It is not good to burn animal dung directly as a fuel because

(i) Animal dung contains important elements like nitrogen and phosphorus which are required by the soil to support crops.

So, burning of dung in the form of dung cakes, destroys the useful nutrients which can otherwise be use as a manure in agriculture.

(ii) They produce a lot of smoke on burning which causes air pollution.

(iii) They do not burn completely, they produce a lot of ash as residue.

(iv) They have a low calorific value.