Rumen phytate degradation in dairy cows is generally high, but it should not be assumed that all phytate-bound phosphorus is fully available.
The amount released can vary with the feed ingredient, processing method, rumen pH stability, rumen passage rate and microbial activity. For vets and nutritionists, this means phosphorus supply needs to be assessed in the context of the complete ration rather than relying on total phosphorus concentration alone.
How much phytate is degraded in the rumen?
Dairy cows can access a large proportion of the phosphorus held within phytate because rumen micro-organisms produce phytase enzymes.
Research has reported high rates of phytate degradation in cattle, while also showing meaningful differences between feeds and diets.
This distinction is important because, if all phytate phosphorus is assumed to be available, usable phosphorus supply may be overstated. Conversely, if phytate phosphorus is underestimated, the ration may be supplemented with phosphorus unnecessarily.
With our high-energy diets and grasses with high soluble sugar content, which require higher levels of dietary phosphorus, it is becoming more common to see signs of deficiency at key stress times. Simultaneously, the pricing of supplemental phosphorus is increasingly making it an expensive part of the feed.
A more useful approach is to firstly maximise rumen performance in order to increase the natural phosphorus sources, only then supplementing any shortfall identified in the feed or blood data.
Why is feed type important?
Phytate is not present in the same form across all feed ingredients. It may be held within the seed structure or associated with proteins and minerals. These differences can influence how easily rumen microorganisms and microbial enzymes can access it. As a result, feeds containing similar amounts of total phosphorus can have different rates of phytate degradation.
This becomes particularly relevant when diets contain a mix of cereal grains, oilseed meals, soybean products, distiller’s grains and other processed plant feeds. The source of the phosphorus is important when looking at the numbers in the dosage.
How does processing affect phytate degradation?
Feed processing can alter the structure of plant material and the accessibility of phytate. Grinding, pelleting, extrusion, drying, and heat treatment can all change the way phytate is presented to rumen microorganisms.
Heat treatment is particularly variable. In some feeds, heating can alter plant structures in ways that make phytate easier to access. In others, heating may increase associations between phytate, proteins and minerals, making microbial breakdown more difficult. The result depends on the ingredient, temperature, heating time and moisture.
Processed feeds should therefore not be assumed to have either fully available or poorly available phytate phosphorus without considering how that ingredient has been handled.
How does rumen passage rate affect phytate breakdown?
It’s important to look at the time feed spends in the rumen. Rumen microorganisms need time to associate with feed particles and for microbial phytase to act on phytate.
Fast rumen turnover (or passage rate) may mean there’s less opportunity for complete phytate degradation before feed leaves the rumen. This is a common problem in high-intake, high-producing cows or diets which cause rapid feed passage through the rumen. These rapid rumen passage rates increasingly become a problem when high-production, rapidly-growing pasture is fed. These pastures often have a high moisture content, inadequate levels of functional fibre and high natural sugar content.
It does not mean these cows cannot use phytate phosphorus. It means the degree of degradation may differ between diets, animals and stages of lactation. This is one reason phosphorus availability should not be treated as a fixed percentage across all feeding situations.
How does rumen condition affect this?
Rumen phytase activity is microbial, so it sits within the wider rumen environment. Stable intake, adequate fibre and normal rumen function support the microbial population responsible for fermentation and digestion. Changes in intake, acute or sub-acute acidosis, feed sorting, sudden dietary changes or poor rumen function can alter microbial activity.
These changes affect more than phytate alone, but phytate degradation should not be considered separately from overall rumen function. When assessing phosphorus availability, we need to take into account the state of the rumen alongside the feed itself.
When might added phytase be useful?
Exogenous phytase is phytase produced outside the animal and added to the diet. It is widely used in pig and poultry feeding because those animals have less opportunity for microbial phytate breakdown before the small intestine. Its role in dairy cattle is more specific because the rumen already provides substantial phytase activity.
Research suggests added phytase can improve phytate and phosphorus digestibility in some dairy diets. Its potential value is likely to depend on factors such as:
- The level and source of phytate in the ration
- How the feed has been processed
- Rumen passage rate
- The cow’s phosphorus requirements
- Existing phosphorus supply
- Overall rumen function
Added phytase should not be treated as automatically necessary. It should also not be assumed that an increase in phytate digestibility will translate directly into more milk or improved production. The relevant question is whether incomplete phytate degradation is limiting phosphorus availability in that ration.
Does phytase change the need for phosphorus supplementation?
Potentially, but the decision still needs to start with the complete diet. Good phosphorus management means supplying enough available phosphorus to meet requirements without routinely feeding more than the cow needs. When assessing a ration, consider:
- Total phosphorus supply
- The feeds contributing that phosphorus
- The proportion likely to be present as phytate
- Feed processing
- Intake
- Passage rate
- Stage of lactation
- Rumen function
- Existing mineral supplementation
Where phosphorus requirements are not being met, direct phosphorus sources such as SoluPhos or monocalcium phosphate may have a role within a balanced mineral programme. The decision should be based on a genuine phosphorus supply gap rather than assuming rumen phytase will release all plant-bound phosphorus.
It is also worth separating a phosphorus shortage from other issues that can produce similar concerns, including poor intake, wider mineral imbalance or another health challenge.
What should vets/nutritionists take from this?
The main point is not that phytate-bound phosphorus is poorly available in cattle, but rather that availability is variable. Cows generally use phytate phosphorus well because rumen microorganisms produce phytase. That natural capacity should be recognised when assessing the ration.
At the same time, phytate degradation is influenced by the feed ingredient, processing, rumen retention time, passage rate and microbial activity. The most useful approach is therefore to assess phosphorus supply within the context of the complete diet rather than treating either total phosphorus or phytate phosphorus as a fixed indicator of what the cow can actually use.
Contact one of the Agvance team for specific advice around how to measure current phosphorus status and how to use this information to ensure your herds are performing at their potential.
Recommended products:
Sources
- Effect of a biosynthetic bacterial 6-phytase on the digestibility of phosphorus and phytate in midlactating dairy cows.
- Determination of in situ ruminal degradation of phytate phosphorus from single and compound feeds in dairy cows using chemical analysis and near-infrared spectroscopy
- Effect of grain source and exogenous phytase on phosphorus digestibility in dairy cows.
- Meta-analysis of factors that affect the utilization efficiency of phosphorus in lactating dairy cows
- Phytase activity of anaerobic ruminal bacteria.
