Dairying is a vital enterprise and important source of income generation for rural families of many developing countries of the world. Although, Holstein is imported by many tropical countries with the idea to improve the milk production but their performance in these environmental conditions is considerably low compared to the temperate environments. The expression of the inherit genetics worth differs from one environment to another and is significantly swayed by non-genetic factors. Environment and genetic (G x E) interaction plays key role in the expression of full genetic worth of an animal. The range of comfortable temperature in lactating Holstein cattle is between 4-24oC. Above 24oC, heat stress effects can be observed on the cattle, and milk yield markedly decreases above 27oC. In most of the tropical and subtropical countries the ambient temperature rises above 40°C, which is obviously suboptimal for Holstein to maintain their health and production performance. The present study was designed to investigate the effect of various environmental and genetic factors on various productive and reproductive traits of this breed under sub-tropical conditions.