Accuracy of Manual Rectal Palpation
Performed by a skilled operator, manual rectal palpation is close to 100 % accurate for diagnosing cows pregnant or empty. The accuracy for ageing the pregnancy is dependant on the stage of pregnancy.
The most accurate results are obtained with pregnancies between 6 and 12 weeks in calf, and even up to 16 weeks. Better than 90 % of the estimated stage of pregnancies will be within 2 weeks of the actual stages. Greater accuracy can be achieved if good breeding records are available at the time of preg testing, allowing the stage of pregnancy to be matched to the day of conception.
If cows are preg tested at 4 to 6 months of pregnancy, it is not always possible to differentiate between stages of pregnancy 3 weeks and sometimes even up to 6 weeks apart.
At more than 6 months of pregnancy, errors of 4 to 6 weeks will be made in up to 25% cases, making it difficult to confidently dry cows off at late stages, according to these results.
If you are wanting to dry cows off every 1 to 2 weeks, maximising the milk yield from each cow, you need to be confident of each individual cow's stage of pregnancy.
To minimise errors cows should be preg tested when they are less than 12 weeks pregnant.