Feed Intake & Utilisation

Digested feed clearly has a major effect on nutrient supply, the most important factor governing the extent to which cows can meet their energy, protein and other nutrient needs is the amount of feed they consume. This is especially important in the early stages of lactation when the energy demand for production is higher than intake can support, creating an increasingly negative energy balance which cows have to meet from body reserves, milking off their backs.

Feed Intake

Feed consumption is generally expressed in terms of Dry Matter Intake (DMI) - the weight of feed material consumed excluding the moisture it contains. A large number of different animal, food and management factors affect DM intake. 

Key animal factors affecting DMI include:

  • Cow size - Big cows eat more than small cows.
  • Cow breed - Some breeds of cow eat more for their size than others.
  • Cow age - Heifers eat less than mature cows.
  • Milk yield - Higher yielding cows eat more than lower yielders.
  • Cow condition - Fat cows eat less than thin cows.
  • Stage of lactation - Early lactation cows eat less than those in mid and late-lactation.

Key food factors affecting DMI include:

  • Fibre content - Cows eat less when fibre levels are too high.
  • Protein content - Cows eat more when protein levels rise.
  • Processing - Cows eat more when the feed particle size is smaller.
  • Moisture content - Cows eat most at a ration DM of 45-55%.
  • Diet composition - Cows eat less when rations are poorly balanced.
  • Digestibility - Cows eat more when rations are more digestible.

Key management factors affecting DMI include:

  • Feed access - Cows eat more when feed is available ad-lib.
  • Water access - Cows eat more when palatable water is readily available.
  • Feeding frequency - Cows eat more when fresh feeds are provided more frequently.
  • Ration palatability - Cows eat more the tastier they find the feed.
  • Feed spoilage - Cows eat less when feeds are spoiled by decay or contamination.
  • Ration changes - Cows eat less when rations are changed abruptly.

Substitution Rates Prioritising Energy Balance

The clear limit to the amount of Neutral Detergent Fibre (NDF) cows can consume in a day means intakes of low fibre feeds like wheat (12% NDF) are potentially four times those of higher fibre feeds like good quality silage (48% NDF). As well as having important implications for overall intakes, this means cows will eat less forage when supplements are available, the extent of this substitution depending on the type of supplement.

Concentrate feeds generally displace relatively small amounts of forage from the diet, so supplementation will generally allow daily DM intakes and performance to be increased. A kilogram of wheat (12% NDF) will, for instance, displace only 0.25 kg of 48% NDF silage from the daily intake (12% ÷ 48% = 0.25 substitution).

In contrast, higher fibre feeds have higher substitution rates - a kilogram of sugar beet pulp (32% NDF) displacing 0.67 kg (32% ÷ 48%) of the same silage.

When buffer feeding grazed grass, higher fibre feeds can lead to substitution rates of greater than 1.0, reducing daily intakes and compromising performance.

Energy Balance

Nutrients absorbed from the gut are continually being partitioned within the cow to maintain its body functions and support the production of milk and body reserves. Over and above the nutrients required for maintenance, milk production receives the clear priority in early lactation, with shortages of nutrients from the diet made-up by the mobilisation of body reserves. Thereafter, there is a progressive re-ordering of priorities, with milk production declining and a greater proportion of nutrients being directed to rebuilding body reserves - primarily fat.

The challenge of feeding high yielding cows is underlined by the fact that in a single lactation they are likely to produce more dry matter in the form of milk than their body contains. This challenge is particularly intense in early lactation as milk yields build rapidly to a peak about 4-8 weeks after calving whereas maximum voluntary feed intake is only reached after around 10-12 weeks. Ensuring high DM intakes as soon as possible after calving is a key priority if high levels of both production and fertility are to be achieved. The inevitable rise in energy demand for milk  production ahead of energy intake in the first few weeks of lactation has not been found to cause problems as long as intake catches-up with production by around the sixth or eighth week of lactation, at which time the negative energy balance ceases and cows stop losing weight.

The fact that early lactation intakes have not risen in line with milk yields in recent years has meant increasingly deeper and longer periods of negative energy balance - in excess of 20 weeks in studies with very high yielding herds.

At peak milk yields of up to 40 litres/day most cows have relatively little difficulty consuming sufficient feed to support their production. Once daily DM intakes of 24 kg or more are required by yields much in excess of 40 litres/day, however, increasing problems arise, even with particularly high energy density diets and relatively high daily weight losses. Considerable research into nutrition and fertility performance has pinpointed more pronounced and protracted early lactation energy deficits as a major factor in the lower fertility experienced by many high yielding cows. Research and experience indicates a daily DM intake of 3.5% of body weight should be achieved by five weeks after calving for optimum performance in high yielding herds.

Cow Condition

Body condition scoring is widely accepted as a practical way of assessing body fat reserves, providing a good measure of a cow's energy balance to inform feeding and management. 

A semi-subjective assessment to an 11-point scale of half units, condition scoring is best carried out by the same person on each occasion to eliminate operator differences. As the change in condition score is more important than the absolute value, scoring should be undertaken regularly. For example:  as calving, prior to first service, in mid-lactation and at drying-off.

Research and experience has established the following ideal Body Condition Score targets for key stages in the lactation cycle:

  • At calving: 2.75-3.0
  • Prior to first service: 2.25-2.5
  • In mid-lactation: 2.5-2.75

As a guide cows should maintain condition during the dry period, lose no more than 0.25 Condition Score to 4 weeks post calving; and, lose no more than 0.25 Condition Score from 4 to 8 weeks post calving.