Establishment of fetal age equations based on ultrasound measurements in cross-bred Holstein cows

Objective: To establish fetal age equations based on ultrasound measurements in cross-bred Holstein cows in Thailand. Methods: The animals were bred for 30-120 d before pregnancy diagnosis using transrectal ultrasonography with real-time B-mode and specify 5 or 7.5 MHz linear array transducer. Parameters examined included crown rump length (CRL), trunk diameter (TrD) and eye diameter (ED). Results: One hundred and two cows had been examined, 80 of them were pregnant and contained at least one measurable characteristic as defined in the methods. Regression analysis and curve estimation were implied. The most frequently visualized parameters were TrD (42/80; 52.50%) and CRL (41/80; 51.25%) whereas ED (13/80; 16.25%) was least frequently detected. In our study, the equations to estimate fetal age (indicated as Y) from CRL and TrD were established with very high correlation coefficients as follows, Y = 22.679+12.005 (CRL)-1.042 (CRL)², R² = 0.950, P<0.001 and Y = 14.583+29.878 (TrD)-3.759 (TrD)²-0.225 (TrD)³, R² = 0.950, P<0.001. The equation from ED was Y=107.582-98.928 (ED)+61.116 (ED)²-9.221 (ED)³ but with very low correlation coefficient (R² = 0.673, P=0.021). Comparing between our equations (CRL and TrD) and the ones embedded in the commercially available ultrasounds, the higher fetal age was estimated based on the same value of parameters (P<0.05). In conclusion, the most reliable equations created in our study were from CRL and TrD. Conclusions: Comparing with others, Thai cross-bred fetus clearly showed slower growth rate and thus, to use our CRL's and TrD's equations for estimating fetal age seems more reliable than to use the ones embedded in the commercial ultrasounds.