If you asked me several years ago about lactose, I would have said it makes up 4.8% of milk and is the osmotic regulator of milk volume and left it at that. Not much interest was paid to lactose for many years as it has been viewed widely to be a low-information trait. However, with routine milk analysis by mid-infrared spectroscopy in DHI and payment testing programs, lactose results have been reported and monitored more frequently. In our milk laboratory, I am amazed at just how different the lactose content (percentage) is among our individual cow milk samples as well as in milk samples from different herds.
In the Journal of Dairy Science, there are a couple of excellent review articles on lactose from both the production and dairy foods perspectives (Costa and colleagues, 2019; Portnoy and Barbano, 2021) that have influenced my current thinking about lactose. Lactose is a major milk solid but has the lowest commercial value when compared with other milk solids such as fat and protein.
Many methods exist to determine lactose concentration with the most relevant methods being enzymatic assays, high performance liquid chromatography (HPLC), and mid-infrared spectroscopy analysis. Portnoy and Barbano emphasized that lactose should be reported as anhydrous lactose because lactose data are being used to make increasingly important decisions in dairy food processing and dairy herd management. In 2017, the USDA federal milk market laboratories started reporting lactose content as anhydrous lactose and discontinued the reporting of lactose by difference. Consistent and accurate methods are critical when developing new dairy products and marketing existing products as well as finding new, value-added uses for lactose. Also, good quality lactose data are needed if lactose is to be used for farm management decision making especially in the context of production efficiency and health status. Understanding factors that affect lactose content is important as there is a challenge of finding economically viable uses for lactose, especially when lactose is a byproduct of cheese, high protein milk, or whey powder manufacturing. Portnoy and Barbano suggested that farmers should focus on high-value milk components such as fat and protein produced per unit of lactose produced by dairy cows.
In a study reported in the Journal of Dairy Science this year, Gargiuio and colleagues explored sources of variation for lactose production in dairy cows. Their dataset included 14 years (2008 to 2022) and 393,772 records from 33,280 cows from 85 herds. They found significant variation in lactose percentage (LP), lactose yield (LY), and milk solids yield (MSY) across breed, parity, stage of lactation, and season. Holsteins had the highest LY and MSY and lowest LP whereas Jerseys had the highest ratio of MSY:LY. The LP, LY, and MSY:LY are moderately heritable with values ranging from 0.24 to 0.33. Gargiuio and colleagues suggested that identification of cows with lower LP or higher MSY:LY ratio is an opportunity for selective breeding to increase the production of economically important milk components while decreasing milk volume leading to a better milk production efficiency. Milk from first lactation cows had higher LP and lower LY than older cows. Interestingly, LP did not follow the usual lactation curve shape of fat and protein percentages with LP being highest in early lactation and lowest in late lactation. As expected, LY was related to milk yield and peaked during early to mid-lactation. Cows with a lower LP had a higher somatic cell count although the correlation was low (-0.3). The LP was highest in the summer and lowest in the fall whereas LY was highest in the spring and lowest in the fall.
By understanding and starting to manage factors that influence lactose production, farmers may be able to improve milk production efficiency and economic return.
— Heather Dann