Abstract
The influence of increase in photoperiods during winter time on plasma levels of prolactin (PRL), growth hormone (GH), and thyrotropin (TSH) in Holstein dairy heifers was investigated. Six heifers in a loose barn were exposed to natural daylight (9.8 to 12.2 hours) as a control group. Another 6 heifers in pens received natural daylight plus supplemental cool white fluorescent light for a total photoperiod of 16 hours from December 4 to March 27 (16L). The mean light intensity of the supplemental white fluorescent light at 1m above the level of the floor was 500lux. The mean overall ambient temperature of the loose barn (control) and the pens (16L) during the experimental period were 6.7 and 10.0°C, respectively. Feed of a mixture of silage, hay and concentrate was given at 8.30 and 16.00. Jugular blood samples were obtained during the period 10.00-12.00 by venipuncture into heparinized evacuated tubes immediately before (basal) and 15 minutes after intramuscular TRH administration (1μg/kg body weight). The mean basal plasma PRL level in 16L 2 weeks after increasing the photoperiod was 5 times higher (P<0.05) than that on day 0 (8.3±3.5ng/ml, Mean±SE). Until the 78th day, the mean plasma PRL level was maintained as high as 50-83ng/ml. From 50.5ng/ml on day 0, the mean plasma PRL level 15min after TRH administration in 16L rose to 216ng/ml 3 weeks after increasing the day length. The PRL levels in 16L were significantly higher in comparison with those of the control (P<0.05-0.005). In the control heifers, no significant elevation of plasma PRL concentrations either 0 or 15 minutes after TRH administration in comparison with values on day 0 occurred until the 56th day of the experiment. Although plasma GH levels in 16L before and after TRH administration did not rise significantly in comparison with the values on day 0, higher mean plasma GH levels in 16L than in the control occurred in the later part of the experiment. No significant effect of photoperiods on plasma TSH was observed. The data indicate that a 16L photoperiod made by addition of supplemental light to natural daylight increases plasma PRL levels in comparison with heifers exposed to a natural photoperiod of 9.8-12.2 hours. Further studies will be needed to establish the effect of day length on plasma GH.
