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Document Type

Thesis - University Access Only

Award Date


Degree Name

Master of Science (MS)

Department / School

Dairy Science

First Advisor

Alvaro Garcia


The objective of this study was to examine the effects of the partial substitution of soybean meal with a product containing slow-release urea and microbial fermentation extracts [(OPT); Optimase®, Alltech Inc., Nicholasville, KY)] in dairy cow diets at two forage concentrations. Sixteen lactating Holstein dairy cows (4 primiparous and 12 multiparous) were randomly assigned to a 4 × 4 Latin Square in a 2 × 2 factorial. High and low forage diets contained respectively 61% and 46% forage, with and without OPT. Forage consisted of 75% corn silage and 25% alfalfa hay (dry matter basis). Experimental diets containing OPT (125g/cow/day) were designed to partially replace soybean meal 48 (0.625 kg) with forage fiber [corn silage (0.225 kg) and alfalfa hay (0.075 kg)], and nonforage fiber [soybean hulls (0.200 kg)]. There were no effects of OPT on ruminal ammonia-N concentration or ruminal pH. Replacing soybean meal by OPT increased total VFA concentration in high-forage diets by 6.4%; however, OPT decreased total VFA concentration in low-forage diets (P value forage × OPT interaction = 0.03) by 4.4%. Feeding OPT decreased ruminal proportions of propionate, isobutyrate, and isovalerate (P < 0.01), increased acetate (P < 0.01), and did not change the proportion of butyrate and valerate. Feeding cows high versus low forage diets reduced DMI, milk, ECM, protein content, and protein yield, but increased fat % (P < 0.05). Feeding OPT decreased (P = 0.03) DMI 1.05 kg/d compared to diets without OPT. Milk yield decreased (P = 0.05) slightly with OPT inclusion; however, 4%FCM and ECM were not affected. Feeding OPT did not affect milk fat percentage and yield, lactose percentage, and total solids percentage and yield. Milk protein yield decreased 3.5% with OPT inclusion (P = 0.013). Feed efficiency (4%FCM/DMI and ECM/DMI) was not affected despite OPT diets reducing DMI and not changing FCM and ECM yield. Under the conditions of this study, OPT fed both in high- and low-forage diets at approximately 125g/day per cow was an effective partial substitute for soybean meal without affecting lactation performance (4% FCM, ECM) and feed efficiency.

Library of Congress Subject Headings

Dairy cattle -- Feeding and feeds
Rumen fermentation
Urea as feed
Feed additives
Enzymes in animal nutrition


Includes bibliographical references (pages 49-60).



Number of Pages



South Dakota State University


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