My goal in this new bimonthly column is to answer specific questions about grazing management. Readers are invited to submit their questions. I may not able to address every question, but I'll try to pick those with broad application and timeliness for the season. Let's get started:
Can dormant pasture be grazed clear to the ground in the winter without hurting the plant stand or next spring's productivity?
Before we can answer the question, we need to know something about how plants survive winter. Some producers seem to believe grass is dead over winter. Summer annual grasses might be dead, but our perennial pasture and range grasses are dormant, not dead.
All living things, plant or animal, require energy for respiration to supply basic maintenance functions. Winter dormant pasture plants survive on energy stored during the growing season.
Most producers know about alfalfa hay management and the importance of fall rest to increase carbohydrate (CHO) reserves in alfalfa's big taproot. We can cut alfalfa to the top of the crown without hurting its regrowth potential because all the stored energy is below ground level.
Because Extension and industry agronomists have done such a good job of teaching the alfalfa model, producers assume all plants store their energy reserves in the root. This is why so many producers think pasture grasses and other legumes can be grazed down to ground level in winter without hurting them.
Unfortunately, the assumption that all CHO reserves are underground is wrong. Alfalfa is actually one of only a few pasture plants with the bulk of their reserves underground. Almost all bunch grasses store CHO in the stem bases, and the reserve zone may extend several inches above ground.
The classic examples of this storage mode are the tall grass prairie species. Big bluestem and indiangrass have CHO reserves as high as 4-6 in. in the stems above ground. If these species are grazed down to ground level in winter, they lose their entire maintenance ration for the winter and can perish. If they do survive, spring growth will be significantly reduced. When grazing native prairie in the winter, plan to leave at least 4 in. of residual.
Most cool-season bunch grasses also have most of their storage above ground, but much lower than tall warm-season grasses. Typically we can graze dormant cool-season grasses much lower than the tall grasses.
Grasses with greater tiller density can be grazed closer than grasses with a limited number of tillers because there are more stem bases packed together closer to the ground. For example, tall fescue or perennial ryegrass can be grazed shorter in the winter than orchardgrass or timothy.
In the eastern half of the U.S., stockpiling tall fescue for winter grazing is an increasingly popular practice. Tall fescue can be grazed shorter in the winter than any other pasture grass we have.
If your pasture is predominantly fescue and you want to introduce other species into the pasture, grazing fescue down to less than 1 in. will weaken it a little and open up bare ground for inter-seeding. This is one of the few times we can justify grazing winter dormant pasture to the ground.
But be careful with winter management in mixed pastures. Grazing fescue this short will hurt other desirable species you want to maintain in the pasture more than the short grazing will hurt the fescue.
We have talked primarily about bunch grasses so far. Sod-forming grasses are a little different because they have rhizomes, or underground stems, where energy can be stored. Switchgrass is an example of native, warm-season grass with rhizomes, while smooth bromegrass is a good example a rhizomatous cool-season grass.
Does this mean these species can be grazed to the ground? No, because there is still a significant portion of CHO above ground in the stem bases.
Besides weakening the desirable pasture species, grazing to the ground in the winter also greatly increases the opportunity for weeds to invade the pasture. Very few weeds actually have the ability to invade a healthy pasture and crowd out the desirable species. Usually weeds are in a pasture because grazing or fertility management gave them an opening.
The only safe conclusion we can draw is dormant pasture and range should not be grazed “to the ground” even in winter. For tall grass native range, plan to leave a minimum of 4-6 in. residual. For mid or short grass range, leave at least 3-4 in. On mixed cool-season grass-legume pasture, 3-4 in. is good. On tall fescue-dominant pastures, feel free to take it down to 2 in.
Jim Gerrish, of American GrazingLands Services, is a grazing management consultant and educator based in May, ID. Prior to his retirement from the University of Missouri (UM) in 2004, he headed UM's Forage Systems Research Center in Linneus. He can be reached at 208/876-4067 or firstname.lastname@example.org.