According to farmers, the tails of sheep are docked to keep them healthy.

Answer

Remember and understand that it is the obligation of the Muslim to submit to Allah's Law, the Shariah. It is not permissible to seek cancellation of the Ahkaam of the Shariah with our logic and reasoning. Regardless of tailless animals being healthier, the Shariah regards the docked sheep as defective for purposes of qur'baani.

In view of the command of the Shariah, reason has to be put aside, and submission to the Shariah is Waajib. It is not permissible to make qur'baani of sheep whose tails have been lopped off. If valid sheep are not available, goats and cattle are available. All of us make qur'baani every year. We have always found animals, even sheep, with tails available.