The META: Keywords tag is the most commonly misused META tag when it comes to SEO. The temptation to cram it full of as many keywords as you can think of, and repeat the most important ones a few times for good measure, is strong. It's even stronger when you remember that the META tags are one of the first things, after your TITLE tag, that search engines look at. It only makes sense to make the most of it.
But too many keywords can be just as harmful as too few. Less is more when it comes to the META tag. So how do you choose which keywords to keep? How many keywords should you include?
Ideally, your keywords tag should have ten or fifteen words, twenty at the very most. You may repeat your three most important keywords once, if you like, but repeat them more than that and the search engine is likely to ignore them, or, worse, not list your page at all.
Use one of the free tools littering the web to figure out which keywords are more popular than others; for example, are more people likely to look for "cow" or "bovine"? Or, better still, are more people likely to look for "cow" or "heifer"? Knowing which keywords to use cuts down drastically on the number of keywords in your META tag. You don't have to list "cow", "bovine", "heifer", "bull", etc. Stick with the most common search terms, and you'll get more traffic from one word than you got before from three.
Also, remember that keywords should be page-specific, not site-specific. Don't ever put the same keywords on each page of your website. If you have one page about breeding cows and one page about milking them, keep keywords related to milking cows off the page related to breeding cows. If someone is looking for information about breeding cows, they'll find your page on breeding cows. They don't need to find your page on milking cows, unless, of course, they want information about milking cows—in which case, why would they be looking for information on breeding them?
Limiting the scope of your keywords like that helps you to narrow down your choices, and makes it even easier to pick just the ten or fifteen most important keywords. (Ten, by the way, is better than fifteen, but you don't really want to drop below that.)
Using your META tags properly means the difference between good SEO and lousy SEO. Resist the temptation to cram them full of keywords. Choose keywords that are both relevant to your site, and frequently searched. The right keywords make all the difference. And never forget: often in SEO, less really is more.