Dr. Jeff McCutcheon, OSU Extension Educator, Knox County
After the dry growing season last year many sheep producers are asking what they could do to improve pasture yields. Other than improving soil fertility there is one thing you can do during the month of May that will improve yields. In fact most experienced graziers I know get pretty fanatical about this task. The task is simple; remove the reproductive tillers before they produce seed. In other words do not let your pasture plants have sex.
The reason you do not want to see reproductive tillers is because letting the grass plants produce seed will decrease yield. Reproductive tillers reduce yields in two ways. First, reproductive tillers elongate above the other tillers on the plant and shade out the vegetative tillers. This competition for sunlight means that the vegetative tillers are growing less. Less sunlight reaching the crowns of the grass plants also means that fewer vegetative buds on the crown will develop into tillers. Yield in pastures is heavily influenced by the density of the pasture. More tillers are better than more height.
The second way reproductive tillers reduce yield, is by producing hormones as the seeds mature that retard or inhibit the development of other vegetative tillers. The grass plant will not produce more vegetative growth until the seeds completely mature or the reproductive tiller is removed. Remember, the plant’s priority is to put its resources into the development of a mature seed, not to grow high quality vegetative material.
When should you remove the reproductive tillers? To keep our pastures vegetative, reproductive tillers should be removed between boot and flowering. During spring we have warming temperatures with increasingly longer days and shorter nights. Each grass plant has multiple tillers. Initially in the spring all the tillers produce vegetative growth. For each grass there is a range of light to dark time where those tillers induced to flowering when they were developed last fall, switch from vegetative growth to reproductive growth. The tiller elongates and stem formation begins. The tiller now shifts its resources to seed development and further leaf production on the tiller stops. The seed head develops and is pushed up and out of the tiller. Boot stage is when the seed head is just about ready to emerge from the last elongated node area and is still wrapped in a leaf sheath. After the seed head emerges from the leaf sheath the reproduction continues with flowering, seed development, seed growth and seed maturation.
Unlike vegetative tillers that have the growing point near the soil surface, the growing point in a reproductive tiller is generally found just below the last completed node. This growing point is vulnerable to grazing or clipping. If the growing point is removed then re-growth will come from the development of new tillers.
One caution, there can be more than one reproductive tiller on a plant. Even though our cool season grasses produce seed heads in the spring, those tillers were actually developed during the preceding fall. Cool season grasses require a period of cold temperature with long nights to develop flower buds on the crown of the plant. Once reproductive growth is initiated in the spring, one reproductive tiller will dominate. If reproductive tillers are removed and light conditions are still right to initiate reproductive growth another tiller initiated last fall will start reproductive growth. I have seen orchardgrass three inches tall with a flowering seed head.
Because of this some sheep producers may want to wait until most of the reproductive tillers have developed seed heads before clipping later in the grazing season. You could wait but you will lose yield. Two of my colleagues have said that clipping after flowering is either cosmetic or for revenge, not to stimulate vegetative growth.
Every tiller on a grass plant is not a reproductive tiller. Only those tillers induced the preceding fall have the capacity to produce a stem and seed head; the new tillers developed in the early spring will not produce a seed head. Eventually the light conditions will change, no longer initiating reproductive growth, and all the growth for the rest of the year will be vegetative.
Removing reproductive tillers can be accomplished with grazing livestock or mowing. Livestock can best do this with high stocking densities and moving livestock quickly from one paddock to the next. The goal is to use the livestock to just graze the top couple of inches of the grass. To effectively do this a heavy stocking density and small paddock size is needed. If the grass is not grazed uniformly enough to avoid seed head formation, then some mechanical clipping will be needed.