Dr. Jeff McCutcheon, OSU Extension Educator, Knox County
Fall is one of the most crucial time periods for our cool season pastures. The most important activity a livestock producer should be doing to help the pastures survive winter and remain productive next year is to avoid over-grazing.
Why is fall a critical time for our cool season perennial forages? The grass plants in your pasture are perennial plants. They survive from year to year. One way perennial plants survive is to develop buds located at the crown of the plant and store energy in the form of carbohydrates to be used by those buds when they start growing.
Leaf tissue that grows in the fall will die over winter. Next years growth comes from the buds developed the previous fall. The buds and roots of the plant are the parts that remain as living tissues over the winter. While not growing, they are respiring and burning energy. If carbohydrate reserves are not adequate then the plant can die before spring. If the plant survives but carbohydrate reserves are low, then initial spring growth is slow and the overall vigor of the plant is reduced.
In the fall, we have short day, long night periods with temperatures above freezing to about 70 degrees F. It is that combination that triggers buds to be initiated and formed on the crown of the plant. In the cooler temperatures leaf growth is slower while photosynthesis does not slow down. This increases the reserve carbohydrates in the plant, which is then used for bud and root development. In the spring new growth comes from these buds. Initial spring growth draws upon carbohydrate reserves stored in the roots and or crowns of the plants. Those buds and stored carbohydrates start next year’s growth.
Healthy leaf tissue is needed for this to happen. Overgrazing removes too much leaf tissue. A grass plant with too little leaf area after grazing has to use carbohydrate reserves from the roots to re-grow. If the plant does not grow enough leaf tissue to gain carbohydrates before it is grazed again then the plant will continue to deplete its carbohydrate reserves. In fall the growing season will eventually come to an end and the plant will not have a chance to recover. Plants struggling to grow leaves will not develop buds. Overgrazed pastures that go into the winter with low carbohydrate reserves are very slow to green up in the spring and exhibit slow growth rates once they do green up.
Overgrazing is not caused by having too many animals in a field. It occurs when you keep animals in a field too long or bring the animals back before the forages have recovered. The length of time is determined by plant growth and how much is there at the start of grazing. The animals should be removed before plants they initially grazed start to re-grow. They also should be removed before they eat all of the leaves. Viable leaves need to remain after the plants are grazed. Plants should also be given enough rest between grazing events. This will allow enough leaf area to be re-grown before the animals are allowed to graze that field again.
Overgrazing can be avoided by paying attention to forage residual, grazing time and rest. You should leave at least 1200-1500 lbs. of DM per acre or 2-3″ of green forage when you pull animals from a field. You should remove the animals before the forage starts to re-grow. The pasture should recover to above 2400 lbs. of DM acre or 6-8″ before turning the animals into a field.