Effects of plane of nutrition and season of birth on the age and body weight at puberty of British Friesian heifers

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Abstract
1. Four groups of British Friesian heifers born in April (no. = 30), July (no. = 29), October (no. = 30) 1977 and January 1978 (no. =27) were allocated to one of six treatments (planes of nutrition) at a mean age of 91 days. From 91 to 365 days of age heifers on treatments 1 to 5 were given individually, according to body weight, a proprietary pellet to 126 days of age, and thereafter a diet of dried lucerne and barley. Heifers on treatments 1 and 2 were fed at the same rate whereas heifers on treatments 3 to 5 were fed at successively higher rates. Heifers on treatment 6 were fed ad libitum the proprietary pellets to 126 days of age and thereafter a 'barley beef diet. Mean live-weight gains between 91 and 365 days of age for treatments 1 and 2 combined and 3 to 6, respectively, were 0·58, 0·68, 0·75, 0·82 and 1·06 kg/day.2. Plasma progesterone concentrations were determined twice weekly in each animal. Puberty was assumed to have occurred 2 days before the start of the first period in which the progesterone concentration exceeded 1μg/1 for at least 10 days.3. There was no significant difference between treatments in mean body weight at puberty (242 kg), but age at puberty (347, 305, 288, 301 and 239 days for treatments 1 and 2 combined and 3 to 6, respectively) was inversely related to rate of live-weight gain between 91 and 365 days of age. However, within each treatment group the fastest growing heifers reached puberty at the same age but at a heavier body weight than the slowest growing heifers.4. There were significant seasonal effects on age and body weight at puberty. The most rapidly reared animals (treatment 6) born in January and April were, on average, 52 days younger and 54 kg lighter at puberty than those born in July and October, but the effects of season of birth in other treatments were different.5. There was no significant difference between treatments in the number of conceptions (mean 0·67) achieved at first service by an Aberdeen Angus bull introduced when the heifers were 350 days of age.