DEVELOPMENT OF LEGHORN PULLETS SUBJECTED TO EARLY LIGHT STIMULATION

Abstract

Leghorn pullets of a commercial strain were reared in cages maintained in rooms providing either 8 or 14 h d−1 light. At 92 d, 12 birds identified as light (L) or heavy (H) weight from within each room were blood sampled for assay of plasma luteinizing hormone (LH). Subsequent samples were taken at 3- or 4-d intervals to 120 d of age. Another group of 12 L and 12 H birds were blood sampled at 92 d from within the 8 h light environment. These birds were again blood sampled at 99 d and then moved to laying cages and photostimulated with 14 h light per day. Birds were blood sampled for LH assay after 3 d and 7 d and then at 7-d intervals to 140 d of age. This general procedure was repeated with birds photostimulated at 102, 106, 109, 113, 116, 120, 123, 127 and 130 d of age. Thus, 10 groups of heavy birds and 10 groups of light birds were blood sampled prior to, at time of, and subsequent to light stimulation. Mean LH values for birds maintained on 18 h light throughout rearing were 1.5–2.0 ng mL−1 whereas with 14 h light throughout rearing values of 1.5–2.5 ng mL−1 were observed. Photostimulation had the same general effect on plasma LH regardless of body weight or bird age. After 3 or 4 d light stimulation there was a dramatic increase of some 100% in LH levels. Earlier photostimulation resulted in earlier age at 1st egg. For light weight birds, maturity was linearly advanced by 0.44 d for each day advance in photostimulation; a quadratic response was seen with heavy birds. Although photostimulation caused almost immediate changes in LH (P < 0.05) age at first egg lagged by some considerable time. These data do not indicate an absolute physiological (chronological) time limit for early sexual maturity, although it is likely close to 100 d of age. Key words: Pullet, luteinizing hormone, sexual maturity