Younger animals are more likely to try new things than older ones and females are a good choice because they can teach their young what to eat. I suggest starting with year to a year and a half old heifers. Working with cow calf pairs is also a good choice. I've found that even when the cows don't eat all of a food, calves may think they have, and they pick up the new food very quickly.

Your animals' nutritional state is key to this process. You may be able to starve an animal into eating something, but the result is reduced weight gain, reduced profits, and an animal who is less likely to try other new foods. So never starve your animals! Cows I have trained have always gained weight at expected rates and suffered no harmful effects from eating weeds.

This is enough black mustard to feed 50 heifers.

Think about how many weeds you want to spend time picking when you decide how many animals to train.  I work with only as many animals as I have time to feed and harvest weeds for. 

Besides, it's more efficient to let trained animals teach their herd mates for you. Research shows that the trained cows will teach the untrained ones! I had first hand experience with this in 2007 when 12 trained cows trained 120 cows and calves how to eat Italian thistle in pasture. Think of that! It's ten cows trained for every one cow I worked with!

The process goes faster if the cows you are teaching have known you for a while and are accustomed to being fed by you.