Why is "30 head" singular instead of plural in this sentence? Can you explain more about it?
Dave Phillips says: In BrE we would always say a head of cattle (meaning the whole herd) but we wouldn't give a number of those head/heads.
While I would concede that the usage "X head" is not unknown here, the use of 'head' as a synonym of the collective noun 'herd' is probably more common in the UK.
Because 'cattle' is a plural noun with no singular (though it almost grades into a mass noun notionally in constructions such as 'raising cattle costs a lot of money' – cf clothing, furniture, and especially poultry – which, however, take singular concord), there is the problem of what to call a single – er, beast. Neat. The very familiar cow doesn't always work. Perhaps because of the notional massness of cattle, we don't say 'three cattle' etc either – it doesn't accept all normal quantifiers.
This is probably one reason why the term 'head of cattle' has been drafted into use. 'x head of' is then a compound quantifier usable with 'cattle'; the variant 'a single head of cattle' is also available.
Though this is a strange construction even for a quantifier, other more familiar quantifiers are known to take zero plurals (30 dozen ...).
Why don't you check your dictionary.
For example, the Merriam-Webster dictionary says the plural form for this definition is "head".
head noun meaning #4b http://www.merriam-webster.com/dictionary/head