Let's take a quick review of ruminant physiology.
Based on how many times I've heard someone say they are feeding supplement to "stretch the grass," I think there are some misconceptions about what is happening when you supplement protein.
This is where the ruminant physiology comes in. How much a cow can eat is determined by two factors; how much her rumen can hold (rumen capacity,) and how quickly the rumen is emptied so she has room to eat more (rumen passage rate.)
Passage rate is affected by diet quality. The lower the quality of the diet, the slower the passage rate. That's one of the unfortunate things about feeding cows. They can eat more high quality feed than they can eat low quality feed, even though it takes less of it to meet their requirements. Now you're thinking – "I'm not sure I needed a biology lesson, what's your point?" My point is that when you increase diet quality by supplementing, you increase passage rate and thereby increase the amount she can eat.
Supplementing low quality forage with protein increases animal performance, but it does not "stretch" the grass. It actually allows them to eat more of it.
The same is true for hay, not just for grass. Providing adequate protein will increase the amount of low-quality forage a cow can eat. That is good for two reasons. It allows us to use lower quality feeds than we would be able to use otherwise.
In years of cheaper hay, that is probably the most important benefit. But in years when hay is expensive there is one other benefit that comes into play.
We can often "limit-feed," that is to feed a cow less than she would be able to consume and still meet her nutrient requirements. This usually results in less feeding waste if hay is fed on the ground.
Protein supplement only stretches feed when intake is limited. Protein supplementation increases passage rate, which means that cows can eat more.
Not only can the cows eat more, but they definitely will eat more if allowed to eat all they want.
Jay Jenkins, Nebraska Extension Educator
University of Nebraska–Lincoln