A trait exhibited by a cow herd or individuals within a herd that saves time and money is referred to as a "convenience trait." Examples are polledness, parasite resistance, heat tolerance and calving ease. Docility is another good example. Think about all the time and money flighty, aggressive animals cost us at gathering, working, sorting, calving, etc. You know the ones - they keep things stirred up in the pen; they are inclined to jump out or tear out, hurt themselves, other animals or you! There are at least three factors that can contribute to a cow's bad attitude.
First, she learns flighty or aggressive behavior from her dam and other animals in the herd. How often have you heard, "Yeah, and her mother was just as crazy as she is!" In turn, she will teach poor behavior to her calves and other animals in the herd. Poor disposition is learned and taught, and passed down from generation to generation.
Second, it's in her genes. Disposition has moderately high heritability. Roughly 40 percent of an animal's craziness is explained by its genetics. This heritability estimate means that one can make fairly rapid progress toward a more docile herd through selection and culling. When choosing females and bulls, docility should be equal to any other criteria you may use.
Third, there is the human factor. In their guidelines, the Beef Improvement Federation (BIF) has suggested a six point scoring system to describe an animal's disposition with number one being docile and number six being very aggressive. Further, BIF suggests scoring animals at weaning or as yearlings to minimize the effects of prior handling experiences. That's because an animal's disposition can be strongly influenced by its human handlers, including any companion animals that may be used to help gather or work the cattle. A wise cowman once told me, "Gentle handling makes for gentle cattle. Aggressive handling begets crazy, wild-eyed, high-headed wenches." The impact of poor handling is significant and can create disposition problems where there otherwise would have been none.
To read the entire article, link to Clay Wright's Simmer Down Your Cowherd at the Samuel Roberts Noble Foundation.