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Section of Dairy Husbandry, Michigan Agricultural Experiment Station East Lansing
ABSTRACT
The nutritive value of roughages for dairy cattle depends upon the amount of dry matter consumed, the chemical composition and the coefficient of digestibility of the dry matter.
Dry matter consumption is affected by factors inherent in the animal and those inherent in the ration and environment. There is a wide variation in dry matter consumption of cows on the same roughage. The palatability of roughages is not a reliable criterion to use in estimating the amount which cows will consume. The trend toward the use of properly managed pasture, and hays and silage made from grasses and legumes cut at an earlier stage of maturity has resulted in an increase in roughage consumption per 100 pounds of body weight. Most cows will consume about three pounds of hay equivalent per 100 pounds of body weight which is a 50 per cent increase in roughage consumption over the old thumb rule for roughage feeding.
The chemical composition has been shown to vary with the stage of maturity, soil conditions and climate. Young plants are characterized by their high protein content, low cellulose and lignin content and a high digestibility of their dry matter. The decreased digestibility of the dry matter with increasing maturity is believed to be associated in some way with lignification although the total amount of lignin does not appear to be the determining factor. The greater use of properly managed pasture before the coefficient of digestibility drops makes the use of this class of roughage profitable. The data indicate that highly digestible roughage exerts a productive energy value about equal to that of concentrates on the dry basis.
The use of non-lignified grasses and legumes for winter feeding is very limited. This is due to the low yields and the difficulty encountered in the field curing of such roughages. Fertilization to increase the yield of roughage and its conservation in the silo after wilting make possible the use of highly nutritious roughage throughout the year.
The nutritive value of hays cut at the usual stage of maturity depends upon the nutrients conserved during the curing process. As has been pointed out by several investigators, the loss of nutrients during the field curing of hay is rather high during good hay making weather but the loss of nutrients is very large during adverse curing weather. Better methods of conserving roughages are needed because poor hay making weather prevails in many dairy sections. The cost of artificial dehydration of hays appears somewhat high at the present time.
Recent experiments indicate that grasses and legumes may be conserved economically in the silo with a loss of only about 10 per cent of the dry matter. In several experiments, however, the loss of dry matter in roughage conserved as silage was rather high. Most investigations indicate, however, that the addition of from one to three per cent of molasses is advisable in the making of grass and legume silage. Inorganic acids may also be used to preserve these crops in the silo but molasses has the additional advantage of preserving the crop and contributing to the nutrition of the animal. Of the inorganic acids, phosphoric acid appears to have a slight advantage in silage making. It is easy to apply and it may contribute to the phosphorus requirement of the animal and to the fertility of the soil. Several investigators have reported that grasses and legumes make excellent silage without any treatment providing the ensiled material has a moisture content of about 70 per cent and the trapped air is expelled by fine cutting.
Many of the silage investigations, however, have been conducted either in small containers or in very large silos. In both cases the results may not represent the conditions in the average farm silo of about 50- to 100-ton capacity. In large silos the increased pressure may affect the moisture and carotene content of the silage as well as influence the capacity of the silo. There is very little difference in the dry matter capacity of the average silo for corn, grass or legume silage when the moisture and fineness of cut are about the same, but in large silos, more dry matter can be stored as grass or legume silage. Additional data is needed in regard to the conservation of grasses and legumes in the average farm silo.
Roughage protein appears to be well-balanced from the standpoint of the essential amino acids necessary for growth and milk production. The possibility of protein synthesis in the rumen by the rumen flora has been indicated.
Although grasses and legumes are probably very low in true fat, the results obtained with alfalfa alone suggest that when a liberal roughage program is followed there is little likelihood of encountering a fat deficiency. The possibility of a fat deficiency or of some factors closely associated with fat should, however, be considered.
The conservation of soluble carbohydrates in roughage is of practical importance because unfavorable weather conditions result in their loss in hay making. They may be of importance in maintaining the health of cattle during the barn feeding period. Ketosis has been shown to occur in cows fed rations low in soluble carbohydrates. This is explained on the basis of the new theory of rumen digestion which postulates the conversion of foods which stagnate in the rumen to fatty acids by bacteria. The soluble carbohydrates have a greater chance of escaping bacterial action.
Grasses and legumes are excellent sources of carotene when used as pasture but when used as hay, a large amount of the carotene is lost in field curing and during storage. The amount present in field cured hay is sufficient to meet the physiological needs of cattle for growth and reproduction when roughage is fed liberally. The conservation of carotene in roughages is of importance in the production of milk high in vitamin A. Several investigators have reported the preservation of large amounts of carotene in grass and legume silage by the use of either molasses or inorganic acids. The results obtained by the United States Department of Agriculture, Bureau of Dairy Industry, indicate that a preservative is not necessarily needed for the preservation of carotene in grass and legume silages.
When green material is placed in the silo, the resultant silage is low in vitamin D inasmuch as this vitamin is formed in the cured plant material during solar radiation. The feeding of four or five pounds of sun-cured roughage per cow daily will meet the physiological needs of the animal for this vitamin during the winter months. Grasses appear to be good sources of vitamin E but whether or not cattle require this factor has not been ascertained. There is no evidence to indicate that the other vitamins in roughage are required in the ration of cattle.
Roughages are usually good sources of calcium and cattle have been shown to utilize about 50 per cent of the calcium in the ration when the supply of this element is rather low. There appears to be very little likelihood of a calcium deficiency occurring among cattle when they are allowed plenty of roughage.
Phosphorus is the mineral element which is most likely to be deficient in the ration of cattle when they are fed a heavy roughage ration because the phosphorus content of most roughage is low. The phosphorus content of roughage, under certain conditions, may be increased by phosphate fertilization but numerous instances have been recorded to show that the ordinary application of phosphate fertilizers to the soil did not influence the phosphorus content of the plant. The soil type and the moisture conditions are frequently the determining factors which influence the phosphorus content of the plant. In sections where the roughage is low in this element, the addition of a phosphorus supplement to the ration of milking cows is indicated.
Roughages grown on soils containing iodine are good sources of this element but on those farms where "big neck" or goiter occur among newborn calves, an iodine supplement should be supplied.
All of the other mineral elements required by cattle have been shown to be adequately supplied by roughage with the possible exception of a few localities where iron, copper or cobalt are needed as supplements.
The high production of dairy cows on roughage alone as reported by several investigators is of great practical importance and also of scientific interest. In these studies cows fed alfalfa alone appeared to make exceedingly good use of the total digestible nutrients for maintenance and milk production. Kellner, Armsby and Fraps have discounted the digestible nutrients in roughage according to its crude fiber content in arriving at "starch values," "net energy values," and "productive energy values." Apparently the low productive energy value of the digestible nutrients in the roughage studied by the many workers was due to some factor other than the crude fiber content. Alfalfa hay as the sole ration in several experiments, however, did not give results comparable to its total digestible nutrient content. In view of the variations in the productive energy values of alfalfa hay which has been reported in the literature, a grain supplement should probably be fed to cows in medium and high milk production. Rules have been suggested for feeding grain with roughage of good quality.
The author wishes to express his appreciation to C. W. Duncan of the Michigan Agricultural Experiment Station for reading and criticizing this paper.
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