During late gestation and early lactation, the mammary gland produces large numbers of secretory cells with the ability to secrete milk.
This is done in response to changes in circulating hormones and growth factors. The demise of the secretory capability of the mammary gland during mid to late lactation is unavoidable in the pregnant cow and ends with the process of involution when the cow is dried off. The rate of decline is known as persistency.
Persistency of lactation will influence profitability as it dictates the milk yield potential of the herd. It also dictates the energy requirement per kg of milk produced. A herd producing 40kg of milk uses less energy per litre than a herd giving 30L due to the dilution effect on maintenance requirements. Heifers tend to be more persistent than cows (Figure 1). This may in part be due to the fact that heifers calving at a younger age are still growing. Stimuli for growth acts on the mammary gland as well as on the rest of the body causing continued growth of the secretory cells in the mammary gland supporting a flatter lactation curve.
On a herd basis, persistency will, of course, be governed by herd demographics and genetics. However, persistency will also be governed by things such as forage quality, transition, health, environment, milking frequency, season and energy availability. I am sure we have all seen the effect of these in our herds. Quite often two or more of these factors work in cojunction with each other to produce a 1 + 1 = 3 effect.
Here are some examples:
- For financial reasons, we have stopped milking the cows three times and turned the cows out for the summer (changes are – seasonal, dietary, milking frequency)
- We continue turning cows out once they are pregnant throughout the summer until the herd is housed (changes are – seasonal, group change, dietary)
- We have had to move on to feeding the poorer third cut silage which is very low in energy because our first cut is at the back of the clamp (changes are – dietary, energy intake)
- We are autumn calving beginning in August and if the weather is good we keep them out grazing (changes are – seasonal, energy intake)
- We keep the cows in the fresh cow group for around 50 days and then move into the main group (changes are – social, dietary)
In all of these examples, the persistency of lactation will be affected. I refer to these as traumatic episodes or TES. They are traumatic in the sense that they impact on the longevity of the secretory cells in the mammary gland. Interestingly young heifers tend to have the ability to bounce back after a traumatic episode, unlike cows where they will impact persistency for the duration of the lactation. At a cellular level, these traumatic episodes exert their effect through three main mechanisms.
1. Nutrient availability
The secretory capacity of the mammary gland will be in balance with the nutrients available for lactation. It is part of the reason why non-pregnant cows will milk on continuously for an extended period of time. In situations where TES occur and nutrient supply to the mammary gland is compromised the mammary gland will reduce its secretory capacity to match by shedding secretory cells. The continued turnover of secretory cells in the heifer mammary gland means they will be able to recover from this kind of TES. Cows in early lactation would be similar, but from peak onwards, the persistency of a cows lactation is likely to be compromised. Early lactation nutrition is key to establishing a successful lactation. Minimising negative energy balance through good transition management and ensuring adequate levels of energy and metabolise protein in the diet will ensure that the number of secretory cells in the mammary gland is maximised.
Persistency of lactation will influence profitability as it dictates the milk yield potential of the herd
Normal cellular metabolism creates what is known as free radicals. Two of the most familiar free radicals would be the reactive oxygen species (ROS) and reactive nitrogen species (RNS). Free radicals cause damage to cells and cell function and can even cause death of mammary secretory cells. Within the body, antioxidants keep free radicals at
level where they do not cause damage. However, during TES the rate of free radical production can overwhelm the antioxidant capacity of the body and damage to cells, including mammary cells will occur (Figure 2). Oxidative stress
can be caused by the following:
• Heat stress
• Butyric silage or silage high in nitrates
3. Milking frequency and milk removal
Milk accumulation within the mammary gland initiates a negative feedback loop suppressing the production and secretion of milk above the capacity of the udder. This is very similar to the balance that is reached between available nutrients and secretory cell number refereed to in point 1. This is why more frequent milking results in increased milk production and persistency of lactation. Milk production by the udder will be in tune with the volume of milk that can be removed from the udder and the availability of nutrients.
The secretory capacity will be altered through cell death. On a positive note though, more frequent removal of milk in early lactation will have a stimulatory effect on the synthetic capacity of the mammary gland. This will result in improved persistency.
Consistency of management is the key to preventing the occurrence of TES that affect lactation persistency. Some of the points to consider are as follows;
1. Provide a consistent diet
2. Maintain consistent energy intakes in milking cow diets
3. Avoid group changes
4. Reduce the risk of oxidative stress
5. Avoid feeding poor quality silages
6. Focus on transition
7. Ensure adequate levels of dietary antioxidants
From a nutritional perspective, adequate energy and metabolise protein in early lactation will result in the development of a mammary gland capable of high milk production. Consistent management will ensure the longevity of the mammary cells within the mammary gland and a persistent lactation. This can be achieved with Wynnstay’s AminoMatch nutritional approach.