Nitrogen Management on Dairy Farms
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Fermentation of Feed Carbohydrate and Protein Fractions by Rumen Bacteria

In efficient dairy cow diets, over half of the protein requirements of the animal are met with bacterial protein produced during the fermentation of feed carbohydrates. Bacteria ferment sugars, starch and fiber in feeds into volatile fatty acids, which are absorbed and used as the primary energy source by the animal. During fermentation, the bacteria grow and re-produce, and pass out of the rumen with the feed particles. The bacteria produced contain an average of 10% N.

Although there are many species of bacteria in the rumen, they can be classified into two groups; those that grow on fiber and those that grow on non fiber sources of carbohydrates (primarily sugars and starch). The bacteria degrade protein into amino acids and ammonia, which is used for bacterial growth. Non protein N sources (urea, N compounds in silage) are fermented to ammonia. The bacteria that grow on fiber require ammonia. The bacteria that grow on non fiber carbohydrates can utilize either ammonia or amino acids for growth. Bacterial protein produced by non fiber bacteria can be increased by as much as 18% when adequate amino acids are available. In most feeds, part of the protein will not be degraded (to amino acids and ammonia) in the rumen, because the protein is resistant to degradation by the bacteria or is in a matrix with slowly digested carbohydrates (fiber, slowly degraded starch). This protein will pass to the intestine.

The absorbed energy and amino acids available to meet requirements from ruminal fermentation depend on accurate determination of dry matter intake (DMI), the content of carbohydrate and protein fractions in diet ingrediets, microbial growth on the fiber and non fiber carbohydrates consumed, and the unique rates of digestion and passage of the individual feed carbohydrate and protein fractions that are being fed. The interactions of DMI, digestion and passage have several implications. First, the growth rate of each microbial pool that digests respective available carbohydrate fractions, and absorbable microbial amino acids produced, will depend on the special characteristics and intake of the feeds being fed, which in turn determines the demand for the N source required by each pool. Second, the percentage of cell wall that escapes digestion will change, depending on digestion and passage rates. Third, the site of digestion and, depending on the rate of whole tract passage, the extent of digestion will be altered. Variable rates of digestion and passage have similar implications for protein fractions in feeds. Those readily available will be degraded in the rumen, while those more slowly degraded will be partially degraded in the rumen and partially degraded post-ruminally, the proportion depending on rates of digestion and passage of the protein fractions in the feeds.