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Obtaining the most efficient use of feed nutrients is essential for profitability on dairy farms. Optimum efficiency of nutrient conversion into milk and milk components occurs when both the nutrient requirements of the cow and rumen microbes are met and in synchrony. Understanding the interface between the cow and her rumen microbes remains one of the major challenges in dairy nutrition today.
Rumen microbes convert feed into volatile fatty acids, protein, carbon dioxide (CO2) and methane. When volatile fatty acid production is increased and methane and CO2 are decreased, more metabolizable energy is available to the cow for milk and milk component production.
Many feed additives have their beneficial effects by altering rumen fermentation to reduce methane and carbon dioxide production and increase volatile fatty acid production. Nutritionists often recommend various feed additives to take advantage of improvements in rumen fermentation efficiency.
Two recent studies by the U.S. Dairy Forage Research Center look at the changes in rumen microbial populations of cows when diets with and without Rumensin are fed.
In the first study, scientists fed 18 rumen-cannulated cows diets of slowly fermentable starch (24.9% from corn silage only); rapidly fermentable starch (28% from high-moisture corn and corn silage); diet 2 plus Rumensin; and diet 2 again following removal of Rumensin.
Some cows responded with milk fat depression when going from slowly fermentable starch to the rapidly fermentable diet. Those that did not were switched to the Rumensin diet; some responded with depressed milk fat. More than 88% of the bacterial communities in these cows' rumens were identical, and a mere shift in a few bacteria families was responsible for milk fat depression.
The study also showed that once the bacteria populations and rumen conditions causing milk fat depression were established, it took more than four weeks for milk fat to return to normal after switching back to a non–milk fat depression diet.
The second study measured rumen pH and bacteria population changes in eight cannulated cows fed the same diet containing Rumensin. The results indicate that rumen bacteria population in cows with a similar pH can be quite different and cows with a widely different rumen pH can have a similar rumen bacteria population.
This study also found that milk fat depression is not related to rumen pH, but more to rumen bacteria populations, with the bacteria species Megasphaera elsdenii primarily responsible for milk fat depression. Why this species proliferates in some cow's rumens and not others remains unknown.
Although the microbiology of the rumen remains mystifying, there are ways for dairy producers to maximize nutrient utilization into milk and milk components.
- Maintain consistency in feeds and the feeding program. Dairy cows respond to nutrient supply, and feeding a well-balanced diet that optimizes feed intake every day should keep the rumen microbial population consistent. Although cows may differ slightly in their rumen bacterial populations, it takes a dietary challenge such as excess fermentable carbohydrate feeding for bacteria populations associated with negative production responses such as depressed milk fat or acidosis to proliferate. A healthy stable rumen microbial population will provide a constant pattern of nutrient supply to the animal and optimize efficiency of nutrient usage.
- Know the nutrients in the feed supply and the amounts of feed cows are consuming to accurately formulate diets and minimize losses from nutrient waste.
- When feed changes or challenges to the feeding program occur, allow several weeks for cows to return to desired production levels.