensin/cow/d and 30 mg virginiamycin/cow/d) and Control + 15 g/cow/d of a yeast culture (YC). No feed additives affected DM intake, milk yield, milk composition or milk FA composition, including CLA (12.9 g/kg milk FA), in mid lactation primiparous or late lactation multiparous dairy cows fed fresh alfalfa, and there were no differences between primiparous and multiparous cows in milk FA composition.