Nutrition and milk fat percentage

Milk composition is affected by many factors, including:

  • body condition
  • stage of lactation
  • number of lactations
  • breed
  • climate.

However, nutrition is one of the most important factors. Improvements in diet composition offer the quickest and, sometimes, the largest potential for lifting milk fat percentage. Any large variation in milk composition indicates an inconsistent diet. A decline in milk fat percentage below 3.3-3.5%  over 2-4 days may indicate a rumen health problem.

Feeding for optimal milk fat percentage

Milk fat percentage of the herd can be maximised by attention to five aspects of the ration:

Dietary fibre

Fibre is important for rumen health and increasing or maintaining milk fat percentage (refer to Managing for healthy rumen function ). A drop in milk fat percentage often occurs (and is therefore a good indicator) when fibre is lacking in the diet. For example, when there are high amounts of readily available starch and/or lush pastures in the diet.

Aim for 28-30% neutral detergent fibre (NDF) in the diet (refer to Balancing the diet ). The rumen requires fibre particles 2-5 cm long that promote cud chewing and rumination to:

  • produce saliva
  • reduce feed size for faster digestion
  • maintain milk fat percentage.

If fibre levels in the diet are adequate, 50% of the herd should be chewing their cud when resting. If less than 30% are chewing their cud when resting, fibre content is too low. If more than 70% are chewing their cud when resting, fibre content is too high.

Forage to concentrate ratio

A low milk fat test indicates a high concentrate/low fibre diet, and the possibility of slug feeding and sub-acute acidosis.

A drop in both milk production and milk fat percentage indicates acidosis. A rumen pH below 5.4 causes fibre-digesting bacteria to die out, and lactic acid-producing bacteria to increase, resulting in acidosis. This can be caused by:

  • abrupt changes in the diet
  • insufficient effective fibre
  • excess rumen-available carbohydrates (usually grain).

To avoid acidosis, digestive upsets and low milk fat percentage, aim for a recommended maximum of 60% concentrate in diet dry matter (DM). Spread high daily grain intake (greater than 6 kg/day) over several feeds for a more steady nutrient supply to the rumen, and to avoid large energy slugs that may lead to acidosis. Move cows onto fresh or conserved forage immediately after feeding grain in the dairy and limit grain fed in the dairy to 3 kg/cow/milking.

Consistent diet

Keep the diet as consistent as possible from day to day and avoid sudden changes in feed types. Rumen microbes take time to recover and build up after sudden feed changes. Fibre-digesting microbes, which provide the precursors for milk fat production, may take 4-6 weeks to build up.

If the diet keeps changing every few days, the required microbes will not be present in sufficient numbers for optimum digestion so the diet should be as consistent as possible.

Rumen buffers

Buffers help stabilise rumen pH, providing a favourable environment for fibre-digesting rumen microbes. Common buffers include sodium bicarbonate and magnesium oxide.

On high grain diets, feeding buffers can reduce the incidence of acidosis. On these diets, buffers help to maintain rumen health and fibre digestion, preventing potential depression of milk fat percentage. Significant milk fat responses have been reported by including dietary buffers when corn silage  is the main forage in the diet.

Provide buffers at 1-1.5 % of ration DM or 150-200 grams/cow/day. Feeding rumen buffers in hot weather may also help maintain/improve milk fat percentage.

Fats and oils

50% of the milk fat synthesized in the mammary gland is derived from fat sources in the diet. However, high levels of fat in the diet (greater than 5-6%) will affect rumen health and potentially reduce milk fat percentage.

Fat sources from feeds such as whole cottonseed (photo at right) have less impact on rumen health because they are broken down slowly in the rumen.