The effects of raising post-oestrus progesterone concentrations on luteal activity in post-partum dairy cows

In order to investigate the rationale of manipulating post-ovulation proges terone (P-4) concentrations, luteal activity was measured in 10 Holstein-Fr iesian cows treated with a progesterone-releasing device (CIDR, 1.9 g P-4) inserted on the morning of day 2 post-oestrus for a period of 7 days and co mpared with 10 untreated control animals. Milk samples were collected daily during afternoon milking from 7 days pre- to 24 days post-oestrus and P-4 concentrations were measured using enzyme-linked immuno-sorbent assay (ELIS A). The milk P-4 profiles were used to assess the effects of early post-oes trus P-4 elevation on five intervals of the luteal phase. In the treated (T ) and control (C) animals, the mean interval from oestrus to P-4 rise (grea ter than or equal to 3 mu g/l), was 2.38 (s.e. 0.18) and 4.90 (s.e. 0.28) d ays, respectively, (P < 0.01). The mean interval from oestrus to the attain ment of peak P-4 concentrations was not significantly different (P > 0.05) between the T (14.75 (s.e. 1.62) days) and C (14.30 (s.e. 0.70) days) anima ls, with cows in the T group showing a more variable (F-7,F-9 = 4.30, P < 0 .05) interval to the occurrence of the peak. The interval from peak P-4 to corpus luteum (CL) regression (when P-4 fell below 3 mu g/l) was not signif icantly different between the T (4.13 (s.e. 1.30 days) and C (5.60 (s.e. 0. 88) days) groups. Furthermore, CIDR insertion did not alter luteal phase le ngth, number of days with P-4 concentrations greater than or equal to 3 mu g/l (T = 16.50 (s.e. 0.80) v. C = 15.00 (s.e. 0.47) days) or the interval f rom oestrus to CL regression (T = 18.88 (s.e. 0.79) v. C = 19.90 (s.e. 041) days). We conclude that treatment with CIDR 2 days after oestrus successfu lly increased the availability of P-4 to cows without significantly affecti ng luteal phase characteristics.