The objective of this study was to evaluate the effect of feeding prepubertal heifers a diet containing a high level of polyunsaturated fatty acids on mammary development and milk production. A total of 116 Holstein heifers were either fed a conventionally formulated concentrate or a high oil (HO) concentrate, using the same formulation but including 20% soybean oil, from birth to 6 mo of age. After 6 mo of age, all heifers were managed identically. Mammary gland development was evaluated on heifers slaughtered at 4 mo (n = 10) and 12 mo (n = 30) of age. Other heifers were bred when they reached 15 mo of age and milk production and feed intake were recorded every day from wk 4 to 18 of lactation. Feeding the high oil concentrate increased the concentration of linoleic acid in blood plasma (176%) and mammary fat pad (78%) at 4 mo of age and mammary fat pad (93%) at 12 mo of age. At 4 mo of age, mammary development was similar in both treatments. At 12 mo of age, total, parenchyma, and stroma weights of the mammary gland were not affected by treatments. However, lipid content was lower and concentration of DNA was higher in the parenchyma of heifers fed the high oil diet. Nevertheless, total parenchymal DNA and dry fat free tissue content did not reach statistical significance despite the fact that they were, respectively, 15 and 21% higher in HO heifers. Milk production and composition was not affected by treatments. In conclusion, feeding prepubertal heifers with a high oil concentrate slightly improved the mammary development but effects were too small to be translated into better lactating performances.

Ten Holstein cows were treated with 30.9 mg.d-1 of recombinant bST from 15 to 41 d of lactation. The Latin square design included three infusion periods of 6 d each with 3 d of rest between infusion periods. Infusions were physiological saline, glucose (50 g.h-1), and insulin plus glucose (12.5 IU.h-1 + 50 g.h-1). Blood was collected continuously during the last 24 h of each infusion period. Statistical analyses of data for energy balance, milk yield, and DMI were performed on the last 3 d of each infusion period. Production data before and after infusions (i.e., no recombinant bST) estimated that recombinant bST increased milk yield of cows infused with glucose and saline by 3.1 and 3.6 kg.d-1, respectively. Net energy intake was not affected by infusion, but glucose infusion resulted in higher BW loss than did saline infusion (2.33 vs. 0.08 kg.d-1, respectively), and insulin plus glucose infusion resulted in BW gain (0.65 kg.d-1). Milk yield was 39.9, 39.6, and 37.6 kg.d-1 for cows infused with saline, glucose, and insulin plus glucose, respectively. The insulin plus glucose infusion increased milk protein 11 and 14% compared with response to saline and glucose infusions, respectively; no change occurred in the proportion of casein and whey proteins. Serum bST was increased 109% with exogenous recombinant bST. Serum IGF-I was lower for cows infused with glucose than for those infused with saline (21.03 vs. 27.44 ng.ml-1) and increased to 46.55 ng.ml-1 for cows infused with insulin plus glucose. Serum concentrations of insulin and glucose were 13.7 and 56.7, 18.5 and 61.9, and 30.5 muIU.ml-1 and 39.4 mg.dl-1 for cows infused with saline, glucose, and insulin plus glucose, respectively. The results of this study suggest that low concentrations of plasma insulin in early lactation may limit the IGF-I response to recombinant bST (uncoupling). Despite higher IGF-I, milk yield was lower, probably as a result of low blood glucose. These results suggest that, in early lactation, insulin is still anabolic because the BW gain of cows increased. However, milk yield was still higher than that for cows in late lactation with similar insulin concentrations.

. 2005. Lactation response of cows to different levels of ruminally inert conjugated linoleic acids under commercial conditions. Can. J. Anim. Sci. 85: 231-242. Dietary conjugated linoleic acid (CLA) supplements have been shown to reduce milk fat synthesis in dairy cows. A rumen-inert source of CLA is required for commercial feed applications. The conversion of dietary lipids to a calcium salt is considered as a method to counter the extensive hydrogenation of dietary lipids that occurs in the rumen. Our objective was to determine whether feeding calcium salts of CLA under commercial conditions would affect milk production, milk composition and blood metabolic profile. A total of 240 dairy cows from eight farms were blocked according to the calving date, and randomly assigned to four treatments providing CLA at 0, 8, 16 and 32 g d -1 . Milk production was recorded and milk was sampled on day 0, 7, 14, 28 and 42 of the feeding period. Blood samples were taken on day 42 from early-lactating cows (< 157 d in milk) to determine the metabolic profile. Milk fat yield was decreased 11, 20 and 28%, and milk fat concentration was reduced 13, 22 and 28% (linear; P < 0.001) when cows received 8, 16 and 32 g d -1 of CLA, respectively. Milk yield, milk protein and blood metabolic parameters were not affected by experimental treatments. Calcium salts of CLA can be used as an effective tool to manage milk fat content on commercial dairy farms. dans l'alimentation diminue la synthèse du gras du lait chez la vache. Les ALC alimentaires peuvent cependant être hydrogénés par les microorganismes du rumen. La formation de sels de calcium est considérée comme une méthode pour prévenir l'hydrogénation des ALC au cours de leur passage dans le rumen. Afin de déterminer les effets des ALC sous forme inerte sur la production et la composition du lait et sur le profil métabolique sanguin en conditions commerciales, 240 vaches laitières ont été réparties selon un dispositif en blocs, selon la date de vêlage. Les traitements consistaient en quatre différentes doses d'ALC (0, 8, 16 et 32 g j -1 ) distribuées de façon aléatoire à l'intérieur de chaque bloc. La production laitière a été enregistrée et des échantillons de lait ont été recueillis aux j 0, 7, 14, 28 et 42. Des échantillons sanguins ont été prélevés au j 42. La production de gras a été diminuée de 11, 20 et 28%, et la teneur en gras diminuée de 13, 22 et 28% par rapport au témoin lorsque les animaux ont reçu 8, 16 et 32 g j -1 d'ALC (linéaire, P < 0,001). La production laitière, la production et la teneur en protéine du lait et les paramètres sanguins n'ont pas été affectés. Les sels de calcium d'ALC pourraient être utilisés en conditions commerciales pour contrôler efficacement la teneur en gras du lait.

No abstract

LBoNano, M., Gnllo, M., Gallo, G. eNt Br-ocr, E. 1990. Effects of a28-day sustained-release formulation of recombinant bovine somatotropin (rbST)