Association of chosen environmental and animal factors with gestation length and lactation of dairy cows in two Slovak herds

The aim of the study was to assess the effect of season of birth of females, their season of calving, as well as sex of calves and incidence of twins on the gestation length and milk yield during their first pregnancy and lactation. Data, 277 records, were collected from two farms located in northern and western Slovakia. The data in the herd “A” were collected in years 2006–2017, in the herd “B” in years 2014–2018. The herd “A” consisted of Slovak Spotted breed (127 records), the herd “B” consisted of black Holstein cows (150 records). Among observed environmental and animal factors were included: season of birth, season of calving, sex of the offspring and incidence of twins. Factor that reached at least to some degree statistical significance in relation to gestation length was season of calving in the both herds (P < 0.03; P < 0.06). 305-d milk yield tended to be influenced by season of calving (P < 0.09). Impact of season of calving on gestation length differed between studied herds. The herd “A” reached longest average gestation length at spring calving (286.66 ± 3.26 days), the herd “B” at autumn calving (278.49 ± 1.35 days). In the herd “A” calving season was associated with the highest milk production in summer (5876 ± 380 kg), however in the herd “B”, the most productive milk yield was reached in spring (9865 ± 269 kg). The physiological mechanisms of these changes are not clear, hence more research in larger scale is needed.