The SNP IGF-1/SnaBI has been studied in about 30 cattle breeds, including dairy and beef breeds [23, 24, 27, 28, 35–38]. In the current study, allele frequencies of the SNP IGF-1/SnaBI estimated for the UHFxNZHF group (A: 0.52, B: 0.48) as well as for the total sample (A: 0.59, B: 0.41), are in agreement with those previously published in North American (NA) , Polish , and Uruguayan  HF populations (A: 0.55, 0.52 and 0.60; B: 0.45, 0.48 and 0.40, respectively). On the other hand, allele frequencies in the sample of UHF cows tended to present a different distribution (A: 0.63, B: 0.37), which is similar to that (A: 0.64, B: 0.36) reported by Ge et al.  in Angus beef cattle. In our study, the sample of UHF cows tends to deviate from HWE due to a heterozygote excess. Reasons for this tendency could be genetic drift or the action of selection acting on this sample; however, no action of selection on the IGF-1 locus was evidenced in our study. Likewise, deviations from HWE were reported for the IGF-1 locus in both beef and dairy cattle [39, 40] and, in particular, for the SNP IGF-1/SnaBI in HF populations [27, 36]. Furthermore, evidence of selection acting on the IGF-1 locus were found in Swedish and Polish HF populations , as well as in the brazilian Canchim beef cattle breed . However, evidence of selection acting on this locus might be even greater provided it has not been investigated in most published works on cattle IGF-1 polymorphisms.
Our results from association analyses of the SNP IGF-1/SnaBI with milk production traits show no significant effects of this SNP on either MY, FCM or TSY within the first four months of lactation. This is in accordance with previous reports that failed to observe any significant association between the SNP IGF-1/SnaBI and dairy production traits in HF cattle [24, 28]. On the other hand, results from Mehmannavaz et al.  proved a significant effect of the SNP IGF-1/SnaBI on estimated breeding values (EBV) for milk production traits in Iranian Holstein bulls, as animals with AB genotype had higher EBV for milk and fat yields than homozygous genotypes. In addition, Siadkowska et al.  and Bonakdar et al.  reported associations between the AB genotype and higher percentage of milk fat and protein in Polish and Iranian HF cows, respectively. Likewise, previous studies from our group have shown that AB cows tended to yield more FCM than primiparous BB cows at early and middle lactation and than multiparous AA cows over the lactation period . No other reports were found in the literature concerning the effect of the SNP IGF-1/SnaBI on milk production traits.
The results of the present study further indicate a significant association of the SNP IGF-1/SnaBI with interval to CLA. Cows with the AA genotype present a shorter interval to CLA and are more likely to resume ovarian cyclicity in the early postpartum than AB and BB ones. Potential mechanisms for an association between the SNP IGF-1/SnaBI and the traits of interest may lie in its location in the promoter region of the IGF-1 gene and its possible influence on transcription factor binding sites and on gene expression [28, 41, 42]. Higher IGF-1 blood levels have been found in cows exhibiting shorter CLA intervals [13, 43]. Association studies between the SNP IGF-1/SnaBI and plasma IGF-1 concentration in cattle are contradictory. Ge et al.  reported lower IGF-1 levels in young BB Angus bulls, while Maj et al.  and Mirzaei et al.  found higher IGF-1 concentrations in BB Polish and Iranian HF cattle, and Ruprechter et al.  reported no association in Uruguayan HF cows. Inconsistences among studies may be explained by IGF-1 having different effects according to breeds, stages of growth, physiological states and nutritional status, as it is known that IGF-1 gene expression is developmentally and physiologically regulated .
Circulating IGF-1 concentrations are temporarily affected by the negative energy balance, which in dairy cows dramatically changes during the transition period. Meikle et al.  reported that, under grazing conditions, lean cows with reduced serum IGF-1 concentrations presented prolonged postpartum anestrus, showing that circulating IGF-1 levels are good indicators of the re-initiation capacity after calving, which is in accordance with others [45, 46]. In our study, although AA and AB cows presented shorter anestrus periods than BB ones, no differences were observed in body condition at calving among them. Moreover, differences in body condition change during the experimental period were not evidenced among genotypes, which suggests that energy balance measured by changes in body condition do not explain the differences observed among the SNP IGF-1/SnaBI genotypes on CLA interval after calving. In addition, the lack of a significant effect of milk yield on CLA could be consider as a further evidence that the energy balance is not responsible for the outcome of postpartum ovarian resumption in this study.
Although our results showed an effect of the SNP IGF-1/SnaBI on CLA, no effect was observed on traditional fertility measures like CFS and CC intervals. This is in agreement with the fact that CLA reflects more directly the reproductive physiology of the cow, being more influenced by the cow itself than traditional fertility measures , which are highly influenced by management and farmer’s decisions . Nevertheless, sample sizes analyzed in this study for traditional traits are insuficient to get to concluding results. To our knowledge this is the first report describing an association of the SNP IGF-1/SnaBI with an endocrine fertility measure like CLA in cattle. These results are consistent with findings from a previous work of our group in another HF herd , where longer CFS intervals were found in primiparous BB cows. In contrast, other studies have failed to identify any association of the SNP IGF-1/SnaBI with endocrine or traditional fertility traits in cattle [28, 30]. No other reports were found in the literature concerning the effects of this SNP or other SNPs at the IGF-1 gene on fertility traits in cattle.