Dairy nutrition is about understanding the nutrient requirements of dairy cows at various stages of lactation and combining various feed alternatives to meet those needs in a cost-effective manner.
The nutrients required by dairy cows are energy, fibre, protein, water, vitamins and minerals.
Pasture provides a balanced feed source and generally only provides a profitable response to supplementation if there is a pasture deficit.
Energy is the key driver of milk production. It determines milk yield, milk composition (fat and protein content) and body weight. Energy is measured as megajoules (MJ) of metabolisable energy (ME).
The main feed energy sources are soluble carbohydrates (starch, sugar) and fibre. Protein and fats in feeds can also be used for energy sources.
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In practice high protein pastures meet the protein requirements for high milksolids production. The table below shows when protein may limit production for cows fed high quality pasture.
Nutrients first-limiting milk production on high quality pasture diets
kg milk/cow/day Approx. kg MS/cow Nutrient first limiting milk production 20 1.6 Energy (protein in pasture> 18%) 25 2.0 Energy (protein in pasture> 24%) 30 2.4 Energy and protein 35 2.8 Protein It would be rare in Kenya for cows to be deficient in protein unless more than 50 percent of the diet was a low protein supplement (e.g. maize silage at 7.5 % crude protein). Pasture has 50 to 100 percent more protein than is required by the cow.
Protein to fat (P:F) ratio
The P:F ratio is calculated by dividing the milk protein % by the milk fat %. There are three main reasons why P:F ratio is considered important:
- Protein to fat ratio is considered by some nutritionists as an indicator of energy balance, with a lower than average ratio reflecting underfeeding and a higher than average ratio reflective of a well-fed cow.
- Milk protein is worth approximately twice as much as milk fat in current milk payment systems, primarily because of the value of milk powders relative to butter and cheese. Therefore, a higher P:F ratio will result in a greater milksolids price.
- Cows with a higher P:F ratio are more likely to get pregnant.
If using the P:F ratio to determine whether or not to feed cows with supplement, the following points should be considered:
- P:F ratio can be influenced by diet. In particular, cows consuming starch will have a higher P:F ratio than cows consuming fibre.
- P:F ratio is not a sensitive measure of energy status and should not be used alone to make decisions on whether or not to supplement cows.
- P:F ratio can be used in conjunction with postgrazing residuals and milk yield/cow to determine, if it is appropriate to provide cows with a supplement.
- Although a high P:F ratio is associated with increased pregnancy rates, there is no evidence that increasing P:F ratio through nutrition has this effect; in fact, there is compelling evidence of a negative effect of starch-based feeds on pregnancy rates.
Related information
FeedRight InfoSheet: Protein to fat ratio Technical Series (November 2011): Is protein supplementation needed during summer? pages 6-10
Nutritional Guidelines