Prediction of daily total digestible nutrient requirement of beef cows in northern climates.

Calculated energy consumed in 284 cow yr by individually fed Angus, Charolais and reciprocal cross cows was partitioned into m~intenance, weight change and milk production requirements. Total digestible nutrient (TDN) consumed in 94 heifer yr was partitioned into maintenance and weight change requirements. The cow year was subdivided into midgestation, late gestation and lactation periods that were 69 + 1, 90 + 0 and 203 -+ 1 d long, respectively. Variables affecting TDN intake (P<.25) in all periods studied were mean body weight (MWT), weight change and age of dam. Previous parity influenced TDN consumption during midgestation and milk production influenced TDN consumption during lactation. Mean body weight was evaluated by three methods as one of the variables influencing TDN consumption; MWT 1"~176 MWT to a fractional exponent (MWTx) calculated from the data and MWT with the regression line forced through the origin (zero intercept) were substituted in the multiple regression model. Equations were used to predict daily TDN for cows over a range of weights. Results indicate that method of analysis influenced predicted daily TDN requirements. Use of MWT 1"~176 and MWTx resulted in similar predictions, while the zero intercept technique underestimated energy requirements for small cows and overestimated energy requirements for large cows. Inclusion of an intercept allowed the best fitting regression line to intersect the Y axis at the point determined