Rationing essentials for dairy diets

Successful rationing involves achieving the best possible balance between a number of key nutritional, practical and financial factors.

The most important priorities in rationing are to:

• Deliver an adequate supply of the nutrients required to meet the cows’ needs
• Achieve a balance of ingredients that will optimise rumen function and nutrient utilisation
• Ensure the ration can provide the required nutrition within the cows’ dry matter intake capacity
• Make the most of lowest-cost (often home-grown) feeds for the greatest economy
• Provide rations which can be fed effectively with the equipment and facilities available

The amount of feed DM cows eat is the single most critical constraint in rationing as it determines how concentrated the ration needs to be to deliver the amounts of energy, protein and other nutrients required.

The primary determinant of dry matter intake in dairy cows is body weight.

Table 1. Daily DM intake estimator

Typical Holstein Friesian cows can eat approximately 3–3.5% of their body weight in DM each day.

This means a 600 kg cow has a DM intake of around 600 x 3 ÷ 100 = 18 kg/day or 600 x 3.5 ÷ 100 = 21 kg/day.

Higher-yielding cows in early to mid-lactation may consume 3.5–4% of their bodyweight in DM.

Breeds like Jerseys and Guernseys also tend to achieve similar, relatively high DM intakes for their size.

Milk production also has a major effect on intake – cows producing more milk at any time having greater appetites.

In addition to cow size and yield, daily DM intake varies with a number of other cow, food and management factors.

Important additional intake factors

• Body condition score
• Stage of lactation
• Forage quality
• Feed protein level
• Method of feeding
• Feed access time
• Diet presentation

Of the various nutrients cows require, metabolisable energy (ME) is the first consideration in rationing.

Performance and wellbeing also depend upon achieving sufficient levels of protein, minerals and vitamins in the right balance.

Energy is, however, almost invariably the primary performance-limiting factor in most cases.

Cows’ daily energy requirements depend on their specific requirements for maintenance, reproduction, milk production and body reserves.

Cows need a certain amount of ME each day to support their basic body functions.

Typical Holstein Friesian cows require 10% of their bodyweight, plus 10 MJ of ME each day for maintenance.

This means a 600 kg cow requires (600 x 10 ÷ 100) + 10 = 70 MJ of ME/day for maintenance.

The amount of ME required to support pregnancy depends upon the stage of gestation, increasing markedly in the final few months with the growth of the unborn calf.

Typical Holstein Friesian cows require around 40% more ME for maintenance and pregnancy each day in the dry period.

This means a 600 kg dry cow requires 70 + (70 x 40 ÷ 100) = 98 MJ of ME/day for maintenance.

The ME required for milk production increases with milk solids levels.

Typical Holstein Friesian cows producing milk at 3.8% fat, 3.2% protein require around 5.2 MJ of ME for every litre.

This means a cow giving 30 litres requires 30 x 5.2 = 156 MJ of ME/day for production.

Body reserves provide a balancing mechanism between the amount of ME cows need and the amount available in their daily diet.

Whenever intake is insufficient to support production – as in early lactation – energy is mobilised from body fat via the liver, causing cows to milk off their backs and lose condition.

Equally, whenever intake is greater than the immediate needs of production – as in later lactation – it will be directed towards body fat, increasing liveweight and condition.

The complexity of the metabolic pathways involved means it requires more ME to gain weight than that released by losing it.

Typical Holstein Friesian cows require 32 MJ of ME for every kg of body weight gain but only generate 28 MJ of ME for every kg of body weight loss.

This means that a 600 kg cow needs 360 MJ of ME over a lactation to end up in the same condition she started in.

Because liveweight changes are impossible to assess without weighing stock, body condition score is commonly used as a measure of body fat reserves.

Typical Holstein Friesian cows gain approximately 10% of their body weight for every unit increase in body condition score.

As well as sufficient energy overall, cows need to have their ME in a balance of forms, which optimises digestive health and efficiency.

They must have the right balance of dietary fibre with fermentable metabolisable energy (FME) to maintain the best possible rumen fermentation.

Although these requirements are important for the most efficient utilisation of feeds and performance, they clearly remain secondary considerations to overall ME supply.

Over the years, a number of different systems have been developed for assessing the protein requirements of dairy cows and meeting them through rationing.

Herds with modest yields on simple feeding systems using traditional ingredients are likely to find crude protein (CP) a sufficiently good basis for their needs.

Although this system fails to take account of the different ways in which protein is involved in ruminant nutrition and the different types of protein provided by feeds, it has the virtue of simplicity and near universal use.

Regardless of their type or source, feeds will almost invariably have a stated, analysed or assumed CP content.

Rumen degradable protein (RDP) and undegraded dietary protein (UDP) split crude protein into the fractions available and unavailable for use by microbes in the rumen.

This system allows more sophisticated rationing to ensure the best value from both rumen fermentation and normal digestion.

While low-yielding animals are generally able to fulfil their protein requirements from RDP alone, higher yielders require the best combination of RDP and UDP.

RDP/UDP is an essential minimum in rationing high performance herds.

Protein requirements can best be assessed by two simple equations:

• RDP requirement (grams) = Energy supplied (MJ) x 11.8
• UDP requirement (grams) = (RDP ÷ 100) x yield (litres)

Developed from the original RDP/UDP system, metabolisable protein (MP) is an even more sophisticated system.

In addition to the different types of protein, it takes account of a number of dietary interactions which affect protein utilisation, most notably the amount and form of energy supplied.

MP forms the basis of the Feed into Milk rationing software and is widely used in rationing programs.

Cows also need to be provided with sufficient minerals and vitamins to avoid deficiencies or imbalances.

Although deficiencies can have a disproportionate impact on performance, the fact that most feed ingredients – especially forages – provide reasonable levels of most minerals and vitamins means they only need to be considered as a final check in rationing.

Overfeeding of minerals is a source of serious financial wastage in many herds and toxicities caused by excess minerals can be as harmful as deficiencies.