Growth and body composition of 121 Holstein heifers (4.6 to 18 mo and 129 to 407 kg) reared on pasture, drylot and pasture-drylot systems were evaluated in 6 experiments. Heifers were switched from drylot to pasture or the reverse to study carry-over effects from previous treatments; both systems were satisfactory. Alternate 28-day periods of supplement feeding to grazing heifers did not affect final body weight nor wither height, but ADF and heart girth varied during 2 yr. When switching drylot heifers previously fed low and high TDN, to pasture, gains were best for heifers fed the lower TDN diet; also, gains on pasture were best for light heifers. Gains by all heifers grazing mainly orchardgrass-clover pasture were acceptable, but supplementing with a 19% CP concentrate or lasalocid, usually improved gains. Heifers with lowest BW during grazing made compensatory gains in drylot. Urea space estimation technique showed that compensatory gains were mainly fat. Lasalocid feeding increased daily gain and subcutaneous fat deposition but reduced feed intake and ribeye area. When moving grazing heifers to drylot a total mixed ration with fishmeal or soybean meal as protein sources gave similar responses. DM intake of grazing heifers ranged from 8.1 to 10.1 kg/d, vs 7.5 kg/d for arylot. Supplementing grazing heifers with degradable or undegradable protein gave similar responses in growth and body composition. Based on growth and body composition, seasonal grazing of Holstein heifers may reduce costs for rearing replacement heifers. A corn silage-alfalfa silage-orchardgrass hay mixed ration without concentrates when fed ad libitum to heifers in drylot resulted in gain of 934 g/d. Pasture alone heifers gained from 368 to 755 g/d depending on drought and heifer age. Calculations of costs of rearing Holstein replacement heifers were prepared accounting for observations of response to grazing, supplements to grazing and drylot diets. Well managed grazing reduces costs of rearing. / Ph. D.