Chapter 2: Nutrients

Nutrients are substances obtained from food and used in the body to promote growth, maintenance, reproduction and production. Feedstuffs contain the nutrients animals require to perform normal body functions (such as breathing, pumping blood, fighting diseases, growing, gaining weight, reproducing) and to produce milk. The feedstuff must be digestible and the products (nutrients) absorbed if the feed is to be useful to the animal.
Some components of a feedstuff have no nutritive value because they are indigestible and not absorbable (e.g. some woody plants) and pass out through faeces. In addition, some plants contain compounds that are toxic to the animal.
The essential nutrients are energy, proteins, minerals, vitamins and water.

• Energy

 
The energy portion of the feed fuels all body functions, enabling the animal to undertake various activities including milk synthesis. This is the major nutrient (in terms of quantity) that dairy cows require.
 

• Functions

 
Maintenance: Simply to maintain itself, an animal requires energy. The body weight does not increase or decrease, the animal does not produce; this energy is only for survival and the amount is affected by body size and the environmental temperature.
Growth and weight gain: Gain is especially important for young animals, who need to attain the recommended weight for a particular age.
Reproduction: A cow requires more energy during pregnancy for the foetus to grow and develop normally.
Milk production: The energy requirement of a lactating cow increases with increase in milk production and butter fat content of the milk.
 

• Sources

 
Energy can be obtained from several types of feedstuffs that contain either carbohydrates or lipids (fats and oils).
Carbohydrates are the major source of energy in the diet of dairy cows. They are found in the staple foods consumed by humans (e.g. rice, maize, wheat, potatoes). Carbohydrates constitute between 50% and 80% of the dry matter in forages and grains.
 
Feeds contain three major types of carbohydrates:

• Sugars: Sugars are soluble in water, making them readily available to the animal.

Sources are molasses, sugar beets and sugar cane.

• Starch: Starch is the main form of carbohydrate stored in plants. It is the main component of cereal grains and some roots (potato tubers).
• Fibre: Forming the structural part of plants, fibre is present in large quantities in roughages. The fibre is broken down by microorganisms in the rumen (microbial enzymes) into products that the animal can use. It is also important in maintaining high levels of milk fat. Sources include grasses, fodder crops and crop residues.

Lipids (fats) contain about 2.25 times more energy than carbohydrates per unit weight. Generally, plants are good sources of oils while animal products contain fats. Most plant seeds contain a small amount of lipids. The exception is oilseed plants, which may contain as much as 20% lipids (cotton, sunflower and soybean seeds) and are better sources of lipids than animal fats.
 

• Consequences of energy deficiency

 
The most obvious sign of energy deficiency is poor body condition due to excessive weight loss. Lactating cows are unable to reach peak milk production in early lactation resulting in low lactation yields.
 

• Consequences of excessive amounts of energy

 
Cows consuming too much energy become too fat, resulting in low conception rates. They are prone to difficult calving, retained placenta, and higher incidence of milk fever and ketosis.
In early lactation, feeding too much energy, especially in the form of grain, may lead to too much acid in the rumen (acidosis), increased risk of displaced abomasum, depressed feed intake and low milk fat percentage.
 

• Special considerations

 
Usually forages are high in fibre and low in energy, and concentrates are low in fibre and high in energy. Therefore there is need to balance the two, as too much forage limits the intake of energy while too much concentrate results in milk fat depression, rumen acidosis and other health problems.

• Protein

 
Protein is quantitatively the second most important nutrient in feeding the dairy cow. Proteins are made up of building blocks referred to as amino acids.
 

• Functions

 
Proteins provide the building material for all body cells and tissues (e.g. blood, skin, organs and muscles). Proteins are also major components of products such as milk and meat. Lack of protein therefore adversely affects milk production.
 

• Sources

 
Good sources of protein for dairy cows include:

• Oilseeds and oilseed cakes: Residues after the oil is removed from oilseeds, e.g. cottonseed meal or cake, whole cottonseed, whole soybeans (cracked) or meal and sunflower meal or cake.
• Products of animal origin: Such as fish meal, blood meal, meat and bone meal, feather meal and by-products from milk processing (e.g. skim milk and whey).
• Herbaceous legumes: Such as lucerne, desmodium and fodder trees (e.g. calliandra and sesbania).
• Non-protein nitrogen: Cows can obtain protein from sources that do not contain true proteins, such as urea and poultry waste (contains uric acid). These sources are referred to as non-protein nitrogen sources. Microorganisms in the rumen use the nitrogen in urea to synthesize protein for their own growth.

 

• Consequences of protein deficiency

 
For lactating cows, there is a sudden drop in milk production if the amount of protein in the diet is suddenly reduced. Severe deficiency may cause excessive weight loss in lactating cows, reduced growth rate in calves and heifers, and result in underweight calves being born.
 

• Consequences of feeding excess protein

 
Protein is an expensive nutrient and feeding excess is a waste of money as protein is not stored in the body but is broken down by microorganisms in the rumen and excreted in the form of urea.

• Protein and rumen microbes

 
Most of the protein in feed is broken down by microorganisms in the rumen (rumen- degradable protein) and re-synthesized into bacterial protein. Bypass proteins are proteins resistant to microbial breakdown in the rumen (undegradable protein), and pass intact to the small intestines where they are digested and absorbed directly into the body.
 

• Protein and milk production

 
Milk contains approximately 3.2–3.5% protein. Thus a cow producing 25 kg milk per day secretes 800–900 g protein daily. Cows have little ability to store protein in the body
and so it must be supplied in the diet daily to maintain the milk yield. Protein should be 15–18% of the total ration of a dairy cow depending on milk yield.
 

• Minerals

 
Minerals are nutrients required in small amounts in the feed. They are required for the body to function properly, i.e. remain healthy, reproduce and produce milk (Table 2.1). Some minerals are required in large quantities in the ration dry matter (macro-minerals) while others are required in small quantities (microminerals). Some minerals are stored in the body (e.g. iron in the liver and calcium in bones) while others are not (e.g. sodium, potassium) and have to be supplied in the diet all the time.
 
Table 2.1. Minerals required in ruminant diet
 

Macrominerals	Microminerals
Calcium	Cobalt
Chlorine	Copper
Magnesium	Iodine
Phosphorus	Iron
Potassium	Manganese
Sodium	Molybdenum
Sulphur	Selenium
	Zinc

 
 

• Functions

 
Specific minerals may have different functions in the body but the minerals are generally required for

• bone formation
• formation of components of enzymes, vitamins and red blood cells
• production of hormones that control body functions
• control of water balance in the body
• milk synthesis

Requirement for minerals is affected by several factors:

• Age: Mineral requirements for young growing animals are higher.
• Physiological status: Pregnant animals require more.
• Level of production: High-producing cows require large quantities of calcium; deficiency is more likely to occur in early lactation rather than late.

 

• Sources

 
Although roughages and concentrates contain minerals, the types and amounts vary widely and hence may not meet the requirements. During ration formulation, macro- minerals calcium, phosphorus and magnesium are taken into account. Roughages will supply adequate amounts of potassium and common salt can adequately provide sodium.
Some ingredients (supplements) are added to supply a specific mineral (e.g. limestone, salt, magnesium oxide).

• Consequences of mineral deficiency

 
Signs of mineral deficiency may not be obvious but they include

• poor fertility: lack of heat signs and low conception rate
• low milk production
• poorly developed bones in young animals (rickets)
• health disorders, for example, milk fever
• poor body condition, which may be accompanied by a change in coat colour

• Vitamins

 
Vitamins are nutrients in the feed required by the body in tiny amounts for normal functioning of the body, through their involvement in many body processes. Some are synthesized by rumen microbes and/or stored in the body of the animal while others must be supplied in the diet. The vitamins that must be supplied in the diet include A, D and E; those that are produced in the body include B complex, C and K.

• Functions

 
Important functions of vitamins include

• maintenance of healthy protective tissues such as skin, stomach, intestinal and cell linings (vitamin A)
• improvement of appetite, hence feed intake (vitamin B)
• production of red blood cells, hence preventing anaemia (e.g. vitamins B6 and B12)
• enhanced calcium and phosphorus utilization, hence play a role in bone forma- tion and growth (vitamin D)
• enhancing immunity (vitamin E)
• help in blood clotting (vitamin K)

• Consequences of vitamin deficiency

 
Vitamin requirements of dairy cows are normally met through diet, rumen microbial synthesis or tissue synthesis. Deficiencies are rare under normal conditions but may occur under certain conditions, shown in Table 2.2.
Table 2.2. Situations in which vitamins may become deficient in dairy cattle
 

Situation	Affected		vitamins
	A	B	D	E
Feeding poor quality roughage (overgrown), frost- damaged maize silage or long-term stored forage, heat- damaged forage; periods of prolonged drought	X
Very high grain-to-forage ratio	X		X	X
Cattle kept indoors throughout			X
Feeding of milk or milk replacer rations alone	X		X	X
Crop residues as a major component of diet	X		X
Prolonged stress period or disease	X	X	X	X
Very young calves (rumen not functional)		X
Deficiency of cobalt		X

 
Adapted from Wattiaux MA. 1999. Dairy essentials. Babcock Institute for International Dairy Research and Development, Madison, Wisconsin, USA.

• Consequences of feeding excess vitamins

 
There is no important health consequence of excess vitamins as the body is able to get rid of the excess. However, vitamin supplements are expensive and hence feeding too much is an economic loss to the farmer

• Water

Water, though not classified as a nutrient, is essential for life in all animals. Water accounts for 74% of the calf’s weight at birth and 59% of that of a mature cow. Every 100 kg of milk contains up to 87 kg of water.
The amount of water consumed at free will is influenced by several factors:

• moisture content of feed. When the diet has a large proportion of dry feeds,

• e.g. hay and grains, the cow drinks more water than when the diet has a larger proportion of young or succulent feeds, e.g. young grass and legumes
• amount of dry matter consumed
• milk yield
• environmental temperature
• salt intake

An increase in any of the factors above increases the water requirement for lactating cows. The amount of water consumed will also depend on the quality and quantity of water provided (Table 2.3).
Table 2.3. Estimated water requirement by calves, heifers and lactating dairy cattle

Dairy cattle type	Level of milk production (kg milk/day)	Water requirement, average range (litres /day)
Dairy calves (1–4 months)		5–15
Dairy heifers (5–24 months)		15–40
Milking cows	14	65–85
Milking cows	23	85–105
Milking cows	36	115–140
Milking cows	45	130–155
Dry cows		35–50

 
Adapted from McKague K. 2007. Fact sheet on water requirements of livestock. Ontario Ministry of Agriculture, Food & Rural Affairs, Toronto, Canada.

• Concept of dry matter

Dry matter is what remains of a feed when all the water has been removed and it contains the nutrients described above (Figure 2.1). This dry matter portion can be divided into organic matter comprising energy, protein and vitamins, and inorganic matter comprising minerals. Animals must consume enough dry matter to obtain the required nutrients to keep them healthy and to let them grow, reproduce and produce milk.
Dry matter is expressed as a percentage of fresh feed; hence a feed comprising 40% dry matter means that for every 100 kg of the feed, only 40 kg is dry matter. It is from the 40 kg that the animal will obtain its nutrient requirements. If the same feed contains 10% crude protein, the amount supplied by the feed will be 10% of 40 kg, which is 4 kg. Banana stem containing 10% dry matter means that if you feed 100 kg of fresh stems, the cow will have only 10 kg dry matter and 90 kg water.

Figure 2.1. Difference in bulk of feeds before and after water is removed.