"Manage for maximum leaf production to maximize forage quality." This rule of thumb has been used for years. The principles that make this true are really quite simple and worth a short discussion.

The basic unit of forage production is a tiller, which is composed of leaf blades and leaf sheaths, stem, and sometimes a seedhead. Most tillers originate from the crown of the plant (near the soil surface), and new leaves are pushed up through surrounding sheaths of older leaves. Most forage grasses will produce between 5 and 10 leaves per tiller. The last leaf to emerge is the flag leaf. The flag leaf precedes the emergence of the seedhead and is recognized by its peculiar orientation, generally parallel to the ground.

However, not all tillers become reproductive and produce a seedhead. Seedhead production varies from species to species and seasonally within species. Tall fescue makes a good example. Most spring growth tillers, in response to temperatures and day length produce seedheads, while fall growth tillers generally remain vegetative. Bahiagrass, on the other hand, produces seedheads throughout its growing season (contrary to popular opinion bahiagrass will not produce a new seedhead within 1 hour of mowing, it generally takes at least a day.) In some cases there are also varietal differences in seedhead production within a species, as common verses Coastal bermudagrass.

New tillers originate from basal buds, a form of specialized plant tissue. If growing points are removed by grazing or cutting, no more tillers are produced. Many of the forage grasses have growing points at or slightly below the soil surface; most notably bermudagrass, bahiagrass, and tall fescue.

Even on the perennial forage plants individual tillers are relatively short-lived. Individual tillers often live for only 5 to 10 weeks, then die. An easy to visualize example is a corn tiller (stalk). The tiller grows, producing leaves, then turns reproductive, produces a seedhead, matures the seed, then the tiller dies. A bermudagrass, switchgrass, or tall fescue tiller does the same thing, with the exception that some of the tillers die without becoming reproductive. This gives rise to another management principle, "Use it or lose it."

Influence on Forage Quality

As the grass tiller changes from vegetative growth (leaf production) to reproductive growth (seedhead production) the plant goes through rapid physiological changes. Typically seedheads are located high on the plant which allows seeds to be dispersed over a wide area. Plants evolved with this characteristic to allow them to reproduce more effectively as the wind, birds and animals help disperse the seed. This is seen as the stem elongates, called jointing in small grains. To hold the seedhead up the stem must become more rigid, stronger, stiffer, or tougher. These words should be telling you that digestibility or forage quality is decreasing. Fibers in the stem are converted from more digestible material to lignin, which is very indigestible. This general process of fiber conversion is occurring in all forages as they mature or age, even if the individual tiller does not become reproductive.

If the tiller is producing a seedhead, several other changes are occurring in the plant. The nutrients to fill the seed are translocated (moved from) the leaves on the tiller. These nutrients include protein, minerals, and carbohydrates such as starches and sugars. The bottom, or oldest leaves on the tiller are the first to have nutrients translocated to the seedhead. When producing a grain crop, such as grain sorghum or wheat, we speak of the bottom leaves "firing." That means the leaves are senescing or dying. The translocation of nutrients is a great process when producing grain such as corn, wheat, or grain sorghum. However, most grass seed, including grain sorghum and the small grains, but especially the forage grass seeds, are relatively indigestible when fed whole and are generally passed through the grazing animal and consumed by birds!

This gives us two management principles to help keep forage quality high, although they are saying almost the same thing. First is to harvest (graze or cut for hay) before seedheads are produced. The second is to utilize the forage in a way that maximizes the leaf:stem ratio. This simply means to maintain a greater proportion of leaves compared to stem.

Influence on Forage Quantity

On a per tiller basis, forage quantity (tonnage) increases as new leaves emerge. In general, maximum dry matter yield per tiller will occur somewhere between flag leaf and flowering (keep in mind that the plant must flower before the seed is formed so we are talking about the time before grain filling, soft dough, etc.). However, maximum digestible nutrient yield almost always occurs at or before flag leaf.

Yield per acre increases as tillers grow, but mainly yield increases as the number of tillers per acre increases. New tillers are produced in response to several actions. Generally, removal of top growth will stimulate tillering, as long as the basal bud is protected. Proper fertility is needed for maximum tiller development, as is reasonable moisture.

Energy, in the form of carbohydrates stored in roots and the lower stem bases, is used by the plant to develop new tillers. The new tiller uses this stored energy to ‘feed’ the new growth until it develops enough leaf area to produce its own energy or food from the sun by photosynthesis. At that point the depleted root energy is replaced. Depleted root energy reserves will slow new tiller development. Therefore proper defoliation management to keep root energy reserves replenished will maximize new tiller development and increase yield per acre.

In most forage grasses some sunlight needs to strike the basal bud to initiate new tiller development. This principle is the reason yield per acre can actually decrease if the defoliation period is too long. The grass actually mulches itself so to speak. Heavy growth does not allow sunlight to the growing points, bottom leaves are senescing, seedheads are forming, and with no defoliation and removal total yield per acre decreases; and forage quality has already gone to pot.

This knowledge then gives us two management principles to increase yield. Do not defoliate so frequently that root energy reserves are not replenished; stated another way, allow the forage plant time to grow with no grazing so that energy is moved to and stored in the roots. The second principle is to defoliate before the plant becomes decadent and few new tillers are being produced. This usually coincides with seedhead formation, and/or when a good percentage of the bottom leaves are senescing.

The Compromise

From the above discussion it should now be obvious that you cannot have both MAXIMUM yield and BEST quality. However, the fact that yield increases with time (maturity) and forage quality decreases with time does give us a management principle to meet livestock nutrient requirements. A dairy cow producing 50 lbs milk/day, or stocker steers gaining 3 lbs/day must be consuming immature vegetative forage to meet these production goals, and you will not be able to produce maximum tonnage of forage.

At the other end of the spectrum are your out of breeding season bulls, and that slightly overweight pleasure horse. These animals can do just fine on older more mature pastures, or hay that was cut late. The nutrient requirements of these classes of livestock are lower and therefore the pasture can be managed for a higher yield; or as may often be the case, the low quality hay can be fed to these animals.

Understanding tiller growth and development is the key to proper defoliation management. Pastures should be grazed and hay should be cut to produce the desired or needed forage quality, with an understanding of the influence that defoliation has on forage yield. It is also important to try to provide the best quality forage to animals which have the highest nutritional requirements.

A final admonishment: there is never a good reason to manage your pastures and hay fields at the extremes. In very immature stands, do not cut or graze before root energy reserves are replenished; and further, do not let stands become so mature that yields are not increasing or even worse, have started to decline.

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Clemson University Cooperative Extension Service Web Based Electronic Forage Fact Sheet 1, 1997, Revised 2000.

This web site is maintained by Dr. Bruce Pinkerton