The relationship of rheological behavior of the vaginal fluid at the time of insemination to the pregnancy rate in dairy cows

The objective of this study was to determine whether rheological properties of the vaginal fluid at the time of insemination could be used as an indicator of potential fertility in dairy cows. Thixotropy and flow behavior were evaluated. Vaginal samples were collected from 347 cows at the time of insemination, within a 12- to 24-h interval after the onset of a natural estrus. Logistic regression analysis showed no significant effects of thixotropy measurements on the pregnancy rate, while flow behavior and the index of consistency (K) had a significant effect. The decrease in the consistency index (K) was related to the increase in the pregnancy rate (odds ratio=0.99; P=0.014), and the risk of nonpregnancy was higher (odds ratio=0.46; P=0.24) in cows in which vaginal fluid had a Newtonian behavior. Non-Newtonian behavior was recorded in 294 samples (85%) while 53 behaved as Newtonian fluids. For samples from all cows, the mean value of the consistency index was 310 ± 28.2 milliPascals.second(n) (x ± SEM) and ranged from 0 to 3881 milliPascals.second(n). In Newtonian samples, the consistency index mean was 2.6 ± 0.06 milliPascals.second(n) and 367.6 ± 30.4 milliPascals.second(n) for non-Newtonian samples. Of the 347 cows, 131 (38%) became pregnant. For flow behavior classification, 15 (28.3%) cows in which samples had Newtonian behavior and 116 (39.5%) cows with non-Newtonian samples became pregnant. Therefore, the highest probability of pregnancy was shown by cows in which samples had non-Newtonian behavior and the lowest consistency index values. Our data suggest that flow behavior measurement of the vaginal fluid at the time of insemination may be an indicator of potential fertility in cattle.