First of all, we need to define biology so as to understand what a biological field may entail. Biology can be defined (loosely) as the study of living things; plants and animals. Hence, a biological field is a field that involves studying plants or animals or even both.
Agricultural science is regarded as a biological field because it <u>involves the studying of cultivating of crops (plants) and rearing of animals for use (in the form of food and fibre)</u> by man. From this, it can be deduced that agricultural science also deals with a study aspect of plants and animals.