Environmental and managerial factors affecting the growth of Friesian heifers from six months of age to calving

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Abstract
The growth of 120 Friesian heifers born into one herd over a period of 12 years was studied for the period from 6 months of age to calving at about 30–35 months of age. Live weights were recorded weekly and rates of live-weight gain were calculated for each 3-month period for each animal. Δt 6, 12, 18, and 24 months of age the heifers weighed 360, 590, 800 and 950 lb. The coefficients of variation of live weight did not vary with age and averaged ±10·0%. Coefficients of variation of live-weight gain increased from ± 20% at 6–9 months of age to ±60% at 24–27 months of age. Growth rate declined by 0·02 lb/day from 12 months to 30 months of age for animals under constant managerial conditions. Heifers wintered indoors grew faster than those wintered outdoors. The growth of younger animals was decreased more by outdoor wintering (a reduction of 0·6 lb/day at 15 months) than was the growth rate of older animals (a reduction of 0·3 lb/day at 21 months of age). When live-weight gain was low in winter there was marked compensatory growth at grass the following summer. Pregnant heifers grew 0·1 lb/day faster in early pregnancy and 0·6 lb/day faster in the last 4 months of pregnancy than did barren heifers.There was no appreciable difference between the live-weight gain of spring- and autumn-born heifers. With an autumn calving policy spring-born heifers were served and conceived 135 days earlier and weighed 106 lb less at calving than did autumn-born heifers. There was little difference in the milk yield of the two groups in 305 days of the first lactation.The regression equation of live weight (lb) on age (months) to 3rd calving was W = 1449 – 2027 e-0·042t. It was concluded that in the interests of economic rearing the dairy farmer could with advantage exercise greater control of the relationship between nutrition and growth during high cost winter periods and low cost summer periods.