Short Communication: Influence of Dietary Tallow and Fish Oil on Milk Fat Composition

Four midlactation Holstein cows in a 4 × 4 Latin square design were fed one of four diets that contained 3% added fat (dry matter basis) as 100:0, 67:33, 50:50, or 33:67 tallow:fish oil.Feed intake and yield and composition of milk were not affected by treatment.The proportion of fatty acids 18:0 and cis-18:1 decreased and trans-18:1, conjugated linoleic acid, 18:3, 20:4, and 20:5 increased in milk fat with increasing fish oil.The efficiency of transferring dietary 20:5 and 22:6 to milk fat was < 7%.(Key words: fish oil, tallow, fat, dairy cow)Abbreviation key: CLA = conjugated linoleic acid.Compared with cows fed no supplemental fat, fish oil supplementation usually does not affect milk yield of low producing cows but has consistently reduced milk fat percentage (1, 4, 7).Fish oil could replace other fat sources in the diet, but most studies on fish oil used control diets with no added fat.The objective of this experiment was to compare the effects of fish oil with dietary tallow, a commonly used fat, on production and milk fat composition of dairy cows.Four multiparous Holstein cows at 138 DIM (± 11 DIM) were used in a 4 × 4 Latin square with 28-d periods.Four total mixed diets were formulated to contain 30% corn silage, 29% alfalfa silage, 7.4% alfalfa hay, 30.6% corn grain and soybean meal-based concentrate, and 3% fat supplement (DM basis).The supplemental fat was: 100% tallow, 67:33 tallow: fish oil, 50:50 tallow:fish oil, and 33:67 tallow:fish oil.The fish oil was light, cold-pressed, feed grade Menhaden oil (Zapata Protein Inc., Reedville, VA).Diets (DM basis) contained 15% CP, 31% NDF, 21% ADF, and 5.6, 5.3, 5.3, and 5.1% fatty acids, respectively.Milk yield and DMI was