The influence of early and late breeding of dairy cows on fertility, weight changes and on milk production of the present and subsequent lactation

In a Holstein herd (125 cows) the influence of early and late breeding post partum on reproduction and production was investigated. At calving the cows were randomly assigned to two groups. The early bred group was scheduled to be bred at the first visible heat following 50 days post partum which resulted in an average interval of 73 ± 18 days from parturition to the first service. The average interval from parturition to conception was 88 ± 33 days. The cows in the early bred group conceived after an average of 1.50 (range: 1 to 3) services per conception. The late bred group was scheduled to be bred at the first visible heat following 80 days post partum which resulted in an average interval of 93 ± 17 days from parturition to the first service. The average interval from parturition to conception was 120 ± 40 days. The cows in the late bred group conceived after an average of 1.96 (range: 1 to 5) services per conception. The number of services per conception was significantly (P£.05) higher for the late bred group. The early post partum reproductive activity was monitored with milk progesterone analyses. A sample of milk strippings was analysed by a radioimmunoassay technique every second day from 6 days post partum until conception occurred or the animal was removed from the herd. Cows which did not conceive were excluded from the statistical analyses. The average length of the first estrous cycle was 17 ± 7.5 days. The first progesterone detected heat occurred 33 ± 10 days post partum compared to the first visible heat which occurred 48 ± 26 days post partum. Calving associated problems and the calving during the pasture season delayed conception in the late bred group but not in the early bred group. The late bred group produced more fat corrected milk (FCM) in the 305 day lactation and in the first 150 days of the subsequent lactation and also gained more (P≤.05) weight between calvings. Cows calving during the pasture season produced more FCM in the 305 day lactation but gained significantly (P≤.01) less weight between calvings than cows calving in the non-pasture season. There was no difference between early and late bred group, health status groups or groups according to the calving season in terms of average daily milk and FCM yield calculated over the entire period of this experiment (beginning of current lactation to day 150 of the subsequent lactation, including the dry period). Calving difficulties and early post partum reproductive problems were not related to production traits.Land and Food Systems, Faculty ofGraduat