The Effect of Extended Photoperiod on Growth and Reproductive Performance of Crossbred Ewe Lambs Exposed for Fall Lambing

Inconsistencies in the efficiency of spring born ewe lambs bred for fall lambing continue to be a challenge for researchers and producers alike. The objective of this research was to evaluate the effect of extended photoperiod on growth and reproductive performance of crossbred ewe lambs exposed for fall lambing. Trial one utilized spring born¼ Finnsheep, ¼ Dorset, ½ Targhee (FDT, n=146) and½ Hampshire, 1/8 Finnsheep, 1/8 Dorset, ¼ Targhee (HFDT, n=46) ewe lambs. These ewes were randomly assigned within breed to either a control (natural ambient photoperiod) or to extended light (EL) treatment from January 4, 1993, until February 19, 1993. Light was provided to the EL treated ewes from 1700 to 23 00 hours, approximating 16 h of light (≈ 16. 5 ft candles at eye level). These ewes received similar diets during treatment. Ewes were then co-mingled and received similar diets once again. Lambs in the EL group had higher (P=.02) overall weight gains than the control ewes (28.2 ± .70 vs 25.9 ± .67 kg) and were heavier (63.3 ± .88 vs 60.6 ± .84 kg, P=.03) on April 6, 1993, just prior to breeding. FDT ewes were heavier (35.9 ± .45 vs 34.0 ± .80 kg, P=.04) than HFDT ewes at the start of the trial September 29, 1992. Overall weight gain was greater (27.9 ± .85 vs 26.2 ± .47 kg, P=.08) for the HFDT than the FDT. There were no differences between breeds for intermediate weights. Blood samples were collected weekly starting March 18, 1993, on 48 lambs from each group to determine serum progesterone (P4) levels. Blood work was discontinued May 27, 1993. Mean serum progesterone (P4) levels were higher (P=.02) for EL ewes than C, .45 ± .04 vs .33 ± .04 ng/ml, respectively. FDT ewes had higher (.54 ± .03 vs .24 ± .05 ng/ml, P4 levels than HFDT ewes. All ewes were exposed to teaser rams 14 days prior to a 3 5-day breeding season which began April 15, 1993. More (P=.12) ewes exposed to EL lambed (n=8) compared to controls (n=3). One HFDT (2.2%) lambed compared to 10 for the FDT (6.8%) ewes (P=.23). The number of lambs per ewe lambing did not differ (P=.26) between control and EL ewes. Trial two utilized 171 FDT and 37 HFDT ewes. They were randomly assigned to treatment groups identical to trial one. Control ewes were exposed to ambient day length, while EL ewes received additional light from 1600 to 0100, approximating 18 h of light, from December 1, 1993, to February 10, 1994. Ewes were then co-mingled and managed as a single group. Initial weights were similar (P=.50) for EL and C ewes at the start of the study. Overall, lambs gained 27.5 ± .71 and 25 .6 ± .73 kg (P=.06) for EL and C, respectively, but were not different (P=.51) in weight prior to breeding. Initial weights were similar (P=.28) between FDT and HFDT ewes at the start of the trial. Prebreeding weights were greater (