Long-term effects of human growth hormone-releasing hormone and photoperiod on hormone release and puberty in dairy heifers.
Forty-eight Holstein dairy heifers (98.9 kg BW; 3 mo old) were subjected for 246 d to twice-daily s.c. injections of saline (CTL) or human growth hormone-releasing hormone (GRH; 5 micrograms/kg BW) and to photoperiods of 8 h of light (L): 16 h of dark (D) or 16L:8D according to a 2 x 2 factorial arrangement of treatments. Jugular blood samples were collected from 16 heifers at 3, 4, 8, and 11 mo of age to monitor prolactin, growth hormone, and estradiol-17 beta. Plasma progesterone concentrations were monitored weekly in all heifers as an index of puberty (> 1 ng/mL). Growth hormone release was induced by GRH (P < .001) throughout the trial; area under the GH curve (AUC) averaged 1,582 vs 3,643 ng.min-1.mL-1 in CTL vs GRH heifers. However, GRH-induced GH response was less (P < .05) after the second daily injection. There was also an interaction (P = .08) between GRH, photoperiod, and days of treatment on GRH-induced GH response; AUC was greater in GRH-16L:8D than in GRH-8L:16D heifers at 3 mo but less at 8 mo of age. The PRL concentrations were similar for both photoperiods at 3 mo (36.4 vs 41.7 ng/mL) and 8 mo (16.2 vs 12.8 ng/mL) of age but were greater in 16L:8D vs 8L:16D heifers at 4 mo (18.4 vs 39.3 ng/mL) and 11 mo (26.3 vs 44.1 ng/mL) of age (photoperiod x day interaction, P < .001). Photoperiod of 16L:8D vs 8L:16D reduced (P < .01) weight at puberty in CTL heifers (251 vs 303 kg BW) and to a lesser extent in GRH-treated heifers (271 vs 284 kg BW; GRH x photoperiod interaction, P = .10). In conclusion, GH response is maintained throughout 8 mo of GRH treatment, and a 16L:8D photoperiod will reduce age and weight at puberty in heifers. Furthermore, refractoriness to photoperiod-induced PRL changes was detected.
Ringuet
H
H
Agriculture Canada Research Station, Lennoxville, Québec, Canada.
Pelletier
G
G
Brazeau
P
P
Gaudreau
P
P
Guilbault
L A
LA
Morisset
J
J
Couture
Y
Y
Petitclerc
D
D
D000375:Aging / Q000378:metabolism; D000818:Animals; D002417:Cattle / Q000254:growth & development / Q000378:metabolism*; D004958:Estradiol / Q000097:blood; D005260:Female; D013006:Growth Hormone / Q000097:blood; D013007:Growth Hormone-Releasing Hormone / Q000494:pharmacology*; D006728:Hormones / Q000097:blood*; D007334:Insulin-Like Growth Factor I / Q000378:metabolism; D017440:Photoperiod; D011374:Progesterone / Q000097:blood; D011388:Prolactin / Q000097:blood; D012741:Sexual Maturation / Q000187:drug effects
D006728:Hormones; D011374:Progesterone; D004958:Estradiol; D007334:Insulin-Like Growth Factor I; D011388:Prolactin; D013006:Growth Hormone; D013007:Growth Hormone-Releasing Hormone