Partial Replacement of Alfalfa Hay with Chopped Wheat Straw in Diets for Lactating Dairy Cows

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Abstract
Two trials were conducted to determine the effect of substituting chopped wheat straw (untreated or ammoniated) for part of the alafalfa hay in rations for lactating dairy cows. As fed, all rations contained 45% forage. In trial 1, three different diets were fed: one-half of the forage was long alfalfa hay in each diet and the remainder was either chopped alfalfa hay, untreated chopped straw, or ammoniated chopped straw. In trial 2, the same three diets were fed, and a fourth diet was included in which all forage was long alfalfa hay. Production responses were consistent across both experiments. Neither DM intake (18 kg/d) nor milk yield (26 kg.d) was affected by diet. Milk fat percentage was increased by substituting straw in the diet and, as a result, yield of 3.5% FCM was increased. Protein and SNF content of the milk generally were not different among treatments. Ruminal acetate increased and propionate decreased when straw was added to the diet. Other than an increase in BW in trial 1, treating wheat straw with ammonia did not affect any of the traits studied. Under the conditions of this study, substitution of chopped straw for 50% of the alfalfa hay in high grain diets for lactating dairy cows maintained milk yields and increased production of FCM because of higher milk fat percentage. Low milk fat on the alfalfa hay diets was related to its lower ADF content and reduced forage particle size compared with the straw diets. Ammoniation of the wheat straw did not affect production of the wheat straw did not affect production responses.