Leptin is a 16-kDa protein synthesized by adipose tissue and is involved in regulation of feed intake, energy balance, fertility, and immune functions. Since evidence of a genetic correlation between start of luteal activity and energy balance, milk yield, and live weights is present, we investigated the association of genetic differences in the bovine leptin gene with these traits. Between 1990 and 1997, a total of 613 Holstein-Friesian heifers of two genetic groups with known pedigree were followed from parturition until d 105 of lactation. During the first 15 wk of lactation live weight, feed intake, and milk yield were measured for 565 cows. The start of luteal activity was set at the first day with a progesterone concentration higher than 3 ng/ml. In addition to the interval between calving and start of luteal activity, analyses were performed for average milk yield, percentage fat, protein, and lactose in milk, dry matter intake, feed intake, energy balance, and live weight over the first 15 wk of lactation. All 613 cows were genotyped for two restricted fragment length polymorphisms and for the BM1500 microsatellite, all located at the leptin gene locus. Significances of the genotype effects were estimated using the approximated F-statistic provided by ASREML. Fixed effects were year-season, genetic group, and a quadratic polynomial for age at calving. Animal was fitted as a random effect including the additive relationship between animals to account for background genes. Firstly, each genotype effect was fitted in turn, and secondly the other restriction fragment length polymorphisms were fitted as a cofactor to take into account effects of linkage disequilibrium. Thirdly, sire x genotype interaction was investigated. Heifers with the RFLP1-AB genotype produce 1.32 kg/d more milk and consume 0.73 kg/d more food compared with the RFLP1-AA genotype. No effects were found for start of luteal activity. When linkage disequilibrium with the other markers was taken into account and DMI was included as fixed effect in the model a 0.96 kg/d higher milk yield was still found. Assuming that no pleiotropic effects on traits such as immunity and milk production in later lactations exist, future breeding programs favoring the RFLP1 B-allele can yield a higher milk production without negatively affecting energy balance and fertility. The prospects are good because in this study the frequency of the RFLP1 AB- and BB-genotypes were only 18.5 and 0.2%, respectively.

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Leptin concentrations in body fluids and tissues undergo dynamic changes during the periparturient period. Polymorphisms in the leptin gene have been shown to be associated with differences in leptin concentration during late pregnancy but not during lactation. As the promoter of leptin regulates the expression of leptin, polymorphisms in this region could play an important role in the differences in leptin expression observed during the periparturient period. We sequenced the leptin promoter and discovered 20 SNP in a 1.6-kbp region of the bovine leptin promoter. Fourteen of these SNP were genotyped for all animals and these were found to be associated with leptin concentrations during late pregnancy but not during lactation. Three of these SNP are located in a 135-bp promoter region and together explained 14.3% of the variance in prepartum leptin concentrations which indicates that this region might be important for pregnancy-induced leptin synthesis. In the association study of the 14 SNP with dairy traits three were separately found to be associated with fertility, energy balance and protein yield. These might serve as markers for future breeding programmes for better fertility and energy balance without significantly influencing milk yield in dairy cattle.

Leptin is a hormone produced by adipocytes, and its expression is regulated by body fatness and energy balance. This study describes the association of four leptin gene polymorphisms in dairy cows (R4C, A59V, RFLP1, and BM1500) with circulating leptin concentrations during the periparturient period. A59V is located at a between-species conserved region of leptin, and R4C might have effect on the tertiary structure of the leptin protein because of the presence of an extra cystein. RFLP1 is an intronic SNP and BM1500 is a microsatellite located 3.6 kb downstream of the leptin locus. The four polymorphisms were genotyped in 323 HF heifers with known pedigree. Leptin concentrations were determined biweekly from 30 days before until 80 days after parturition. The effect of genotype on leptin concentrations was modeled by fitting a spline in ASREML describing leptin concentrations as a function of days relative to parturition for each genotype/allele. Surprisingly, associations were found during pregnancy, but not during lactation. This indicates that the polymorphism could be more effective during pregnancy. If further studies demonstrate that more leptin-binding protein (Ob-Re) is present in this stage, it is hypothesized that a structural difference in the leptin protein could cause a sub-optimal binding stringency to Ob-Re. Free leptin could be cleared faster than bound leptin, and this could result in lower leptin concentrations during pregnancy for the polymorphism. The effects found might be ascribed to R4C. However, more study on the Ob-Re receptor, like binding stringencies between R4C and wild-type leptin and glycosylation during pregnancy, would provide more insight in the results found.