If the cattle are not yours I dont think it is right to offer unsolicited advice.  I see enough criticism of neighbors in the tillage system business. Older folks telling younger folks they need to moldboard plow or they are going to "go broke".

Unless there is an animal welfare issue, I would just be a friendly neighbor and wait for them to ask you for advice before offerring any.  I know your intentions are good but preaching  to them is not going to work. They need to ask you for your opinion and experience. I have a very good neighbor I often call or go to see for cattle advice. 

If they are your cattle then I think you have a clear right to say how you want them fed as you are paying for their services.   jmho.

Jim