An experiment was conducted to examine whether increased CLA in milk of dairy cows fed fresh pasture compared with alfalfa and corn silages was because of ruminal or endogenous synthesis. Eight Holsteins were fed a total mixed ration using alfalfa and corn silages as the forage source in confinement or grazed in a replicated crossover design. The proportion of total fatty acids as CLA (primarily c9, t11-18:2) in g/100 g was 0.44 v. 0.28 in ruminal digesta, 0.89 v. 0.53 in omasal digesta and 0.71 v. 1.06 in milk during confinement feeding and grazing, respectively. Blood plasma CLA was 0.54 v. 1.05 mg/l for the two treatments, respectively. The increased concentration of CLA in milk with grazing likely resulted from increased synthesis through desaturation of t11-18:1 in the mammary gland.

Milk production of fall-calving dairy cows during subsequent summer grazing was evaluated in two consecutive years using a total of 80 mid- to late-lactation Holsteins. Cows calved during September and October and grazed from April to August in the following year. In yr 1, 27 cows grazed a native grass pasture and 13 cows grazed a native grass-clover mixed pasture containing 26% red clover and white clover. In yr 2, 40 cows grazed native grass pasture as one group. Also, cows in yr 2 were administered bovine somatotropin, whereas in yr 1, no bST was used. Grazing cows also were fed concentrate supplements at 6.2 kg/d of dry matter (DM) in yr 1 and 7.9 kg/d of DM in yr 2 to provide 35 to 40% of total intake. Average daily milk during the grazing period decreased 3.6 kg in yr 1 and 7.7 kg in yr 2 when compared with milk yield extrapolated from the lactation curve established 10 wk before being turned out to pasture. Estimated DM intake during grazing was also less than what would have been expected had cows continued on a total mixed ration in confinement. Cows grazing the mixed pasture of grass and clover yielded 1.3 kg/d more milk than those grazing the grass pasture in yr 1. A decrease in milk resulting from the change from total mixed ration fed in confinement to grazing supplemented with concentrates was not avoided with these mid- to late-lactation cows, but the cumulative loss over the lactation was less than with early lactation cows in a companion study. Clover enhances the grazing value of pasture when grown with grasses.

Feeding shelled corn as the primary supplement in cracked dry form or ground high moisture form to grazing cows was studied in an experiment using 32 cows in late lactation. Each of the two forms of corn accounted for 74.7% of the concentrate supplement, which was fed at 9 kg/d (DM). The experiment was 13 wk, during which cows grazed permanent native pasture containing mostly grasses and some clover. Milk yield was 2.4 kg/d higher for cows fed ground high moisture shelled corn than for those fed dry cracked shelled corn. Milk fat content was lower (3.28 vs. 3.67%), but protein content was higher (3.26 vs. 3.15%) for the high moisture corn supplement. Results suggest that high moisture corn can serve as a better supplement for grazing cows than dry corn, likely by providing more metabolizable energy and microbial protein due to more fermentable and digestible starch.

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