Although there is no clear evidence to fully describe the mechanism involved in glucosinolate-related effects on animal reproduction, lowered fertility in animals fed diets with rapeseed meal (RSM) inclusion is related to glucosinolate (Gls) content in the diet (Mawson et al., 1994). The degree of reproduction impairment depends both on glucosinolate content and on the type of animal. Negative effects of feeding high Gls RSM on fertility and poor reproduction traits in cows fed high amounts of very low-glucosinolate rapeseed meal were reported (Tripathi and Mishra, 2007). Therefore, cows fed diet not only high in Gls RSM but also high in low-glucosinolate RSM are sensitive to dietary glucosinolates. Diets containing Gls may cause thyroid disturbances that depressed fertility (Ahlin et al., 1994), such as increased calving to conception period, more numbers of inseminations per pregnancy and more numbers of acyclic and cystic ovaries (Tripathi and Mishra, 2007). Canola is a trademarked quality description of a group of cultivars of rapeseed variants from which low erucic acid rapeseed oil and low glucosinolate meal are obtained. In the current animal feed market in Iran, canola meal is cheaper than soybean meal (320 vs. 482 Toman/kg; US $1=933 Toman). The objective of this experiment was to evaluate substitution of soybean meal with canola meal and measure its effects on follicular dynamic and days postpartum to first ovulation in early lactation Iranian Holstein cows.