Factors influencing age at first mating in purebred Swedish Landrace and Swedish Yorkshire gilts.

The aim of the present study was to retrospectively analyze causes of the variation in age at first mating in Swedish Landrace (L) and Swedish Yorkshire (Y) gilts. Production traits including growth rate from birth to 100kg body weight and backfat thickness at 100kg body weight were also studied. Data analyzed were obtained from 11 L and 11 Y nucleus herds and included gilts born during a 5-year-period from October 1993 until September 1998. The complete data set included information on 14,761 gilts (6997 L and 7764 Y). Traits analyzed included age of gilt at first mating, growth rate and backfat thickness. Seven statistical models were used for analyzing the data. Factors included were gilt breed, birth month, parity number and size of the litter in which the gilt was born as well as their interactions. Compared with Y gilts, L gilts grew faster (571 versus 556 g/day; P<0.001), had a thinner backfat (11.9 versus 12. 3mm; P<0.001) at 100kg body weight and were 12 days younger at first mating (237 versus 249 days; P<0.001). Birth month significantly (P<0.001) influenced age at first mating, growth rate and backfat thickness. Gilts born from smaller litters were mated at younger age than gilts born from larger litters even when age at first mating was adjusted for the effect of growth rate and backfat thickness. Growth rate of the gilts decreased when 'birth litter size' increased. Gilts born from primiparous sows grew slower, had a thinner backfat at 100kg body weight and were older at first mating compared with gilts born from multiparous sows. Gilts with a higher growth rate were younger at first mating than those with a lower growth rate. Gilts with a thicker backfat at 100kg body weight were mated earlier than the thin ones. However, the effect of growth rate on age at first mating was more pronounced in the gilts with a thinner backfat rather than the ones with a thicker backfat.