Subclinical hypocalcemia causes significant adverse conditions in dairy cows that affect productivity, welfare, and physiology. It plays direct or indirect roles in the pathogenesis of many diseases such as ketosis, abomasal displacement, mastitis, metritis, retained placenta, uterine subinvolution, fatty liver, and suppression of clinically critical immune system functions in the bovine (Rodriguez et al., 2017). Calcium is essential for the normal functioning of both skeletal and smooth muscle cells. Reportedly, normocalcemia is theoretically necessary for the contractility of the teat canal sphincter, which is one of the primary defense mechanisms of the udder during lactation and dry periods in dairy cows (Kimura et al., 2006; Goff, 2008; Libera et al., 2021). Some studies have reported that calcium level does not affect milk leakage in dry-off dairy cows (Mahjoubi et al., 2018). However, to our knowledge, no study has investigated the effect of blood calcium levels on teat canal closure in lactating cows. Therefore, this study is the first to examine the impact of blood calcium status on teat closure in dairy cows post-milking.
Information about the post-milking teat canal closure is controversial. While some researchers (McDonald, 1975; Blowey & Edmondson, 2010; Löfstrand, 2018) claimed that two hours was sufficient for the teat canal closure post-milking, others claimed that the teat canal could not completely close at the 3rd-hour (Stadnik et al., 2010; Stojnovic and Alagic, 2012) and not even at the 8th-hour post-milking (Neijenhuis et al., 2001; Wieland et al., 2019). As a barrier, the teat canal plays a crucial role in preventing intramammary infections. Previous studies have shown that inoculation of pathogens directly into the teat canal significantly increases the likelihood of developing an intramammary infection compared to the inoculation into the sinus papillary (Notcovich, 2021). However, even if the teat canal, a mechanical barrier, is not fully closed, the hydrophobic layer formed by keratin and cationic proteins in the teat canal prevents milk flow in the first 30 minutes post-milking. Therefore, the current study investigated the effect of blood calcium status on teat canal closure in the first 30 minutes post-milking.
The study findings showed that pre-milking teat canal measurements (TCL, TCW, and TCV) were not different between NC and SCH cows (P > 0.05). Previous studies (Klein et al., 2005; Çelik et al., 2008; Paulrud and Rasmussen, 2004) reported that the length of the teat canal was around 1 cm (0.3 to 1.8 cm), although it varied according to race, parity, and lactation period. Similar to previous studies, the current study found that pre-milking TCL was almost identical in both NC and SCH cows (0.85 and 0.88, P > 0.05). TCL was similar in NC and SCH cows at the 15th minute after milking, and an increase was observed in both groups at the same rate. Moreover, there was no significant relationship between ΔTCL15-0 and blood calcium level. At the 30th minute post-milking, TCL in NC cows was significantly close to the pre-milking state (P < 0.01) compared to SCH cows, and there was a significant correlational relationship between ΔTCL30-15 and blood calcium level. This situation suggests the vacuum effect created by the milking system at the 15th minute may suppress the sphincter muscle rhythmic contractions surrounding the teat canal (Paulrud, 2005; Witzel, 1965), or the inability of muscles to perform their normal functions may be caused by acute congestion and edema in the region (Hamann et al., 1994b; Wieland et al., 2020). Therefore, it can be concluded that due to the adverse effects of the milking system, the blood calcium level could not affect the TCL at the 15th minute after milking. However, after the milking system's unfavorable impact gradually disappears at the 30th-minute post-milking, the TCL returns to its pre-milking state more rapidly in NC cows. A recent study (Wieland et al., 2020) investigated blood perfusion of the teat pre- and post-milking and revealed time-dependent different blood flow patterns. In this respect, it can be concluded that machine milking delays teat canal closure as it reduces the blood flow to the teat canal and thus the calcium required for contraction of the teat canal muscles.
The current study found that the mean pre-milking TCW in NC cows was 0.2 cm. While Giesecke et al. (1972) stated that TCW was 0.5 cm on average, Melvin et al. (2019) reported that TCW varied between 0.86 mm and 3.45 mm, with an average of 1.94 mm. Teat canal width, an important parameter that reflects the milk flow rate (Paulrud, 2005), is also a contributing factor in the susceptibility of cows to mastitis (Lacy-Hulbert and Hillerton, 1995). Reportedly, there is a strong positive correlation between TCW and somatic cell numbers (Jorstad et al., 1989). Although the data obtained from studies conducted with different measurement techniques differ, they were almost the same (Giesecke et al., 1972; Hamann, 1987; Neijenhuis et al., 2001; Çelik et al., 2008). However, different cow genotypes might be a reason for these differences. Similar to our TCL findings, no difference occurred in TCW between NC and SCH cows at the pre-milking and the 15th-minute post-milking. The TCW recovery was higher in NC cows than in SCH cows. However, the recovery rate of TCW was less than that of TCL. At the 30th-minute post milking, NC cows' TCW recovery was statistically less insignificant than their TCL recovery, despite their higher pre-milking period return rate than SCH cows. This finding may be related to the elasticity of the muscle fibers surrounding the teat canal because the vertical part of the teat can directly adapt to the mechanical forces caused by machine milking than the longitudinal part (Besier et al., 2016; Löftstrand, 2018).
The current study found no difference in TCV at the 15th-minute post-milking in NC and SCH cows. There was a significant recovery in TCV at 30th-minute post-milking in NC cows compared to SCH cows. For the first time in the literature, the authors of this paper suggest that the volume of the cylindrical teat, depending on the length and width of the teat canal, will give a more reliable result in evaluating teat canal closure. Since no similar explaining model has been found in the literature, the researchers are the first to suggest this model. Net-like muscle fibers surrounding the teat canal (Van Der Merwe, 1985) and the machine milking-related interstitial edema at the teat canal tip rather than the upper Furstenberg rosette regions (Besier et al., 2016; Neijenhuis et al., 2001) imply that using TCV might be more advantageous in evaluating teat canal closure.
The current study has, for the first time, revealed that blood calcium status significantly affects post-milking teat canal closure in machine milking. In the study, it was observed that blood calcium did not affect the teat canal closure at the first 15th-minute post-milking, possibly due to the destructive effect of machine milking. However, at the 30th-minute post-milking, it played a significant role while the teat canal regained its pre-milking state. Within the scope of mastitis control program, in addition to strategic applications including teat dipping, teat film formation, and post-milking feeding. the calcium status of the lactating herd should be periodically scanned and a good feeding management should be implemented.