A total of 615 autumn calving Friesian dairy cows in seven herds were allocated to treatment or control groups according to date of previous calving, parity and milk yield. All cows were scored for body condition at the start of the breeding season. A silastic coil impregnated with progesterone with a capsule containing oestradiol benzoate attached was inserted into the vaginas of the 310 treated cows. The coils were removed after 12 days and cows inseminated 48 and 72 hours later. Cows more than 47 days post partum were inseminated on the first day of the breeding season. Thereafter, cows were inseminated on a weekly basis when between 47 to 54 days post partum. The 305 control cows were observed from 42 days post partum and inseminated when observed to be in oestrus. In both groups, cows returning to service were reinseminated at observed oestrus. Pregnancy was diagnosed by milk progesterone assay and by rectal palpation. The mean intervals to first service were 74.8 days for the treated and 90.4 days for control cows. No significant differences were found in the pregnancy rates to first service, services per conception or barren percentage for treated and control groups respectively. The mean calving-to-conception intervals differed significantly, 93.1 days for treated and 107.3 days for control cows. There was no significant association between condition scores and either pregnancy rates to first service, barren percentage or calving-to-conception intervals.
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