. 1999. Plane of nutrition and folic acid supplementation between birth and 4 months of age on mammary development of dairy heifers. Can. J. Anim. Sci. 79: 227-234. Forty-seven dairy heifers of approximately 10 d of age were assigned randomly to a 2 × 2 factorial design to study the effects of folic acid supplementation (0 vs. 40 mg) administered weekly i.m. and levels of feed intake after weaning on mammary development. Folic acid treatment started immediately and all heifers were weaned 5 wk later. Heifers were then either fed ad libitum grass hay and concentrates or restricted to a body weight gain of approximately 700 g d -1 until slaughter at 4 mo of age. Average daily gain was affected by feed intake level after weaning (615 vs. 954 g d -1 ; P < 0.01); however, folic acid supplementation increased weight gain only during the 5-wk period following weaning (P < 0.05). Heifers fed ad libitum were 33 % heavier before slaughter (P < 0.001) but there was no effect due to folic acid supplementation (P > 0.05). There was no effect of treatments on serum prolactin and growth hormone concentrations (P > 0.05); overall, prolactin increased and growth hormone decreased over the 16-wk sampling period. However, serum IGF-1 concentrations were significantly higher in heifers fed ad libitum following weaning as compared with the feed-restricted animals (P < 0.001); overall, IGF-1 concentrations increased linearly between weeks 2 and 16 (P < 0.001). Plane of nutrition did not have any effect (P > 0.05) on the total volume of parenchymal tissue in the mammary gland (61.6 vs. 63.6 cm 3 ); however, ad libitum feeding increased significantly (P < 0.001) the volume of extraparenchymal tissue in the gland (262.0 vs. 1067.6 cm 3 ). After adjusting data for the difference in body weight at slaughter, the amount of parenchymal tissue was smaller in animals fed ad libitum (P < 0.05); this adjustment did not change the effect of plane of nutrition on mammary extraparenchymal tissue. In conclusion, a fast rate of gain after weaning up to 4 mo of age induced a large accumulation of mammary fat, but did not negatively affect the total amount of parenchymal tissue in the mammary gland of dairy heifers.Key words: Plane of nutrition, folic acid, mammary gland, IGF-1 Petitclerc, D., Dumoulin, P., Ringuet, H., Matte, J. et Girard, J. 1999. Taux de croissance et supplémentation en acide folique entre la naissance et l'âge de 4 mois sur le développement de la glande mammaire de génisses laitières. Can. J. Anim. Sci. 79: 227-234. Quarante-sept génisses âgées approximativement de 10 jours ont été distribuées aléatoirement selon un dispositif factoriel 2 × 2. Les deux facteurs étudiés ont été la quantité d'acide folique injectée i.m. par semaine (0 ou 40 mg) pendant 16 semaines et le niveau d'alimentation suivant le sevrage. Suite au sevrage, les génisses ont été alors alimentées à volonté de foin et de concentrés ou restreintes pour permettre un gain de poids moyen de 700 g jour -1 jusqu'à leur abattage à l'âge de quatre mois. Le gain moyen quot...

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.

Forty-eight heifers (3 mo old) were subjected for 35 wk to twice daily s.c. injections of saline or human somatotropin-releasing factor (5 microgram.kg-1 BW) and photoperiod of 8 h (short day) or 16 h (long day) of light daily according to a 2 X 2 factorial design. There was no treatment effect on slaughter weight, withers height, and carcass length. Somatotropin-releasing factor increased thymus weight, metacarpal length, and protein content of the carcass but decreased carcass fat content. Long day photoperiod increased chest girth, pituitary weight, and carcass yield. Effects of somatotropin-releasing factor and photoperiod were not additive on volume of mammary parenchymal tissue; there was also a treatment interaction on liver and lung weights. However, extraparenchymal mammary tissue was decreased by human somatotropin-releasing factor and short day photoperiod. In conclusion, use of somatotropin-releasing factor and photoperiod treatments affected carcass parameters and composition and mammary growth.