Response to lactation induction differs by season of year and breed of dairy ewes.

Lactation artificially induced (ART) by steroid hormones and natural lactation (NAT) after lambing were compared in 2 dairy sheep breeds (Manchega and Lacaune) in 2 experiments conducted during winter and spring. In experiment 1, ART ewes (14 Manchega and 9 Lacaune) were induced into lactation in winter by the standard protocol, which consisted of s.c. injections of estradiol and progesterone administered in 2 portions daily from d 1 to 7. Hydrocortisone acetate was injected s.c. daily on d 18 to 20. Milking was initiated on d 21 and continued for 13 wk. A similar group of NAT ewes was selected for the contemporary comparison of NAT vs. ART lactation. All Lacaune ewes, but only 3 of the 14 Manchega ewes (21%), were successfully induced into lactation. Despite the successful induction of lactation in Lacaune ewes, milk yield was much lower than that obtained in NAT lactation (1.23 +/- 0.14 vs. 2.51 +/- 0.15 L/ d). Milk composition from wk 5 to 13 did not differ between groups, except for whey protein, which was greater in ART than in NAT ewes (1.47 vs. 1.25%). In experiment 2, 19 Manchega ewes were divided into 2 groups and induced into lactation in spring by using the standard induction protocol, similar to that used in experiment 1 (control, n = 9), or the standard protocol modified with bovine somatotropin (bST, 250 mg/ewe on d 11; n = 10). Manchega ewes had an improved response to the standard protocol of lactation induction in spring compared with winter. Milk yield in bST-treated Manchega ewes was 98% greater than that in control ewes (402 +/- 85 vs. 203 +/- 86 mL/d). The use of bST during mammogenesis did not affect milk composition. In conclusion, marked differences between Manchega and Lacaune dairy ewes were observed in their response to lactation induction when using the standard protocol during different photoperiod conditions. The Manchega ewes were unable to establish lactation in winter but were able to do so in spring. The response to lactation induction in dairy ewes seems to be related to their endogenous levels of prolactin and growth hormone, the use of which should be explored more deeply in future research.