## 4 Discussion  
The aim of the present research work was to evaluate the impact of the inbreeding degree in parameters related to production and reproductive parameters in high-producing primiparous dairy cows. Several research groups have found differences in the effect of the degree of inbreeding on the parameters studied, showing adverse effects on different phenotypic and genotypic traits in dairy cattle, mainly due to the reduction of genetic variability [10; 27; 29; 31; 37; 38; 39]. In contrast to other authors who in the mid-1980s claimed that the impact of reduced genetic variability on dairy cattle was negligible; more recent work indicates that reduced genetic variability may have a highly negative impact on livestock productivity [40; 41]. In our study, we observed that the effect of inbreeding has a differentiated impact depending on the specific phenotypic/genotypic parameter studied. The fact that phenotypic parameters are differentially affected indicates the need to go further in the evaluation of the animals using genomic tools to determine the impact of inbreeding depression (ID) thus allowing a more accurate estimation of such impact [39]. It has been estimated that, on average, per every 1% of inbreeding, the mean of a production/reproductive parameter has a decrease of 0.137% [42]. However, there are no studies to find a correct threshold to define a significant value for production and reproductive parameters in high-producing primiparous cows. Therefore, the present research compared animals with different levels of inbreeding (LI: \(<\)2.5% and HI: \(\geq\)2.5 and \(\leq\)5.0) to determine the impact on genotypic/phenotypic, production/reproductive parameters in primiparous cows with lactations adjusted to 305 days. Regarding genomic parameters linked to production, the only observed parameters in this study showing significant differences when comparing inbreeding groups (i.e., LI and HI) were MP305G and MPPROT305G (milk production at 305 days and protein production at 305 days, respectively). In particular, we found a negative effect on milk and protein production adjusted to 305 days when comparing the LI with HI groups of high-producing primiparous cows. This is in agreement with [PERSON] et al. [29] and [PERSON] et al. [9] who showed that inbreeding harmed production performance in different breeds and crosses, particularly in multiparous Holstein cows at 305 days of lactation [39]. Furthermore, it has also been observed that a 1% increase in the inbreeding index is associated with a decrease in milk production yields of around 0.6% on average [28]. Consequently, according to our study and in accordance with previous work, an increase in the levels of genomic inbreeding would be associated with negative effects on milk production yield at 305 days. Similarly to our findings on production phenotypic parameters TFAT and TPROT (significantly affected by the degree of inbreeding), several research groups have found that the degree of inbreeding in purebred multiparous Holstein-Friesian cows and crosses negatively affects production traits, such as a decrease of 12.5% of milk production obtained by lactation and adjusted to 305 days, and in fat and protein percentages, accompanied by an increase in the SCC-log [16; 26; 27]. These results were corroborated in the present study, since we observed that in purebred Holstein-Friesian primiparous cows, although a decrease in milk production at 305 day (total Kg/lactation) was associated with a higher degree of inbreeding, it was neither significantly different between groups nor was the SCC-log. These observations contrast with the work of [PERSON] et al. [41] for milk production and [PERSON] et al. [31] for milk production and SCC%. However, the latter study was conducted on Ayrshire dairy cattle which may be differentially affected due to the breed factor.