In Wisconsin, heifers are reared in different housing types and in diverse climatic conditions. Dairy heifers require more or less dietary energy based on the season of the year and housing type. Specifically, heifer size, temperature, hair coat, resting surface, wind, and solar radiation all have an effect on the energy requirements of heifers. Combinations of environmental conditions affect maintenance energy requirements, and the effects of environment on maintenance energy requirements are more profound on smaller heifers as compared to larger heifers. As heifers grow, they gain body mass and rumen capacity and are more adept at handling colder, more energy-demanding environmental conditions. As a result, energy requirements by season vary more in smaller heifers as compared to larger heifers.
A set of dietary energy guidelines that take into consideration the average monthly temperature and three common heifer housing types in Wisconsin is presented in Table 1. Because not all possible environmental conditions in which heifers are reared could be evaluated, the energy requirements in Table 1 should be considered as educational guidelines only. The energy requirements may have to be further modified for producer-specific heifer rearing conditions. The following methods were used to develop Table 1.
The metabolizable energy (ME) requirements of heifers were estimated for 300, 600, 900, and 1,200 lb heifers using the NRC, 2001 model. The ME requirements were estimated in increments of 5°F, spanning typical Wisconsin temperatures from 10 to 85°F. Heifers reared in an outside housing system were considered to be reared at ambient temperatures. The temperature and respective ME requirements of heifers in bedded pack barns and free-stall barns were estimated at temperatures 7° and 17°F warmer than the ambient temperature. Temperature-based ME requirements were developed for heifers with clean, dry hair coats, not exposed to wind, with night cooling, and not grazing. The ME requirement of all heifer groups was increased 0.35 Mcals to account for miscellaneous environmental energy demand. The ME required of 300, 600, 900, and 1,200 lb heifers were estimated at average daily gains of 2.05, 1.95, 1.85, and 1.75 lb/day and at body condition scores of 2.0, 2.5, 2.7, and 3.0, respectively. A diet of 50% corn silage and 50% alfalfa silage was used to establish baseline effects of diet on maintenance requirements.
Dry matter intake (DMI, kgs/d) was estimated by the equation of Hoffman et al, 2007 (DMI, kg/d=15.36*(l-e(-.022*BW, kg)) and adjusted for temperature effects by the equation (unpublished): DMI adjustment kgs/d= (-.012*Temperature, °C). Dietary energy requirements (Mcals/kg) were developed by dividing daily ME requirements by DMI and subsequently converting ME to digestible energy (DE) by the equation: DE, Mcals/kg)=ME, Mcals/kg+0.45)/1.10. Finally, DE was converted to total digestible nutrients (TDN) by the equation (DE,Mcals/kg/0.04409).
National Research Council. 2001. Nutrient requirements of dairy cattle. 7th Rev ed. Natl. Acad. Sci., Washington, D.C.
Hoffman, P.C., Weigel, K.A., and R.M. Wernberg. 2007. Negative Exponential Models to Predict Dry Matter Intake of Dairy Heifers. J. Dairy Science. 90(Suppl. 1):558(abstr.).
Patrick C. Hoffman
University of Wisconsin-Madison