Milk components (fat, protein, lactose, urea) are important in the dairy industry and many milk contracts are based on financial reward or penalty for specific levels of one or another of these components. Milk quality determined by somatic cell counts (SCC) is also a factor taken into consideration by milk processors.
Aside from the financial implications, milk components and quality can give an indication of cow health and management, especially nutrition.
Impact of non-nutritional factors
Milk fat and protein are the components that are most often used to help identify issues with the ration. However, there are other, non-nutritional factors that can have an impact.
Breed (genetics) is reported to account for ~50% of the variation in milk fat and protein levels, so utilizing genetics to manipulate herd milk composition can be a useful long-term strategy. Stage of lactation, age, health status, season, body condition and physical issues with the milking machinery and bulk tank also have the potential to affect milk components and quality. There is a natural drop in milk solids over the summer months, possibly due, in part, to the reduction in intake during hotter weather. Ketotic animals can sometimes show elevated milk fat.
Disease, in particular mastitis, will directly affect SCC. Elevated SCC is indicative of an infectious challenge and an SCC of more than 250,000 is to be avoided. Acceptable SCC values for bulk tank milk would be <150,000 with values over 200,000 indicating an issue that needs to be addressed. Identify individual animals with elevated SCC and treat them; repeat offenders need to be removed from the herd.
Nutrition is key factor affecting milk composition. (123ducu | iStock.com)
Numerous nutritional factors influence milk component levels, not least diet composition. Milk fat is derived from acetate, the volatile fatty acid (VFA) product from ruminal fiber degradation. Thus, increasing the level of fiber digestion will generally see an increase in milk fat. It goes without saying that any compromise to rumen function will negatively affect milk components.
Any reduction in dry matter intake (DMI) will have an impact, as well as energy density and protein content of the diet. Insufficient forage and/or excess fermentable carbohydrates, e.g. starch, can result in milk fat depression. Several studies have shown that amount and type of supplementary dietary fat can influence milk fat content, leading to the phenomenon known as milk fat depression.
It is not only milk components per se but also the relationship between them that can give clues about cow health and the fat-to-protein ratio (FPR) is the most often cited. The ideal ratio is around 1.3, but actual values will be determined by the type of animal and stage of lactation, etc. A protein percentage higher than the fat percentage, an “inverted” ratio, can be indicative of ruminal acidosis, possibly linked to low DMI, poor diet balance and composition, especially a lack of fiber and/or excess concentrate. In this situation, the protein percentage remains normal but the fat percentage drops markedly.
Conversely, excessive milk fat levels can highlight an issue with ketosis and some researchers have found that using a threshold of FPR of >1.42 (or even 1.35) could be used to screen cows for sub-clinical ketosis.
Milk urea nitrogen
Milk urea nitrogen (MUN) is closely linked to blood and plasma urea nitrogen (N) and thus can be a good indicator of how efficiently dietary N is being utilized.
Excess ammonia coming from the rumen is converted to urea by the liver and can be recycled back to the rumen or excreted in the urine, as well as milk. Additionally, excess protein is also broken down and excreted. Both these processes carry an energetic cost to the animal, can have a negative impact on fertility and increase N excretion into the environment.
Feeding system, time after feeding, breed, season and diet balance all affect MUN levels. Milk urea nitrogen levels should be monitored regularly and elevated or fluctuating levels indicate areas for improvement. Ensure sampling is always carried out at the same time relative to feeding but the target should be 8-12 mg/dl. One of the most effective ways to improve how efficiently dietary N is used and, consequently, MUN, is to ensure a well-balanced diet in terms of energy and protein.
Rumen microbes rely on energy to be able to use the N available to them, thus insufficient energy will lead to wasted N.
Most rationing systems will have parameters allowing the nutritionist to gauge the synchrony between rumen-available energy and protein, for example, metabolizable energy (ME) vs. metabolizable protein (MP) in the Feed into Milk system or PDIE vs. PDIN in the French Inra system.
An overview of the interpretation of milk component levels.
However, it goes without saying that the FPR, MUN and SCC are not the only means by which to evaluate cow health. They are tools that can be used in conjunction with others, including manure sieving, body condition scoring, rumen fill scoring, counting chews per cud and the Penn State Particle Separator.
Good nutritional management should be the priority, and this can include the use of feed additives to boost milk solids.