去掉后就变成: In recent years cattle breeders have increasingly used crossbreeding partly to acquire certain characteristics in their steers and partly because crossbreeding is said to provide hybrid vigor. 原本是两个表示原因的成分在and两边,是两个并列的原因。 如果去掉comma, 就变成and后面的以because引导的原因状语从句前面的主句并列——显然是错误的。