Extensive development of dairy farming in the dairy belt of North America has been possible due to the presence of a combination of physical and economic factors. The region receives a rainfall varying between 55 and 125 cm. The rainfall efficiency is further increased by relatively lower temperature ranging between 18° and 24°C. The cattle yield more milk in a cool climate and the milk can also be kept for a longer time at lower temperature.
Ultrasound refers to a sound wave with a frequency greater than the upper limit of human hearing, which is generally over 20 kHz. In general, humans can hear sounds with a frequency between 20 Hz and 20 kHz.
EXAMIANS