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Research and Development Department, Agway Inc., Syracuse, NY 13221 and Cooperative Research Farms, Charlottesville, NY
ARS-USDA, Cornell University, Ithaca, NY 14850
ABSTRACT
In early lactation, dietary CP and energy can profoundly affect milk yield, but current methods of assessing these nutrients are often inadequate to predict animal performance. Nutrient availability is largely determined by rate and extent of ruminal digestion. Current ruminal availability data for many feedstuffs are plagued with extreme variation. Microbial yields are related primarily to the growth rate that carbohydrate permits, and few data exist on rumen carbohydrate availability. Variations in microbial growth rates (.04 to 25%/h) can cause a 44% variation in yield; use of a constant growth yield across feedstuffs is inappropriate. Microbes can be starved for nitrogen when rumen available protein is low (<30%). If rumen-available protein is greater than 60%, nitrogen losses are excessive, even if rumen-available carbohydrate is also high. Decreases in rumen availability of protein increases milk production when CP intake is marginal (<14% CP), but no response is seen if CP is more than 16%. Manipulation of rumen-available protein appears to be most beneficial when cows are in negative energy balance and sufficient body fat is available for energy demands. Early lactation studies indicate that DM intake and milk yield are highly correlated with carbohydrate digestion rates. Changes in rumen available protein appear to have less dramatic effects on production performance than modifications of energy, but the two are highly interdependent.
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