Analysing your herd's milk docket

The milk docket provides clues about the herd’s diet, though there is no substitute for looking at the herd and farm system to better understand the issues. Milk fat and milk protein values are unique to the herd and looking at past seasons is important and is influenced by both genetics and nutrition.

The relative amounts of protein, watersoluble carbohydrates and fibre available in the rumen at a given time are factors affecting rumen fermentation and therefore milk components. Any diet or management factors that affect rumen fermentation can change milk fat, protein and urea levels.

The normal milk curve for milk protein and fat percentage should follow a ‘Nike tick’ pattern, decreasing after calving as milk volume increases, then steadily increasing as shown in Figure 1.

The main figures on the milk docket that provide give good information on the herd are the milk protein, milk fat and milk urea percentage.

The most common reason for low milk protein percentage is lack of energy. There can be adequate protein in the diet, but the process to make milk protein requires energy, hence a lack of energy will decrease milk protein percentage. Less commonly it is due to a lack of dietary protein. Aim to have the milk protein percentage increasing coming into mating as this indicates adequate energy is in the diet to meet milk production and reproductive requirements.

Ketosis is one cause of an increase in milk fat percentage. In negative energy balance, fat is mobilised from the body to make ketones, lifting milk fat percentage. Increased fibre in the diet is another, as more of the volatile fatty acid acetate is produced. Fat in the diet, such as palm kernel extract and bypass fats, may increase milk fat percentage.

Subclinical or clinical rumen acidosis is a cause of milk fat depression. Rumen acidosis is a risk when fibre is low in the diet, too much water-soluble carbohydrates or a combination of both. Excess fat or unprotected fats in the diet can cause milk fat depression.

Milk urea is a crude measure of protein available to the cow and an indicator of rumen efficiency. It should not be used for making decisions in isolation. Less than approximately 16 may be an indication of a lack of protein in the diet. High milk urea greater than 35 to 40 may reflect high dietary protein or poor rumen function.

To complete an extended feed test of pasture and forages to assess your herd’s diet, contact your local PGG Wrightson Technical Field Representative.