Learn French: Les vaches mangent de l'herbe.

In French, you almost never leave a noun without an article. When talking about a general category or all members of a group like cows, you must use the definite article, which is les for plural nouns.

Why is it de l'herbe instead of just herbe or l'herbe?

Because grass is an uncountable mass here. When you talk about an unspecified amount of an uncountable noun, meaning some grass, you use a partitive article. You cannot use the definite article l' unless you mean specific grass or grass in general as a concept.

Doesn't herbe start with a consonant? Why does it shorten to l'herbe instead of la herbe?

The letter H in herbe is silent. In French, words starting with this type of silent H act exactly as if they start with a vowel, which is why de la herbe must contract to de l'herbe.

How do I pronounce the -ent at the end of mangent?

You do not pronounce it at all. The -ent ending on regular third-person plural verbs is completely silent. You pronounce mangent exactly the same way you pronounce the singular form mange.