Babcock Institute for International Dairy Research and Development University of Wisconsin-Madison
6) FEEDS FOR DAIRY COWS
Michel A. Wattiaux
W. Terry Howard
Department of Dairy Science
INTRODUCTION In general, feeds are classified into one of the following categories: • Forages; • Concentrates (energy and protein feeds); • Minerals and vitamins.This classification is a convenient way to group feeds, but it is somewhat arbitrary. Classifying feeds is not as important as knowing which feeds are available, their nutritive value, and the factors that affect their utilization in a ration. FORAGES In general, forages are the vegetative parts of grasses or legume plants containing a high proportion of fiber (more than 30% neutral detergentfiber). They are required in the diet in a coarse physical form (particles with a length of more than 2.5 cm) to help rumen function. Usually, forages are grown on the farm. They may be grazed directly or harvested and preserved as hay or silage. Depending on the cow's stage of lactation, they should contribute from almost 100% (for nonlactating cows) to no less than 35% (for cows in early lactation)of the ration dry matter. The general characteristics of forages are as follows: • Bulky: Bulkiness puts limits on how much a cow can eat. The energy intake and the milk production of a cow may be limited by too much
forage in a ration. However, bulky feeds are essential to stimulate rumination and maintain the health of the cow. • High Fiber and Low Energy: Forages may contain from 30 to 90%fiber (neutral detergent fiber). In general, the higher the fiber in a forage, the lower the energy content of the forage. • Variable in Protein: Depending on the stage of maturity, legumes may contain 15 to 23% crude protein; grasses typically contain 8 to 18% crude protein (depending on the level of nitrogen fertilization), and crop residues may have only 3 to 4% crude protein (straw). From anutritional standpoint, forages may range from very good feeds (lush young grass, legumes at a vegetative stage of maturity) to very poor feeds (straw, browse). Grasses and Legumes High quality forage can make up twothirds of the ration dry matter with cows eating 2.5 to 3% of their body weight as forage dry matter (for example, a cow of 600 kg, can eat 15 to 18 kg dry matter of good quality forage).Cows usually eat more legumes than grasses at similar stages of maturity. However, good quality forages fed in balanced diets will supply much of the protein and energy needed for milk production. 21
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Dairy Essentials – Nutrition and Feeding
Soil and climate conditions usuallydetermine the type of forage that is most commonly grown in a region. Both grasses (rye-grass, brome grass, bermuda grass, fescue, orchard grass) and legumes (alfalfa, clover, lespedeza, birdsfoot trefoil) are widely spread throughout the world. Grasses need nitrogen fertilizers and moisture conditions to grow well. However, legumes are more resistant to drought. Also, they can add as many as 200 kgof nitrogen/year/hectare in the soil because they live in association with bacteria that convert nitrogen of the air into nitrogen fertilizer. The feeding value of forages is greatly influenced by the growth stage when harvested or grazed. Growth can be divided in three successive stages: 1) Vegetative stage; 2) Flowering stage; 3) Seed formation stage. Usually, the feeding value of a forage isthe highest during vegetative growth and the lowest during the seed formation stage. With advancing maturity, the concentration in protein, energy, calcium, phosphorus, and digestible dry matter in the plant decreases while the fiber concentration increases. As fiber increases, the lignin content within the fiber also increases. Lignin is indigestible and makes the carbohydrates in the fiber less...
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