Long‐term bromocriptine treatment during late pregnancy has differential effects on milk yields of single‐ and twin‐bearing ewes

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Treatment with 2a‐bromoergocriptine mesylate (CB154) was used to determine the effects on ovine lactogenesis of short‐ versus long‐term suppression of circulating prolactin (PRL) concentrations in single‐ and twin‐bearing ewes and whether the effects were associated with changes in plasma concentrations of progesterone or insulin. Ewes were housed under constant photoperiod (18 h light: 6 h dark) from 3–4 weeks pre‐partum until 8 days post‐partum, then grazed outdoors. Ewes were treated daily with subcutaneous injections of 2 mg CB154 for 20 ± 1. days pre‐partum (CB20 group, n = 10), or for 9 ± 1 days pre‐partum (CB9 group, n = 10), or with excipient injections for 19 ± 2 days pre‐partum (Control group, n = 9), and milked daily for the first week, then on Days 14, 21, and 28 post‐partum. CB154 inhibited PRL secretion and delayed lactogenesis with no consistent effects on plasma progesterone or insulin concentrations. PRL concentrations were higher (P < 0.05) in CB9 than in CB20 ewes from 14 days pre‐partum until 1 day post‐partum and plasma PRL concentrations in the two groups converged as they approached parturition (P < 0.01). There was no significant treatment group by pregnancy rank interaction for circulating PRL concentrations. Although the delay in lactogenesis did not differ between CB20 and CB9 ewes, CB 154 treatment was more effective in reducing milk yield in twin‐bearing than in single‐bearing ewes when used for 20 days, but not for 9 days, pre‐partum (P < 0.05). These results indicate that, during the period 20–10 days pre‐partum, CB154 treatment had a marked effect on milk yield in twin‐ but not single‐bearing ewes which can not be explained by corresponding effects on plasma PRL, progesterone, or insulin concentrations.