From the beginning of livestock agriculture, farmers and ranchers have realized that not all forages are equal in their ability to produce animal products. The first assignment given newly established experiment stations as agriculture developed in this and other countries was to develop a system for measuring the differences in forages. For some reason, this difference became known as "forage quality." Measuring quality in forages and other roughages involves more than mere chemical composition. Forage quality is based on the following factors.
The major contribution of forages to the ration are fiber and carbohydrates, which are a source of energy. Energy value is expressed in terms of calories and really is the ability to produce heat when burned. As an example, Table 1 lists some values for the same hay harvested at four stages of maturity.
The four hays are listed according to their digestibility. The hay with the very high digestibility was 67 percent digestible, the high hay was 61 percent digestible, the moderate hay was 51 percent digestible and the low hay was 47 percent digestible.
The first important point is the gross energy values and the heat that would be produced if you burned the hays and measured the heat production. Note that all four hays had the same gross energy. This is true for all plant materials from sawdust to the most digestible fibers.
The second value is digestible energy. Because the cow only can use that portion of the energy that is digested, this measurement has real value. The amount of digestible energy is the gross energy multiplied by the percent digestibility.
The final value is the net energy for lactation a value that is familiar since it appears on forage analyses and in feed programming. This value is that portion of the digested energy available for the cow to use for maintenance and milk production. Remember that all of the values that measure availability are determined by the percent of forage that the cow can digest.
There is a natural tendency to say that the more digestible the forage, the better. Unfortunately, this is not entirely true. Dairy cows work best when the digestibility of the total ration is somewhere between 68 and 74 percent. Above this range, feed may move through the digestive tract too rapidly for good utilization, the cow may not do enough cud chewing and there may not be enough useable fiber in the ration.
Roughage value (cud chewing time)
Forages have the ability to provide three-fourths of the needed ADF and NDF in the total ration and as much as 90 percent of the cud-chewing time required for normal milk composition and good rumen health. This latter contribution is what researchers have talked about for generations as the "roughage value" and "scratch factor."
Researchers have measured the amount of cud chewing time generated by several feedstuffs. Research in several countries has established that cows need to do about 14 to 16 minutes of cud chewing for each pound of dry matter in the ration to maintain optimum rumen conditions for good feed utilization. Such values have limited use in dairy herds unless they can somehow be included in feed programming. To do this, there is continuing research on the role of fiber type and fiber length in the promotion of cud chewing.
When forage is scarce and must be fed in minimal quantities, major emphasis must be placed on its value as a roughage to maintain rumen health and function. Conversely, when a forage will be used with other forages that provide an abundance of cud chewing, the very early cut forage would be the one to choose. Under most conditions where only one forage is used, selection would be based on needed ADF and NDF along with good usable net energy.
Low fill factor
Roughage rarely constitutes the entire ration for milking cows. Generally, adding grain to the ration increases total dry matter intake. The decline in roughage intake for one unit of grain added varies from as low as 0.2 unit of roughage to 1.0 unit. The smaller the decline in roughage intake per unit of grain added, the greater the increase in total dry matter consumed. The ideal roughages are those with smaller reductions in intake when grain is fed. Overall, legumes permit greater intake than grasses at the same digestibility of dry matter.
There is considerable research on the "fill (bulk) factor" of roughage. The fill factor recognizes that individual roughages occupy more or less space (bulk) in the digestive tract per unit of dry matter. Those occupying the smaller amount of space should permit greater increases in total dry matter intake from added grain.
Nutritive Value of Forages
Feeding value of forages is greatly influenced by the growth stage of the forage when harvested or grazed. The feeding value of a forage is the highest during vegetative growth and the lowest during the seed formation stage. Therefore, when forages are grown for the purpose of feeding cattle, they should be harvested or grazed at early stages of maturity.
Most of the nutrients in both grasses and legumes at the vegetative stage are contained in the leaves. As the plant grows, the stems make up a larger proportion of the total dry matter and the proportion of leaves decreases. The stems are fibrous and rather indigestible compared to the leaves, so the vegetative parts of a plant are usually low in fiber and high in protein. However, as the leaf to stem ratio decreases with advancing maturity, the plant contains less protein and more fiber. In addition, as the plant matures, the plant cell wall of the stem becomes more lignified. Not only is there more fiber, but the fiber itself becomes less digestible. In summary, the decrease in nutritive value of a grass or a legume with increased maturity is due to:
· The decreased proportion of leaves and the corresponding decrease in protein.
· The increased proportion of stem and the corresponding increase in fiber.
· The increased lignification of the stem and the corresponding decrease in energy value.