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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Soluble Carbohydrates (starch and sugars)
If cows have sufficient feed (i.e. target residuals are 1500 - 1600 kg DM/ha with good pasture utilisation), supplementing with starch-based concentrates is unlikely to improve reproduction.
If a pasture deficit exists, it is most important to increase the energy (MJ ME) available to the cow. The type of supplement used to achieve this is secondary.
Related information
- FeedRight InfoSheet: Feeding starch or sugar after calving to mating will reduce empty rates – true or false?
- Inside Dairy (October 2011) - Myth Buster page 21
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Fibre
Fibre is the primary source of energy for grazing animals, but is also required to stimulate chewing and saliva production.
Neutral Detergent Fibre (NDF) is the most common measure of fibre used for animal feed analysis. It measures more than 90% of the structural components in plant cells (i.e. lignin, hemicellulose and cellulose).
For cows grazing only high quality pasture the minimum NDF requirements are 28-32%.
There is no benefit of feeding additional fibre (i.e. straw or hay) to lactating cows when pasture makes up more than 60 % of the diet.
In diets containing large amounts of finely ground grain and by-products or precision chopped silages, recommended fibre levels have been further refined to include the term 'effective fibre' (eNDF). This term is used to describe the fibre that is most effective at promoting chewing. For example, the fibre in fresh pasture or silage (pasture, cereal or maize) is very effective and the fibre in cereal grains is 0% effective in promoting chewing. However, currently there is no accurate way to measure eNDF of long forages (fresh pasture, or grass or cereal silage), hence eNDF figures have not been included in the feed table.
Counting cud chews to evaluate dietary fibre and rumen function is not necessary in a New Zealand pasture-based system as New Zealand pastures will always provide sufficient fibre.
Related information
FeedRight InfoSheet: Do lactating cows on pasture require additional fibre?
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Protein
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 New Zealand 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
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Fat
Fat is not limiting in pasture-based diets as fresh pasture contains 3-5% fat.
Fat supplements can be classed into two categories:
- Protected or bypass fats: These are either chemically treated (calcium soaps) or saturated fats (e.g. HyFat) that pass through the rumen unaltered.
- Unprotected fats: These are unsaturated fats that require hydrogenation in the rumen.
Fat supplements are generally expensive and the gains in milk revenue are rarely profitable.
Related information
FeedRight InfoSheet: Feeding Fat Supplements
Nutritional Guidelines
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Soluble Carbohydrates (starch and sugars)
Pasture + Supplement, TMR
(% diet DM)
Maximum total soluble carbohydrate
38
Maximum starch
30
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Fibre
As a general rule for all diets
(% diet DM)
Minimum NDF
35
Minimum effective fibre (eNDF)
17
Pasture + supplements, TMR
(% diet DM)
Minimum NDF
27-33
Minimum effective fibre (eNDF)
20
Minimum ADF
19-21
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Protein
Good quality all-pasture diets
kg milk/cow/day
kg MS/cow/day
Protein content of diet required % DM
20
1.6
18
30
2.4
24
Pasture + supplement, TMR
kg milk/cow/day
kg MS/cow/day
Protein content of diet required % DM
20
1.6
16
(65% degradable, 35% bypass, 32% soluble)
30
2.4
18
(65% degradable, 35% bypass, 32% soluble)
As a general rule for all diets
(% diet DM)
Early lactation
18
Mid lactation
16
Late lactation
14
Dry cow
12
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Fat
Pasture + Supplement, TMR
(% diet DM)
Maximum additional unprotected fat
3
Maximum additional protected fat
3
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Macro Minerals
All diets for high production - (2 kgMS/cow/day)
Mineral content of diet required (%DM)
Calcium
0.6-0.8
Phosphorus
0.3-0.35
Magnesium
0.22-0.28
Potassium
1.0+
Sulphur
0.23
Sodium
0.20
Chlorine
0.25
