The average freezing point depression (FPD) of milk from GB dairy herds has fallen over time from 521m˚C in 2006 to 516m˚C in 2020. On the back of this decline, new standards were set a year ago in May 2021 with GB milk now being acceptable with a FPD of 505m˚C or above for tanker and retail milk. The previous standard was 509m˚C or above. The decline was thought to be a naturally occurring phenomenon, with no known explanation and not due to increased presence of extraneous water in milk.
The FDP is a measure of extraneous water in milk. While water freezes at 0˚C, the freezing point will drop as more solids/particles are dissolved in water. Raw milk has a freezing point in the region of -0.512˚C to -0.550˚C, which is reported as a whole number: 512 to 550m˚C. Therefore, if water accidentally (or deliberately) is added to the bulk tank, the freezing point of the milk will move closer to 0˚C with a FPD less than 505˚C, resulting in penalty.
Many factors can affect the FPD, the obvious one is water contamination from the milking plant. Others include seasonality, with cases more common in spring and summer due to changes in the diet at turnout and higher temperatures or heat stress increasing water intake. Stage of lactation and protein nutrition are also thought to have an effect.
With a low FDP, the first checks should be with the milking plant to ensure that there is no way for water to enter the bulk tank. Prior to milking check that no water has pooled in the bulk tank from condensation. Plate coolers can also be a source of extraneous water in milk due to leakage from corrosion or pinholes. Consider increasing the pressure of air blasts to make sure that all water is removed from the milk lines. Lastly, if there is a FPD issue, it may be worth running the first milk through the lines to waste or feeding to calves to reduce the risk of leftover water in the system ending up in the bulk tank.
There tends to be more cases of FPD failures in the summer under warmer temperatures and the issue can be exacerbated if water is in short supply or cows have insufficient access to water at grazing. This can cause cows to gorge on water when it does become available and if this occurs close to milking time, more water can end up in the milk due to it being very quickly absorbed through the rumen, into the bloodstream and then transported to the mammary gland. A deficiency in certain minerals (especially sodium) can also be a contributing factor with cows losing potassium and sodium when they sweat. Ensure good access to mineral supplements or free access rock salt under high temperature conditions.
Dietary imbalances leading to a low milk protein content (and low urea) have been shown to lower the FPD. Block calving herds are potentially more at risk as the FPD as this tends to be lower or closer to 0 in the first 3 months of lactation. This time also corresponds with dietary changes at turnout and warmer temperatures when cows are at peak lactation and have a higher water requirement, along with reduced milk solids.
The first port of call for an issue with FDP is to carefully check the milking plant, including the bulk tank, to pinpoint any areas where water could enter. Check availability of water, especially at grass and ensure troughs are refilling quickly. During periods of hot temperatures, access to water is even more important. A tell ale sign of insufficient space is cows crowding round water troughs when taken in for milking. Take steps to mitigate the effects of heat stress https://www.fas.scot/article/how-are-your-cows-coping-with-the-warmer-weather/ and review nutrition to ensure mineral supplementation and protein supply are adequate.
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