A selection experiment with Angus cattle, established in 1984/85, has achieved a difference in age at puberty in heifers of 70 + 6 days (17%) between an `Age Plus` (i.e. high age) selection line and an `Age Minus` line. Currently three heifer calf crops (born in 1993-95) are being retained until 5.5 years of age, so that pregnancy rates in 5 mating seasons (and calving data in 4 seasons) on unculled females can be recorded. Genetic correlations between pubertal traits (from either sex) and heifer or cow reproductive traits have been calculated. Similar estimates were also derived from a pilot beef-cattle trial in 1980-85 using paternal half-sib data (230 sire groups), but no selection. The objective of this paper is to evaluate the merits of selecting on pubertal traits to improve reproduction, and to assess the possibilities of this type of selection in dairy cows. Correlated responses in the `Age Minus` herd, relative to `Age Plus`, include earlier pubertal development in half-brothers (+1.6 cm (5.2%) in scrotal circumference (SC) at 8, 10 and 12 months of age, P<0.001), an earlier mean calving date in cows by 3 days following an unchanged start-of-mating date, a 3-day reduced postpartum anoestrous interval (PPAI, adjusted for the earlier calving date), and a higher mean pregnancy rate (+4.7%, based on 1083 matings) in cows 2 years of age or more (P<0.05). All the genetic correlations between age at puberty in heifers and cow reproductive traits in our two studies so far have favourable signs (e.g. negative between age at puberty and pregnancy rate, or positive between age at puberty and PPAI), so that selecting heifers for earlier pubertal age would be consistent with increasing cow reproduction. Because selecting heifers for puberty is not very practical, selecting bulls could be more realistic. All the genetic correlation estimates between SC and cow reproductive traits in our two studies so far also have favourable signs. Additionally, genetic correlations between SC in bulls and age at puberty in heifers are negative (-0.21 ± 0.09 and -0.19 ± 0.20, respectively in the two trials cited above), so that bull selection for higher SC would reduce age at puberty in heifers and improve cow reproduction. Thus direct selection for precocious puberty in our Angus herd (predominantly by bull selection) has led to more fertile beef cows. It is suggested that the genetic correlations observed could be an initial model for dairy cattle, showing how selection-index traits on dairy animals at puberty could be used to correct for the inferior reproductive genes of some dairy cows.
Proceedings of the New Zealand Society of Animal Production, Volume 59, , 169-172, 1999
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