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Document Type

Thesis - University Access Only

Award Date


Degree Name

Master of Science (MS)


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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