Elizabeth Plunkett, Graduate Research Associate, Department of Animal Sciences, The Ohio State University
The modern dairy cow is a metabolic marvel, capable of breaking milk production records thought to be unachievable just decades ago. While records were meant to be broken and the economic benefits of increased milk production and a more persistent lactation are self-explanatory, this increase in productivity is not without consequences.
Newton’s Third Law of Motion states that for every action, there is an equal and opposite reaction. While Sir Issac Newton certainly was not describing the lactation performance of a dairy cow, his law offers a useful framework. Forces do not exist in isolation, and the outcomes that arise from their interactions depict how nature operates through balance. Therefore, as we select genetically superior cows and increase lactation persistency, thus increasing late lactation milk yields, we have created a circumstance that does not exist in isolation and will impact the proceeding portion of a cow’s life cycle: the dry period.
The goal of the dry period is to replace senescent mammary secretory cells before the next lactation, as milk yield is dependent upon the number of secretory cells and their mean secretory rate (Capuco et al., 2001). Immediately following the cessation of milking, intramammary pressure increases, resulting in shortening and dilation of the teat canal. Ultimately, reducing the efficacy of an important physical barrier, allowing bacteria to enter the mammary gland. Maximum fluid accumulation within the mammary gland occurs 2 to 3 days after dry off (Nickerson, 1989). As involution progresses, total fluid volume in the mammary gland decreases as immune cells are recruited to phagocytose residual milk components (Sordillo and Nickerson, 1988). During the early dry period, the mammary gland is highly susceptible to intramammary infection, which can impair mammary involution and decrease milk production in the subsequent lactation (Bramley, 1975; McDonald and Anderson, 1981)
Current National Mastitis Council (NMC) guidelines recommend that milk production should be below 15 kg/day at the time of dry off. Production levels over 15 kg/day can impair mammary involution and increase the risk of intramammary infection during an extremely fragile tissue state. Today's cows produce well over 15 kg/day at dry off, leading to exacerbated and persistent inflammatory responses and reduced animal welfare. These challenges raise an important question: How can we use management to prepare the mammary gland for involution and ensure a successful transition into the dry period for our high producing cows? A review by Cattaneo et al. (2023) critically evaluated many strategies aimed at reducing milk production prior to dry-off. Below are some highlights.
Moderate Feed Restriction
Restricting feed is not something we often think of as having a positive effect on dairy cows. However, moderate feed restriction can be a useful tool to help lower milk yield prior to dry off. Late lactation cows are often in a positive energy balance, and modulating intake by either reducing the total amount of feed offered or lowering the energy density of the diet can help decrease milk yields by 40 to 60%. By reducing energy intake, a cascade of hormonal changes ensues, creating a more catabolic state that is focused on sending limited nutrients to vital organs and not the mammary gland. The feed restriction method should be approached with caution, as reducing intake too much raises animal welfare concerns and may impair immune function. Additionally, the effects of minor feed restriction on the calf in utero have yet to be investigated. While reducing energy intake may be beneficial for reducing milk yield in high producing cows before dry off, it may be detrimental to the developing calf.
Additive Effect of Feed Restriction and Decreased Milking Frequency
The combination of moderate feed restriction and a decreased milking frequency has been shown to effectively reduce milk yields in late lactation cows. Feed restriction decreases the amount of glucose available for the mammary gland and decreases the expression of glucose transporters on the mammary secretory cells. Whereas lengthening the milking interval acts within the mammary gland to decrease glucose uptake and decrease the expression of key proteins involved in lactose synthesis. Reducing milking frequency in high producing cows around dry off will still cause milk to accumulate in the mammary gland even when only removing one of the previous milkings per day. This will cause milk leakage, indicating the teat is dilated and open, increasing the risk of an intramammary infection prior to dry off. While both strategies have their limitations, when you induce moderate feed restriction and decrease milking frequency together, you are able to regulate milk production at the systemic and local levels, creating more of a transitional and complementary change within the system.
Change is vulnerable. So, it's no surprise that we see increased disease incidence during periods of tissue transition (i.e., weaning, the transition period into lactation, etc.). Tissue remodeling and a shift in primary tissue function induce a variety of systemic changes, resulting in homeorhetic adjustments in metabolism to support this new tissue physiology. As we continue to change cows’ physiological limits, we need to constantly change our mindset and remain vigilant in creating profitable and sustainable solutions for problems yet to occur. Continuing to challenge existing protocols and recognize the biological influence we have during these transition periods to make or break her next lactation will help drive our success as an industry.
References
Bramley, A.J. 1976. Variations in the susceptibility of lactating and non-lactating bovine udders to infection when infused with Escherichia coli. J. Dairy Research 43:205–211. https://doi.org/10.1017/S0022029900015752.
Cattaneo, L., A. Minuti, G.E. Dahl, and E. Trevisi. 2023. Graduate Student Literature Review: The challenge of drying-off high-yielding dairy cows. J. Dairy Science 106:6416–6426. https://doi.org/10.3168/jds.2022-23113.
Capuco, A.V., D.L. Wood, R. Baldwin, K. Mcleod, and M.J. Paape. 2001. Mammary cell number, proliferation, and apoptosis during a bovine lactation: Relation to milk production and effect of bST. J. Dairy Science 84:2177–2187. https://doi.org/10.3168/jds.S0022-0302(01)74664-4.
McDonald, J.S., and A.J. Anderson. 1981. Experimental intramammary infection of the dairy cows with Escherichia coli during the nonlactating period. https://doi.org/10.2460/ajvr.1981.42.02.229.
Nickerson, S.C. 1989. Immunological aspects of mammary involution. J. Dairy Science 72:1665–1678. https://doi.org/10.3168/jds.S0022-0302(89)79278-X.
Sordillo, L.M., and S.C. Nickerson. 1988. Morphologic changes in the bovine mammary gland during involution and lactogenesis. Am. J. Vet. Res. 49:1112–1120. https://doi.org/10.2460/ajvr.1988.49.07.1112.