Animal energy is a renewable and sustainable source of energy. It is renewable because the animals can be reproduced by breeding and rearing the required number of animals. It is sustainable because the animals derive their energy for work largely from agricultural by-products. In addition, there are other environmental contributions of the working animal stock — consider replacing it by agricultural machinery run on fossil-fuel. Animal energy saves natural resources, fossil fuel and prevents green house gases emission. The fossil-fuel equivalent of the animal energy used in the Indian agriculture has been found pretty large, as much as 19 million tonnes of diesel in 2003. If this much amount of fuel were to be burnt through combustion to run tractors in the absence of the working animal stock of over 60 million, it would have released about 6 million tonnes of carbon dioxide.