Nutrient Requirements

Cows require four main groups of nutrients to live, grow, produce and reproduce - water, energy, protein, and minerals and vitamins. 

Water Requirements

Comprising 50-80% of a cow's body, depending on age, and essential for all cellular functions as well as milk production, the transport of nutrients and excretion of waste products, water is the single most important dairy nutrient. It is also vital to the regulation of body temperature.

Cows require at least 60 litres of water/head/day and may need 100 litres or more depending upon yield. Cows also have a good sense of smell and taste it is important to ensure water supplies are of sufficient quality; poorly-sited wells or bore holes can become contaminated by slurry stores, septic tank outflows, carcase burial pits and even landfills. Salmonella and other coliform bacteria can survive for long periods in leach into otherwise clean water supplies from some distance. 

Water Quality Testing

All non-mains water should be tested annually for pH, total dissolved solids, total coliform bacteria,  faecal coliform bacteria, total plate count and key minerals   using clean, sterile sample containers from a testing laboratory. Samples for bacteriological testing must be refrigerated, insulated, and delivered to the laboratory within six hours. Other samples can be delivered or mailed using a standard overnight service.

Energy Requirements

50-80% of the energy cows require to power all their bodily functions comes from Volatile Fatty Acids (VFAs) produced by the fermentation of feed carbohydrates in the rumen, with the remainder derived from carbohydrates, fats and proteins that escape rumen degradation.

Ruminant energy requirements and feed energy supplies are generally expressed in terms of Metabolisable Energy (ME) - the energy available to the cow after accounting for losses in digestion, gases and urine. 

Fermentable Metabolisable Energy (FME) is the proportion of the ME potentially available in the rumen.

Bacteria cannot use either fermentation acids or fats/oils as an energy source, so the right balance of dietary sugar, starch and fibre is essential for efficient rumen function.

Imbalances of the main energy sources can cause the following problems:

  • Sugar and starch: Too high risk of acidosis; fat cows. Too low risk of low milk protein; thin cows.
  • Fibre: Too high intakes drop. Too low risk of acidosis; displaced abomasums.
  • Fat: Too high poor fibre digestion and low intakes. Too low risk of low milk yield and fat.

Protein Requirements

Essential to every aspect of body maintenance, reproduction and milk production, so called Metabolisable Protein (MP) is supplied to the cow as a combination of microbial protein from the rumen and dietary protein that passes through it undegraded.

Ruminant protein requirements and feed protein supplies are generally expressed in terms of Crude Protein (CP) which includes non-protein nitrogen as well as true protein.

Rumen Degradable Protein (RDP) describes the protein supply available to the rumen microbes, while Digestible Undegraded Protein (DUP) is the protein available from the feed which escapes rumen degradation.

Although not considered to be a reliable guide for fertility purposes, Milk Urea Nitrogen (MUN) concentrations can provide useful indications of the efficiency with which protein is being utilised in the rumen.

Mineral & Vitamin Requirements

Minerals are inorganic compounds needed for a whole host of regulatory and structural functions in the cow. They are provided in different quantities are supplied in a range of feed supplements

Macro Minerals

Required in relatively large amounts - grams/cow/day - and expressed as a percent of ration dry matter, include calcium, phosphorus, magnesium, potassium, sodium and sulphur. Sodium, potassium and sulphur salts are ionic and affect the acid-base balance in cows, critical to the maintenance of many bodily functions.

When dry cows are fed rations high in potassium producing positively-charged ions, for instance, the availability and absorption of magnesium can be reduced, leading to milk fever type symptoms. There is good evidence that feeding anionic (negatively-charged) chlorine or sulphur salts using a Dietary Cation-Anion Balance (DCAB) approach helps prevent milk fever in these circumstances. If dry cow rations contain potassium at over 2% in the forage DM, however, it is often better to change the forage rather than adding anionic salts, since their poor palatability can depress appetite.

Key Trace Elements

Are only required in relatively small amounts and measured in milligrams/day include cobalt, copper, iodine, iron, manganese, selenium and zinc. They can be supplemented in either inorganic (eg, zinc oxide) or organic (eg, zinc methionine) forms.

Inorganic minerals are most commonly used because they are less expensive and more concentrated than organic minerals. The many interactions between minerals and the fact that some can be toxic at relatively low levels makes providing them in excess of requirements as harmful as failing to correct deficiencies. 


Organic compounds needed in small amounts for a variety of chemical reactions in the body. Fresh forages are good sources of fat soluble vitamins but dried, stored and ensiled forages have little vitamin content remaining so diets based upon them must generally be supplemented. 

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