Protein and Energy as an Integrated System. Relationship of Ruminal Protein and Carbohydrate Availability to Microbial Synthesis and Milk Production

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 rurnen 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