请教一个语法问题

aaronlaurel

og12上的一题，a study commissioned by the department of agriculture showes that if calves exercise and associat with other calves, they require less medication and gain weight more quicky than those raised in confinement. 这句话than those raised in confinement这儿之前为什么不用加do，我知道是主语间的比较，为什么不用补出动词do呢？呢？og上说是the comparision is between calves and those, the verb do ungrammatically interrupts the comparision and should be eliminated.    
搞不明白了，我记得主语间的比较不是都要补出谓语动词的吗，望大侠赐教~

stucash

曾经看过以前老帖子里NN和前辈说过, OG里面比较要看结构,
S+V和S+V比较就不用补出谓语动词了;
S+V+O和S+v+O则需要补出, 因为怕有歧义, 比较对象不明确, 害怕是S1和O2比较.

有什么不对的..请NN指正....