Influence of Photoperiod on Growth, Puberty and Plasma LH Levels in Gilts

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Abstract
Thirty-six Yorkshire gilts at 100 days of age (average 39.5 kg) were housed in six pens to study the effect of three photoperiods on growth, puberty and ovulation rate, pineal weights and plasma LH concentrations. Gilts in complete continuous darkness (CD) exhibited puberty (first estrus) later and at heavier weights (P<.05) than gilts in 18 hr cool white fluorescent light with 6 hr dark daily (LD) or those in a natural winter photoperiod of 9 to 10.8 hr daily (NDL). Average weights and ages at puberty were 103.3, 90.3 and 94.8 kg (P<.05) and 193.4, 175.6 and 177.1 days (P<.05), respectively. There were no differences (P>.05) in average daily gain or feed conversion attributable to photoperiod treatments. In gilts slaughtered at no less than 30 days post-insemination no differences were found in ovulation rate (as determined by the number of corpora lutea: 12 to 13) fertilization rate (as determined by the number of fetuses: 10 to 12) or in pineal weights (39 to 43 mg). Plasma LH concentrations increased as the experiment advanced (P<.01) from an initial value of approximately .87 ng/ml to a final value of approximately 3.3 ng/milliliter. Although there was marked fluctuation in LH throughout the experimental period, the trends were similar between the treatment groups. It is concluded that for pigs intended for slaughter, increased light in the barns is of questionable benefit. It would however, be advantageous to select replacement gilts early and rear them in daily photoperiods of sufficient length to avoid delaying sexual maturity. It is suggested that light may be producing its effect on the hormone levels at the hormone sensitive phase of gonadal growth and (or) by advancing the onset of this hormone sensitive phase.