The effect of length of use of teaser rams prior to mating and individual liveweight on the reproductive performance of ewe hoggets

AIM: To determine the effects of liveweight (LW) and duration of teasing with vasectomised rams prior to breeding on both the mating performance and pregnancy rate of ewe hoggets. METHODS: Five hundred and forty-six (1/2) Romney x (1/4) Texel x (1/4) Finnish Landrace ewe hoggets were weighed and randomly assigned to one of three treatment groups: either exposed to five harnessed vasectomised (teaser) rams for 17 days prior to the breeding period (T17; n =181); exposed to five harnessed teaser rams for 8 days prior to the breeding period (T8; n = 184); or not exposed to teasers (unteased; n = 181). Harnessed entire rams of the same breed were then introduced for the duration of two oestrous cycles. Using crayon marks, hoggets were identified as having been mated in the first 5 days, Days 6-10, Days 11-17, during the first 17 days only, during both 17-day periods, during the second 17 days only, or not mated. All hoggets that were mated were scanned for pregnancy, using ultrasound, 48 days after the end of the breeding period, and identified as being either non-pregnant, or as singleton- or twin-bearing. RESULTS: Hoggets mated in the second 17-day period only were significantly lighter than those mated in either the first 17-day period only (p < 0.001), or in both the first and second 17-day periods (p < 0.02). At Day 1, twin-bearing hoggets were heavier than singleton-bearing hoggets (p < 0.001) and tended to be heavier (p = 0.06) than non-pregnant hoggets. At Day 18, twin-bearing hoggets were heavier than non-pregnant (p = 0.01) and singleton-bearing (p < 0.01) hoggets. A lesser proportion (p < 0.001) of unteased hoggets was mated in the first 17-day period only, compared to T17 hoggets. Conversely, a greater proportion of unteased hoggets was mated in the second 17-day period only, compared to either T8 (p < 0.01) or T17 (p < 0.001) hoggets. A lesser proportion of unteased hoggets was mated in the first 17 days of breeding, regardless of whether they returned to oestrus or not, in comparison to either T8 (p = 0.001) or T17 (p < 0.001) hoggets. Of those mated in the first 17 days of the breeding period, T8 hoggets were less likely (p < 0.05) to be mated in the first 5 days of that period but more likely to be mated in the last 7 days, compared to unteased and T17 hoggets. T17 hoggets had higher pregnancy rates to the first 17 days of breeding compared with either unteased (p = 0.01) or T8 hoggets (p = 0.06), and were more likely (p < 0.05) to be twin-bearing. In addition, the scanning percentage of T17 hoggets was higher than either unteased or T8 hoggets (p < 0.05). CONCLUSIONS: Heavier hoggets at breeding are more likely to be mated during the first 17 days of breeding and to be twin-bearing. While teasing for just 8 days increased the numbers mated in the first 17 days of breeding it did not increase the numbers of hoggets pregnant to that period. In contrast, teasing for 17 days did increase the numbers pregnant to the first 17 days of breeding and also increased the overall scanning percentage. It is recommended that farmers use a 17-day period of teasing before the breeding of ewe hoggets.