Variability in delivery of nutrients to lactating dairy cows.

Dairy cows consume feedstuffs from which they derive nutrients to meet their requirements. Water and dry matter intake limit overall nutrient intake. The variability in the nutrient content of forages is recognized by dairy producers and is addressed with a laboratory analysis program. Grains, oilseeds, and by-products vary considerably in nutrient content, most often being below values listed by the NRC. Nevertheless, these ingredients are usually not analyzed prior to delivery or prior to consumption by the lactating dairy cow. Commercial feeds are regulated and must provide certain concentrations of major nutrients and list the ingredients on the feed label. Energy cannot be directly analyzed but has been estimated by in vitro, regression, and summative equations. Conversions from digestibility to metabolizable and net energy are also estimated simply from coefficients or by using empirical models based on the nutritional composition of individual feedstuffs or diets. Storage and handling losses must be accounted for, and variability in feedstuffs must be addressed through the supplier or a check sample laboratory analysis program. Knowledge of ingredients and dietary nutrient content is essential for any method of computing and delivering nutrients to meet the requirements of the lactating dairy cow.