In the present study, the conception rate in cows with subclinical mastitis in the second and third lactations was lower than in cows without subclinical mastitis. Meanwhile, there was no significant difference between cows with and without subclinical mastitis in the first and fourth and higher lactation. The results of this study also indicated an increase of subclinical mastitis following the increase of lactation number. Consistent with our findings, Jarassaeng et al. (2012) showed that increase of lactation number causes increase of incidence of subclinical mastitis. These may be due to weakened immune system with aging and a higher exposure to pathogens. Moreover, conception rate in the first and fourth lactations was lower than the second and third lactations (Ansari-Lari et al. 2010). Therefore, the effect of subclinical mastitis in these lactation numbers may be defective. Sensitivity to mastitis with increase milk production has increased (Shanks et al. 1978). Consequently, control of subclinical mastitis in the second and third lactations is more important, but that control must also be considered in the other lactation numbers.
In the present study, number of insemination and rate of non-pregnant cows were higher in cows with subclinical mastitis than cows without subclinical mastitis. Consistent with our findings, it has been shown that subclinical mastitis had effects on different reproductive indices in dairy cows (Schrick et al. 2001). Cows with subclinical mastitis before the first service had increased days to the first service, days open, and services per conception compared with controls (Schrick et al. 2001). It has been demonstrated that high SCC resulted in longer intervals from calving to the first breeding and to conception (Pinedo et al. 2009), as well as in lower the first artificial insemination, conception rate and total pregnancy rate (Santos et al. 2004). Contradictory results regarding the effect of high SCC on reproductive performance have been also reported (Nguyen et al. 2011a). High SCC and lameness reduced the likelihood of ovulation, but the same was not true for animals with a high SCC and without lameness (Morris et al. 2009). It is presumed that high SCC or subclinical mastitis inhibited the release of gonadotropic hormones which delayed resumption of ovarian cyclicity postpartum. So that, Lavon et al. (2010) reported that 30% of cows with subclinical mastitis exhibited delayed ovulation. Also, Nguyen et al. (2011b) demonstrated that cows with a high SCC have a higher incidence of abnormal postpartum resumption of ovarian cyclicity, included delayed first ovulation and prolonged luteal phases. Invasion of the mammary gland lead to the release of cytokines including interleukins (IL-1β, IL-6, IL-10, IL-12, and IL-1α), interferon-β, tumour necrosis factor-α (TNF-α) (Hansen et al. 2004). It has been demonstrated that certain cytokines such as IFN-β decrease the secretion of luteinizing hormone (LH) (McCann et al. 2000). IL-6 blocks the secretion of estradiol (Alpizar and Spicer 1994) which can lead to reduced LH secretion, which is responsible for ovulation. Cytokines can also cause increased secretion of other molecules such as prostaglandin F2α (PGF2α) (Hoeben et al. 2000) and nitric oxide (NO) (Athanassakis et al. 2000). PGF2α may reduce fertility by interruption to oocytes maturation and pre-mature luteolysis (Hansen et al. 2004; Soto et al. 2003). Elevated concentrations of NO have also been associated with embryonic death (Soto et al. 2003). Also, mastitis results in increased blood concentrations of cortisol, a hormone that blocks the release and the peak of LH (Hockett et al. 2000). Also, the direct association between subclinical mastitis and endometritis has been suggested by Bacha and Regassa (2010). Endometritis increased service per conception and decreased pregnancy rate. Therefore, subclinical mastitis may cause increased service per conception and decreased pregnancy rate in dairy cows by interruption in normal function of hypothalamus, pituitary, ovaries, and uterus.
It has been reported that the incidence of mastitis can increase in spring and summer (Sabuncu et al. 2013), the others reported that incidence of mastitis can increase in winter and autumn (Batra et al. 1977). Also, the effect of parity on SCC (Nguyen et al. 2011b; Sabuncu et al. 2013) and subclinical mastitis (Jarassaeng et al. 2012) has been shown. In this study, we show that the pregnancy rate in cows with subclinical mastitis in summer in the second lactation and in winter in the second and third lactations was lower than in cows without subclinical mastitis; while, there was no significant difference in the other seasons. Heat stress in summer and high humidity combined cold stress in winter could be responsible for reducing the pregnancy rate (Li et al. 2021). In the mid lactation, only in the spring, high SCC was leading to high service per conception. In winter and spring, high SCC decreased pregnancy rates in the first insemination. In summer, high SCC decreased pregnancy rates in the second insemination. So it appears that the subclinical mastitis reduced conception rate during winter and spring in the mid lactation. In this study, increasing of subclinical mastitis in winter and spring and milk production in mid-lactation could be responsible for the reduced pregnancy rate. In addition, most of inseminations were done in the mid lactation stage and it is also expected time for setting days open in dairy farms. Moosavi et al. (2014) reported that in spring and winter, occurrence of clinical mastitis in primiparous cows increased in early lactation stage and in multiparous cows increased in late lactation stage. Occurrence time of clinical mastitis was not different between three different parities (1, 2 and ≥3) in mid lactation stage (Moosavi et al. 2014). They concluded that the most important stage of lactation which is in high risk for mastitis occurrence depends on the parity of cows. The early lactation is critical stage for primiparous cows in winter and spring while it was important for the third parity in winter and autumn. The late lactation is critical stage for primiparous cows in summer and autumn while it was important for the second-parity in autumn and winter but in spring for the third parity (Moosavi et al. 2014). It seems that occurrence of clinical and subclinical mastitis can be happened in mid lactation especially in the second and third parities. As a result, control of subclinical mastitis is more important in the mid lactation stage. It is also important to control the rate of subclinical mastitis in the second and third parities especially in summer and winter. However, control of subclinical mastitis in the other seasons and lactation stages and periods also must be considered.