Different plants respond differently to being grazed. Knowing how, and why, can help you decide how you are going to manage your pasture and hayfields.
To sum it all up, tall plants grazed short will have to rely on their roots to feed them for regrowth, but low-growing plants have leaves that will provide energy for regrowth from photosynthesis.
To quote Geoff Brink of USDA-ARS:
“Plants would prefer to grow new leaves by producing carbohydrates with old leaves than by moving stored carbohydrates. It’s easier and more efficient.”
In other words, leaving some leaves to help the plant regrow is best. Even though old leaves are less efficient at photosynthesis, having them around to feed the plant is more efficient than dragging stored energy out of the roots and using that.
To apply that concept, it helps to know which plants, grazed too short, will have bigger problems than others. One big determinant is the growing point. The graph of “Morphological Characteristics” gives you a good idea of which plants are considered tall or short. Tall plants have more regrowth problems when grazed to low. Short plants have more leaves at lower heights, so when grazed short, will still be able to depend on photosynthesis.
It is the jointed grasses, with non-flowering stems that will have to rely on energy stored in the roots as carbohydrates if grazed too short, and that’s easy to do. Think: timothy, smooth bromegrass, reed canary grass. Jointed grasses have growing points that move up the stem with regrowth. Grazing them shorter will have even more of an impact.
Non-jointing grasses keep their growing point at the crown, even when they are grazed. That keeps the growing point farther from the mouths of grazing livestock and blades of mowers. These include orchard grass, tall and meadow fescue, perennial rye, Kentucky bluegrass.
It gets even trickier when the breeders get involved. Orchardgrass could be thought of as a tall grass, but some orchardgrass cultivars (Topeko and Benchmark) have been bred to survive shorter grazing.
Timing of the first harvest or grazing, the frequency of grazing, and the amount of pressure, can all impact the stand’s ability to recover. Picking the point of the first harvest will help determine the quality of the forage. In some cases, you might rely on boot stage, but for some species that might be too late. Sampling and knowledge of the specific species involved can help.
The shorter you cut (or graze), the more yield you’ll get. For alfalfa, for each inch you lower the mower blades, you can expect about an extra 1/2 ton per acre per year. But that will also lower the Relative Feed Value by 5 points for every inch. Not to mention that the shorter you go, the shorter the stand life may be. The shorter plants will have a harder time recovering, and this will end up tiring out the pasture or hayfield more quickly.
Taller residual height affects regrowth rate; okay, that’s obvious.This is especially true with smooth bromegrass, and also with orchardgrass and timothy. Leaving 4″ of residue instead of 2″ means that your regrowth rate will be almost 50 lbs per acre per day, instead of just 30. In late summer and early fall, the cool season grasses put in tillers, including reproductive tillers. Severe defoliation can set things back. Knowing this, overgrazing can be used strategically in the fall to help add new species with a spring planting into an established stand.
However you choose to graze, or harvest, knowing what you’ve got and how it grows is half the battle. Then the fun of figuring out how to best utilize it is a game of strategy.