why don't my cows eat the lush grass under the oak trees?

At our property in Sonoma County, CA we are in the first season of a serious rotational grazing plan. Since getting back on grass in late January we have been moving our small herd every day. Herd is composed of some black Angus and Highland crosses.

One thing we have noticed in our oak savannah ecosystem: the cows avoid the lush green grass that grows directly under the Coastal Live Oaks. We expected them to go nuts for it but they really don't touch it.

I'm not positive of the grass species involved, I think it's mostly the non-native wild rye that dominates the hillsides here. But the species under the trees is not different. Just greener. To my human palate it tastes the same as the other grass!

My best guess is that the oak leaves change the acidity of the grass? Maybe the leaf mould underlying the grass? It seems like a shame that they won't eat it. We've noticed this phenomenon also on the neighbors property. Any ideas about the origin and what we can do to work with nature here? We're thinking about planting some kind of shade-loving berry or other perennial directly under the oaks if the grass isn't going to get used.

1- Grass species vary in palatablity. Some are preferred when lush, some not.
2- Do the cows dung under the trees? Cows tend to rest in the shade under trees, thus manuring the area. My own animals prefer to avoid eating the grasses adjacent to their dung.
3- Check the pH of the grass. If that species is normally acidic when growing in the sun, but becomes alkaline when growing in the shade, that could be the problem. Where I am I have a couple of grasses that do that, thus the sheep will only eat the shaded grass during a drought period when the pH changes.

There could be other circumstances involved, but these are the ones that I have to deal with.

As Su mentioned, it might be less preferred but stockpiling out until a drought.  Which can be a good thing.

But, maybe you aren't stocking them dense enough.  Part of the system is to cause a panic eating so the graze and trample everything.  Eat a third, trample a third, leave a third.

chris pelfron wrote:
I'm not positive of the grass species involved, I think it's mostly the non-native wild rye that dominates the hillsides here. But the species under the trees is not different. Just greener. To my human palate it tastes the same as the other grass!

I'm still learning my grass identification.  I find the rye grass and fescue to be similar enough to be confusing, although the fescue is always a shade darker/more saturated.  Fescue blades have a saw-tooth like edge while the Rye does not. Fescue also likes shade. Cows dislike fescue and will only eat it under duress - which in rotational grazing terms is something like 50,000 lbs of cattle per acre? (please don't quote me on that number ... the point is its a LOT of meat per acre.  My ickle herd maybe weighs in a 5000 lbs, but they're just not enough of them to induce a "mob" even on a postage stamp).

So it might be tall fescue?

Thanks folks. I will knuckle down with the grass identification and really try to figure out if that's a different species under the trees.

This was grazing at about 20,000 lbs per acre.

Do let us know what you learn even if there is no definitive conclusion!

The other thing I notice in your photo is that there are some clumps of darker grass in the foreground - it looks like the cows are skipping those as well.  I have similar clumps of fescue in the middle of some pastures - I have to mow them down after rotation to ensure they don't get imperialistic.

it is possible that shaded grass is less palatable due to lack of vitamin D2 and other nutrients which only emerge within the leaf cells as a result the plants of being exposed to direct sun ,however in the case of random super green looking clumps it is a result of the site having been peed or pooped on in the not so distant past (extra nitrogen)  

Some types of fescue have endophytic fungus which is bad for cows.  I don't know if it has anything to do with the oak trees other than the fact that fescue tends to like cooler weather, and perhaps it thrives best under the shade of the trees, whereas out in the full sun, other grass types do better.

https://www.beefmagazine.com/americancowman/pasture-and-range/tall_fescue

Not all cattle are effected by endophytic fungus, but those who are will avoid it like the plague.

Here's another article on grazing fescue:

https://www.beefmagazine.com/mag/beef_fescue_toxicity

The sugar levels in grass respond rapidly to exposure to direct sunlight. The grass in the shade is literally less sweet.

Ok by now more of the uneaten grasses have seeded out and that has given me more to work with.

Current theory is that the stuff the cows don't like is "bromus diandrus roth" AKA "ripgut brome" which when young can look very much like a cereal (per wikipedia) and grows well under "heavy thatch and oak canopies" (per the Field Guide for Common California Rangeland and Pasture Plants). Possibly a different brome.

So the cows are very happy with young wild oats and some kind of ryegrass growing in the sun near the brome. The young brome looks like young cereal, grows well in the shade, but it's different and they don't like it. Now that it's seeded out it's bordering on dangerous but they leave it alone anyway.

I have attached some samples my kids and I collected today. My theory is that sample 493 grows up into sample 999 but before that seedhead showed it looked like a wild oat to my undiscerning eye.

The main flaw in this theory is that the Rangeland guide says that young bromus diandrus is palatable to cows. So maybe I've got that identification wrong. But either way it's probably just a different species under the trees.