The effect of selenium supplementation during the early post-mating period on embryonic survival in sheep

The effect of selenium (Se) supplementation of ewes with blood Se concentrations ranging between 100-200 ng/ml on embryonic survival during the early post-mating period (days 15-35) was studied in 4 trials. In the 1st 2 trials 137 ewes were used in 1991 and 118 in 1992. After being mated as a single flock, these ewes were stratified randomly into 3 groups. One group served as a control, the 2nd was injected with 1 ml Deposel (containing 50 mg Se as Ba selenate) and the 3rd group injected with 1 ml containing 1 mg Se as Na selenite. During 1991, supplementation was administered immediately after the mating period. It was postponed by 14 days in 1992. Parenteral Se supplementation reduced (p < 0.10) the number of ewes that lambed by > 19% during 1991 but not during 1992. The number of ewes producing twins was unaffected. In Trials 3 and 4 there was a consistent indication that parenteral Se supplementation of pregnant ewes between 15-35 days after mating resulted in a reduced (22-40%) embryonic survival rate, although significant (p < or = 0.10) differences were only observed after the pooling of treatments receiving parenteral Se supplementation. Drenching of ewes with 50 mg Se as Na selenite resulted in a similar tendency. Biochemical appraisal of the blood, kidney and liver Se status of ewes failed to reveal toxic levels. The possible mechanisms involved in impaired embryonic survival are unclear. Supplementation of ewes during the 1st month of pregnancy with parenteral Se preparations is not recommended.