Nutritional and developmental regulation of plasma leptin in dairy cattle

Leptin is thought to play a critical role in regulating energy metabolism throughout mammalian life. In growing dairy cattle, plasma leptin has been proposed as a partial mediator of the effects of nutrition on reproductive and mammary development. However, the developmental stage at which the plane of nutrition increases plasma leptin has not been well defined. Further, it is unknown whether the onset of puberty is affected by plasma leptin concentration in dairy cattle. To investigate these questions, two studies were performed. In the first study, neonatal calves were fed a milk replacer at levels supporting an average daily gain of 570 g/d (L) or 1210 g/d (H). Weekly blood samples were obtained until slaughter at 105 kg of body weight. Plasma leptin and adiposity remained constant in the L calves, but started to increase by the third week of age in the H calves. In the second study, 3- to 5-mo-old heifers were fed a total mixed ration supplemented with either calcium salts of palm fat or conjugated linoleic acids at levels sustaining an average daily gain of approximately 1.0 kg/d. Blood samples were obtained until the third postpubertal luteal phase. The fat source had no effects on growth parameters, body composition, age at puberty, or plasma leptin. Therefore, plasma leptin was reanalyzed as a function of age from start of treatment until slaughter. The plasma concentration of leptin remained nearly constant at 2.3 ng/ml until 1 yr of age, when a rise in plasma leptin became obvious. Puberty occurred with equal frequency either around 1 yr of age when plasma leptin was nearly constant or later when leptin was rising rapidly. We conclude that plasma leptin is regulated by nutrition in early postnatal life, but that a sudden increase in plasma leptin is not required for the onset of puberty in dairy cattle.