**1. Introduction**

Since 1960 due to successful genetic selection of Holstein-Friesian cows for higher milk production, the average milk production in the United States has exceeded 11,000 kg/year. Parallel with this, the reproductive performance of dairy cows declined; however, only the conception rate was considered for the comparison to milk production [1–3]. Others could not confirm this antagonistic relationship between milk production and fertility [4–6]. LeBlanc [5] emphasizes that individual (age at first calving, parity, body condition, and diseases) and herd-level (herd size, nutrition, season, environment, herd/reproductive management, and skilled farm personnel) factors may substantially influence

the production of a dairy farm. At the same time, any shortage in individual or herd-level factors in a dairy farm may increase the average number of days open (calving to conception), the number of services per conception, and the number of cows culled for infertility [2]. It is important to emphasize that reproductive performance in heifers was not affected [7]. It is essential to improve our reproductive management practices to decrease the longer lactations and the number of cows culled for reproductive reasons [7].

Concentrated management activities, especially during the first 100 days following calving, are needed to achieve an optimal calving interval (less than 400 days) with higher milk production per lactation and the birth of more calves [8]. Correct reproductive management can significantly contribute to reducing production costs.

The following diagnostic activities should be pursued during the early postpartum period to achieve or approach the optimal calving interval: prediction of the onset of calving, early diagnosis of postparturient uterine and metabolic diseases, accurate detection of estrus, determining the optimal time for artificial insemination (AI), and accurate diagnosis of early pregnancy and pregnancy losses.
