NEW These blue Estrus detectors are being sold in a package of 50. They measure 4 1/8" long by 2" wide. They are made of a flexible adhesive plastic sheet. When is the best time to breed your cow? The Estrus detector gives a clear indication that she is in heat. Each time the cow is mounted a portion of the Estrus patch is rubbed off exposing the brilliant blue undercoat. The Estrus detector can save you money and time breeding your cows. The Estrus detector is self adhesive. Clear and easy instructions are found on the back of the box. The Estrus Alert manufactuer number is 86134.