Think of the cow as a production unit. Procurement is where the cow consumes the feed which must pass a strict quality control test. The feed is passed on to the manufacturing department – also known as the rumen. The rumen produces the raw materials required for the production of milk. These are then passed on to the packaging and shipping department, also known as the mammary gland. In the mammary gland the glucose, lipids, proteins and minerals are packaged and secreted. As nutritionist, consultants and dairy men, we tend to concentrate our efforts on the rumen and digestive tract. However, no matter how many nutrients are produced in the rumen, if the mammary gland is not able to package them they will be deposited as fat, protein and carbohydrate reserves. This is wasteful, energetically expensive and may affect any subsequent lactation.
The packaging and shipping departments are quite dynamic in terms of their capacity. The secretory epithelium alters the number of cells actively producing, in response to the availability of nutrients. Starving these cells leads to the build-up of free radicals and is followed by oxidative stress.
So practically, how do we establish and maintain a highly active mammary gland which maintains its persistency throughout lactation?
The secretory tissue is composed mainly of protein, so we need to ensure sufficient protein is available during the late gestation and early lactation periods – this must also be the correct form of protein. Don’t ask for a specific CP level, the requirement is 1300g of metabolizable protein.
Avoid oxidative stress. Although feeding additional antioxidants such as vitamin E and selenium will reduce the harmful effects of free radicals on cells, always set out with the goal of removing the causes: overcrowding, butyric silage, mycotoxins, ketosis to name but a few. Ensure adequate levels of chelated zinc in the diet of dry and fresh cows; zinc improves the integrity and survival of mammary epithelial cells. Maintain constant energy intakes, as I mentioned earlier, the mammary gland adapts synthetic capacity to the level of available nutrients. Sudden drops in energy intake caused by group changes, forage quality, disease can trigger an unrecoverable decline in secretory cell numbers, which will negatively impact persistency. This has less of an impact in early lactation and in heifers, as the mammary gland in these two groups is able to put cells to sleep and wake them up when energy levels increase. In older cows this is linked to insulin resistance during early lactation, whereas in heifers it may be associated with fact that in most cases they are still growing throughout lactation.
Persistency is the key to efficiency and consistency = persistency.