Nutrition and milk protein percentage

Improvements in diet composition offer the quickest and sometimes the largest potential for lifting milk protein percentage. Milk protein percentage is a particularly important issue in Australia's subtropical and tropical regions, where milk protein tends to decline in the spring and summer months.

Feeding for higher milk protein percentage

Feed intake

(Refer to Feed intake and Factors affecting feed intake )

Maximising daily feed intake has a major impact on milk protein percentage, allowing more energy and protein to be available for milk yield and components.

Aim for a daily dry matter (DM) intake of at least 3% of body weight. High-producing cows can eat more than 4% of their body weight. Refer to Feed intake for ways to calculate/estimate feed intake. Neutral detergent fibre (NDF) content of the diet is a major limitation to DM intake.

Cows that are not leaving any feed in troughs or paddocks are hungry, and are not eating their potential intake. Insufficient energy intake is the most common dietary factor causing low milk protein percentage and yield.

Energy is required to produce rumen microbial protein, which in turn becomes an important source of protein for milk protein production. Most energy for the rumen microbes is sourced from the starch, sugar and fibre in the cow's diet.

Starch

(Refer to Balancing the diet )

Increasing the starch content of the diet is an effective way to increase milk protein percentage. Aim for 22-25% starch in the diet.

Starch sources are degraded by rumen microbes at varying rates (15-80% per hour). Rapidly degraded starch can quickly provide readily available energy; however, starch overloading can cause the rapid production of lactic acid in the rumen leading to ruminal acidosis.

Starch sources include:

  • cereal grains
  • pulses (chickpeas and mung beans)
  • cereal silage (including corn, barley, sorghum and wheat silage)
  • vegetable waste (including potatoes, peas and beans)
  • bread and bakery waste.

Processing changes the speed and extent of starch digestion by rumen microbes. The more highly processed the grain, the faster starch is broken down. Steam flaking and hammer milling increase rumen degradability of starch more than dry rolling and cracking of grain.

Sugar

Both an excess of sugar (e.g. high levels of molasses) and low levels of sugar in the diet can lead to low milk protein percentage. Too much sugar can cause rumen pH to drop (acidosis). Aim for 3-6% sugars in diet (up to 10% is okay on mature tropical grass/molasses diets).

Temperate and lush tropical grasses are good sources of sugars. Their sugar reserves increase throughout the day and peak in the late afternoon. Other sources include molasses, citrus pulp and other fruit by-products.

Sugar is rapidly digested (within a quarter to half of an hour) by rumen microbes, but has proved less effective than starches at increasing milk protein percentage.

Dietary protein

Generally, dietary protein is not the main limiting factor for milk protein percentage. Potential situations where dietary protein could limit milk protein percentage include diets of mature, low-protein tropical grasses fed with energy but no protein supplements, or on silage/hay diets where forage is grown in nitrogen-deficient soils.

A good indicator of low dietary crude protein is when a cow´s body condition increases, often accompanied by lower milk yield, milk protein percentage and milk protein yield. Aim for 13-16% crude protein (CP) in the diet, depending on production and stage of lactation.

For optimal microbial production, available protein in the rumen must balance available energy. Rapidly available protein sources include:

  • lush pasture
  • legume grains
  • protein meals
  • urea.

For high-production cows (greater than 30 L/cow/day), diets should also be balanced for amino acids, particularly lysine and methionine for optimum milk protein percentage. Good quality protein meals, such as soybean and canola meal, also offer some bypass proteins, which are required in these diets.

Dietary fat

High levels of fat in the diet (greater than 5-6%) can affect rumen health and potentially reduce milk protein percentage. Fat sources from feeds such as whole cottonseed have less impact on rumen health as they are slowly broken down in the rumen.

For average feed composition of major feed types, refer to feed tables in the Protein Plu$ checkbook or to the Feed Plu$ feed analysis database. Both are available from Dairyinfo.