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Physiology of lactation in dairy cattle—challenges to sustainable production
C H A P T E R
7 Physiology of lactation in dairy cattledchallenges to sustainable production Geoffrey E. Dahl Department of Animal Sciences, University of Florida, Gainesville, FL, United States
O U T L I N E Current state of affairs
121
Genetic innovations
126
Mammary growth and function
122
Animal health and well-being
126
Nutrition and metabolism
124
Housing and monitoring
127
Reproduction
125
References
128
Current state of affairs Milk is defined as a “fluid secreted by the mammary gland of females for the nourishment of their young.”1 Milk from ruminants, especially cows, provides a significant source of nutrient dense calories in the form of fluid milk and associated products such as cheese and yogurt. Indeed, a growing body of evidence suggests that including animal source foods in the diet overcomes nutrient deficiencies associated with their lack in the diet, particularly physical
Animal Agriculture https://doi.org/10.1016/B978-0-12-817052-6.00007-0
and cognitive stunting.2,3 And, milk protein is the most efficient of all animal proteins when compared on a production per unit of land basis, almost twice the efficiency of poultry meat and rivaling that of maize protein (Fig. 7.1).4 Dairy production ultimately depends on the rate of milk synthesis in the mammary gland, which can be affected by a number of factors. In the US and most developed countries, milk yield per cow has increased tremendously in the past 70 years due to improvements in genetic selection for yield and technical advances in feeding,
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FIG. 7.1 Estimates of square meters of land required to produce 1 gm of human edible protein from various crops or production systems. Copied with permission from Britt et al.4
management and health that support expression of the higher genetic potential. In 2017, the average annual yield per cow was 10,404 kg, and the record yield is 35,451 kg.5 Thus, the modern dairy cow, properly managed, can produce a substantial volume of nutrient rich, human consumable food. And, the potential is more than three-fold greater than currently realized. In developing countries, however, milk yield hovers between 500 and 1,500 kg annually.6 In many countries native breeds serve as the base of the dairy herd, and genetic limitations are apparent in those animals. But, more important to explanation of low yields are management and feeding deficiencies that hamper the ability of even those animals with lower genetic potential to express greater productivity. With the demand for animal source foods increasing in many developing countries as incomes rise, and as awareness of the importance of animal source foods to prevent stunting increases, there is a push to improve the output of milk in total and on a per cow basis. Much of this increase
can be realized with improved feed availability, better management of reproduction and health, and improvements in animal and milk harvest hygiene. Over the next 30 years, continued improvements in milk output will be achieved in developed and developing countries. Rather than a review of mammary biology, the rest of this chapter considers broad areas related to mammary function and cow management that are likely to impact the continued rise in milk yields. Sustainably producing milk in 2050 is likely to be associated with continued increases in output on a per cow basis, but that rise in yield will only occur with improvements in genetic selection, nutrition and management of the herd worldwide. A variety of management interventions are discussed below and some of these factors are summarized in Fig. 7.2.
Mammary growth and function The mammary gland is a modified skin gland developmentally, and secretion of milk components is controlled by the endocrine system.7 Structurally, the secretory epithelial cells are organized into alveoli, which expel milk components into the open area apical to the cell orientation. With regard to structure, there is no difference in the basic secretory unit regardless of productive potential, rather, it is the number of secretory cells that are linked to productivity. Whereas udder size is not highly correlated with output on an animal-to-animal basis, per se, as lactation number advances and udder volume increases there is an increase in milk output. There is little reason to envision substantial increases in udder volume in the future, rather, more efficient output is the route to higher yields. That is not to say, however, that mammary epithelial cell number cannot be manipulated. There is constant loss and regeneration of cells during lactation, but cows typically experience a net loss of about
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FIG. 7.2 Depiction of the relationship of mammary cell number and milk yield throughout the lactation cycle of a cow, with emphasis on factors that can alter cell number and activity across the lactation and dry period. The lactation curve is characterized by a peak in cell number and then activity during the initial 6e8 weeks in milk, as depicted in the open curve. The solid black curve depicts the typical milk yield response observed in cows in many emerging and developing country systems, characterizing the lower peak and overall yield of milk, as well as the shorter duration of lactation. Early in lactation, cell number increases and that can be further stimulated by more frequent milking (IMF) in the first 3 weeks in milk. Following the peak of lactation, cell number and thus, yield, decline as lactation advances. A number of factors such as poor nutrition, sub-clinical mastitis as indicated by high SCC, heat stress and incomplete milk removal can limit the peak and accelerate the decline in yield. Conversely, management interventions such as bovine somatotropin (bST), increased milking frequency, and long day photoperiod can be used to slow the decline in even increase the yield of well-fed cows (illustrated as the shaded curve). The dry period is characterized by involution and some cell loss followed by regeneration of mammary cells as parturition approaches. Dry period interventions that increase mammary development and subsequent yield (i.e., the shaded curve) include short day photoperiod exposure and heat stress abatement. Therefore, many management factors can be manipulated throughout the lactation cycle to further enhance mammary gland output. These factors are discussed throughout this chapter. Adapted from Capuco and Ellis.8
50% of the secretory cells they begin lactation with as that lactation advances.8 Indeed, one area of particular interest is mammary stem cell biology and how that may be harnessed to increase the number and duration of activity of mammary epithelial cells in a given lactation. Mammary growth or mammogenesis begins in utero and development mirrors overall body growth until puberty. Estrogen and progesterone are the dominant hormones related to mammary growth, although adrenal steroids, somatotropin, prolactin and a number of local growth factors also play roles.9 At the onset of estrous cyclicity (or menstruation in primates) the waves of estrogen and progesterone during the cycle drive progressive increases in ductular
expansion into the mammary fat pad, as well as some alveolar development. With pregnancy, mammary growth accelerates under the synergistic effects of greater circulating concentrations of estrogen and progesterone. This is particularly evident with regard to secretory tissue development. But, growth in itself does not mean that milk secretion begins without the process of lactogenesis, which occurs around the time of parturition in response to the periparturient surge of prolactin. That increase in prolactin activates the cellular mechanisms responsible for lactose (and casein) production, and lactose secretion from the mammary epithelial cell results in fluid movement into the alveolar lumen, thus milk secretion. Maintenance of
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milk secretion, i.e., galactopoiesis, depends on continued milk removal from the gland, and endocrine system support via somatotropin, prolactin, thyroid hormones and cortisol; the gonadal steroids have limited effects after lactation is established. With regard to mammogenesis, there are a number of management factors that can be associated with reductions in ultimate productivity, likely because of impaired mammary growth and development. One of the most studied is that of excessive fat accumulation prepubertally and postpubertally, which limits the extent of parenchymal infiltration of the mammary fat pad and, ultimately, the volume of secretory tissue in the gland. Before puberty, when mammary growth proceeds in an allometric manner, nutrient supply appears to be critical to maximize growth. Specifically, when high volumes of whole milk are fed to developing heifers, mammary growth increases and subsequent first lactation yields are improved,10 but these effects appear to be independent of skeletal growth. After puberty, when mammary development returns to an isometric growth rate, neither nutrient supply nor stimulation of the somatotropic axis alters parenchymal accumulation, although overall body growth was improved.11,12 Thus, it appears that prior to puberty, nutrition limits maximal mammary growth whereas that influence recedes after puberty. Manipulation of photoperiod, or the duration of light that a heifer is exposed to each day will also impact mammary growth and eventually, production of milk. Calves that are raised under long days of 16 h of light and 8 h of darkness will have increased mammary parenchymal development when compared to herdmates housed under a short day photoperiod of 8 h of light and 16 h of darkness. Both prolactin and insulin-like growth factor-1 (IGF-1) are increased in heifers housed on long days, and they may be the drivers to increase parenchymal growth and lean body growth in general.13 When those heifers calve, their first lactation yields exceed
those of short-day heifers, further supporting the concept that parenchymal growth is related to milk yield, and that management factors can impact mammary development long before the first lactation begins. Heat stress is the other environmental factor that has significant impacts on mammogenesis and mammary function, both in mature cows and during development. While the effects of heat stress during an established lactation are well described,14 recent studies support the concept that heat stress in late gestation reduces mammary growth in the dry period which leads to lower milk yield in the subsequent lactation and poorer immune status through the transition into lactation.15 Of interest, the developing fetus also suffers negative effects of heat stress, wherein calves born to heat stressed dams subsequently produce less milk in their initial lactation, suggesting that in utero heat stress alters mammary development for life, likely through epigenetic alterations of the genome.16 Because the majority of cows producing milk reside in subtropical or tropical climates worldwide, these findings have significant relevance to future improvements in the efficiency of milk production around the globe.
Nutrition and metabolism The ruminant digestive system offers a significant advantage with regard to feed availability and byproduct utilization. This is particularly relevant when considering the conversion of non-human consumable feeds and byproducts into highly nutritious milk and milk products. For example, crop residues, oilseed meals and other byproducts of food manufacturing comprise at least 40% of dairy diets in developed countries, and are an ever-growing contribution to rations in developing countries.17 In addition, the list of novel alternative feeds is expanding, particularly in developing countries, where scarce land and water resources limit overall
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Reproduction
feed production capacity.18 This is a significant component of the sustainable nature of milk as a food for human consumption. Indeed, by converting non-human consumable byproducts and inedible highly fibrous plant material to milk, dairy cows fill a niche that would otherwise be a drag on waste streams. Further, those byproducts and other roughages would still produce methane as they decayed, so the net production of nutritious food per unit of greenhouse gas (GHG) is likely underestimated because no credit is assigned for the conversion. Despite their ability to convert a variety of forage and byproducts to milk, the volume of feed needed for dairy cows presents an ongoing challenge, one that is exacerbated early in lactation. The metabolic load of a high yielding cow in early lactation represents a significant challenge to maintain normal function while simultaneously secreting 1.5 kg of milk protein, 2.5 kg of lactose and 1.75 kg of milk fat each day, assuming a typical 50 L of milk daily yield. The output of energy dwarfs the maintenance requirements of the rest of the body, and it is perhaps most accurate to say that the cow is an appendage of the gland in early lactation, rather than the reverse. Cows are simply incapable of consuming enough dry matter to meet their energy requirements in early lactation and they enter a phase of negative energy balance, and must mobilize tissue energy reserves. There is a coordination of the somatotropic system such that the catabolic actions predominate to increase mobilization of fat stores,19 and this state continues for 6e8 weeks into lactation when energy intake from dry matter intake exceeds that of milk energy output and energy balance is restored. While substantial literature suggests that the consequences of negative energy balance, including elevated circulating concentrations non-esterified fatty acids and ketone bodies, predisposes the cow in early lactation to metabolic and pathogen-induced disease, it is a normal physiological process that can be monitored
and managed to limit negative effects.20 One of the primary issues is appropriate nutrition during the dry period to avoid excessive body condition at calving. Managing the nutrient intake of cows during the dry period to limit conditioning should be a priority. Significant bodyweight gains, or extended lengths of the dry period, have been associated with greater risk of metabolic disease post-partum. In contrast, dry periods of under 30 days can limit productivity in the next lactation. A number of studies have investigated nutritional management to reduce the nutrient density of prepartum diets to slow the accumulation of body condition in dry cows so that a dry period of 45e60 days can be managed effectively and without overconditioning the cow.21
Reproduction As the final phase of the reproductive process in mammals, normal lactation depends on successful reproduction, and that is particularly important in dairy cattle as milk yield wanes with advancing lactation. Therefore, maintaining annual calving results in the greatest overall milk output. Reproductive performance in heifers is typically quite robust in a well-managed herd. But a steady decline in daughter pregnancy rate (DPR) was associated with increased emphasis on selection for milk yield from 1960 to 2010, and reproductive performance suffered as yield climbed.22 Over the past decade, however, the DPR has increased primarily because of two factors. First, improved genetic selection for DPR has shown significant improvement in reproductive performance.23 Second, the advent of timed breeding protocols, i.e., Ov-synch, have dramatically improved the ability to achieve high levels of reproductive performance in lactating dairy cows.24 The improved ability to attain reproductive success without sacrificing continued milk yield increments, however, may not be limitless. Consumer pressure over the
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use of exogenous hormones for management may affect the ability to take advantage of systems to synchronize ovulation in the future. But, other technologies are being developed and perfected to replace older techniques that become obsolete or unavailable. For example, cow activity monitoring and in-line milk progesterone monitoring25,26 are currently being used to determine estrus in a non-invasive manner. But those approaches require significant financial investments and may not be suitable in many situations in developing countries. Selection for reproductive performance using genomic, and eventually epigenomic methods, may further improve our ability to realize highly fertility along with high milk yield. Sex sorted semen allows for gender selection of over 90%, a substantial boon to the dairy industry with regard to the production of heifers as potentially lactating animals. Obviously the number of females is increased, but with that selection comes a greater ability to improve the overall genetic value of the herd. Alternatively, with the advent of sexed semen, there has been a significant increase in heifer availability throughout the dairy industry in North America.27 As the number of available heifers increases, alternative approaches to breeding the entire herd are realized, specifically the selective mating of the bottom half of the herd to beef sires, in an effort increase the value of those offspring for the beef market. In countries where the dairy industry is less developed, the application of AI is useful to accelerate genetic progress. But, it is important to consider the management and feeding capacity of those countries in concert with genetic improvement, as forage quantity and quality in particular may mask expression of the genetic capacity for production.
Genetic innovations Genomic selection has had a tremendous impact on selection of dairy cattle for superior
traits, but this has been especially true for health and well-being traits relative to production traits.23 As improvements are made in identification of markers for disease resistance and reproductive performance, and even for geographic or management specific traits, it should be possible to select for more resilient animals that are best suited for that production system (e.g., pasture vs. freestall). In addition, better understanding of the epigenomic28 effects on mammary gland function and metabolism will enhance our ability to identify highly productive cows that function well within specific production systems. Indeed, advances in genomic and even epigenomic selection may be of huge impact in developing countries to improve yield in indigenous breeds without sacrificing adaptive traits.29 Selection of superior indigenous bulls may then allow for wider propagation of those improved genetics through coordinated breeding programs using AI and other advanced reproductive technologies. However, it is critical that performance estimates be confirmed under local conditions of adequate management, or improvements will not be realized at the farm level.
Animal health and well-being Udder health is a key component of successful lactation. Mastitis, or inflammation of the gland, is most commonly caused by an invasion of bacterial pathogens, and can cause substantial reductions in yield and quality of milk, and even death of the cow depending on the pathogen and associated complications.30 Despite improvements in milk harvest and sanitation, mastitis continues to be a problem in the dairy industry. Antibiotics are routinely used as a therapy, but a growing body of evidence suggests that many infections will clear-up spontaneously without antimicrobial therapy and the pathogen profile is shifting away from microorganisms that are sensitive to those products available for use in food producing animals.31 Indeed,
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improvements in on-farm identification of particular mastitis pathogens to more effectively target responsive pathogens is likely to realize a reduction in antibiotic use and greater cure rates overall relative to the less discriminate treatment strategies of the past. Moreover, supportive therapies, including fluids and nonsteroidal anti-inflammatory drugs (NSAID’s), to deal with the broader effects of infection with gram negative pathogens such as E. Coli are an important consideration to recovery of the animal and a functional mammary gland.32 Stem cell biology may be useful in overcoming the loss of productivity associated with mastitis, particularly in instances where mammary tissue loss occurs.8 In many cases of severe mastitis, local inflammation of the gland results in loss of secretory tissue and even functionality of individual quarters. Typically, milk secretion from those quarters never recovers, which suggests a loss of regenerative capacity of the mammary epithelial cells. With greater understanding of stem cell biology, it may be possible to develop therapies that overcome the loss of secretory capacity and allow for recovery of milk production from an infected quarter of the mammary gland. Blanket antibiotic therapy for dry cows has yielded great improvements in reducing chronic infections in cows, but is an area of substantial research emphasis currently. There is the potential to dramatically limit the use of antibiotics in the dairy industry. That is due to the fact that many cows may have no infection at the end of lactation, yet all quarters of the mammary gland are treated regardless of infection status. Selecting specific cows or quarters to receive dry cow antibiotic therapy will require improved screening methods to ensure pathogen identification, and a higher level of individual cow management.33 Practices such as gradually reducing milk removal and pharmaceutical interventions34 to rapidly limit milk secretion may sufficiently limit residual milk, even in high producing cows at dry off. This would decrease the potential
for new intramammary infections during the dry period, but field based evidence is lacking to confirm the utility of those approaches. While judicious use of antibiotics will continue to be important for managing mastitis in dairy cattle, consumers and regulators are exerting pressure to minimize the amount and range of antibiotic therapies. Therefore, improvements in host ability to resist pathogen infiltration and establishment of infection offers a potential approach to reduce the volume and necessity of antibiotic use related to dairy management. Transgenic approaches have been used to achieve this goal, but with limited success.35 In addition, these methods would likely face regulatory and consumer acceptance burdens that would limit their rapid translation into industry practice.
Housing and monitoring Evidence continues to emerge that housing affects cow productivity and welfare, and represents an area for further management interventions. Free-stall barns that allow for expression of individual cow behavior for feeding, grooming and mobility are now used widely in developed countries. That will likely continue as the housing method of choice, while tie-stalls and other barn designs that limit animal movement will decline in use. Pasture, as a housing system (vs. nutrition), is an area of interest, as some perceive this as a more “natural” choice for cattle and promote movement toward having all cows having the option for all cows to have outdoor access. However, it is important to consider the effects of heat stress on pastured cattle as there are significant limitations to cooling systems in the pasture setting compared with cows housed in conventional barns.36 As previously mentioned, photoperiod and heat stress can impact mammogenesis and thus alter mammary output of milk. However, there are also direct effects of both environmental factors on milk production. Manipulation of
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photoperiod improves the production efficiency during lactation and the potential for milk yield during the dry period.37 Therefore, appropriate management of the lighting system to which cows are exposed in modern confinement operations can yield significant improvements in efficiency. Heat stress is an example of an environmental insult to productive efficiency across the life cycle of the dairy cow. Lactating cows decrease dry matter intake under conditions above a temperature:humidity index (THI) of 72, and experience a corresponding reduction in milk yield.14 The lower milk yield, however, is more severe than expected from the reduced energy intake, thus the cows are dramatically less efficient. Robotics is another example of an emerging technology that will potentially improve lactation and overall cow management.38 Automated milking systems allow a cow to select the appropriate number of milkings each day, and there is evidence that cows alter the frequency of milking as lactation advances. This is a good example of a technology that will allow animals to express natural behaviors with fewer constraints imposed by the management system itself.39 That, in turn, should improve animal welfare. Individual monitoring of cows is streamlined in automated milking systems and thus “personalized” cow management can be practiced with ease with that technology.40 As monitoring and data reduction become more pervasive in the dairy industry, new opportunities will arise for decision support tools that can be used to optimize individual management of cows regardless of their point in the lactation cycle.
References 1. Webster’s Ninth New Collegiate Dictionary. Springfield, MA: Merriam-Webster Inc.; 1987. 2. Hulett JL, Weiss RE, Bwibo NO, Galal OM, Drorbaugh N, Neumann CG. Animal source foods have a positive impact on the primary school test scores of Kenyan schoolchildren in a cluster-randomised, controlled feeding intervention trial. Br J Nutr. 2014; 111(5):875e886.
3. Iannotti LL. The benefits of animal products for child nutrition in developing countries. Rev Sci Tech. 2018; 37(1):37e46. 4. Britt JH, Cushman RA, Dechow CD, et al. Invited review: learning from the future-A vision for dairy farms and cows in 2067. J Dairy Sci. 2018;101(5):3722e3741. 5. New National Milk Production Record Set. Dairy Herd Management; 2017. https://www.dairyherd.com/ article/new-national-milk-production-record-set. 6. Salmon GR, Marshall K, Tebug SF, et al. The greenhouse gas abatement potential of productivity improving measures applied to cattle systems in a developing region. Animal. 2018;12(4):844e852. 7. Capuco AV, Akers RM. The origin and evolution of lactation. J Biol. 2009;8(4):37.1e37.4. 8. Capuco AV, Ellis SE. Comparative aspects of mammary gland development and homeostasis. Annu Rev Anim Biosci. 2013;1:179e202. 9. Tucker HA. Hormones, mammary growth, and lactation: a 41-year perspective. J Dairy Sci. 2000;83(4):874e884. 10. Moallem U, Werner D, Lehrer H, et al. Long-term effects of ad libitum whole milk prior to weaning and prepubertal protein supplementation on skeletal growth rate and first-lactation milk production. J Dairy Sci. 2010; 93(6):2639e2650. 11. Capuco AV, Dahl GE, Wood DL, Moallem U, Erdman RE. Effect of bovine somatotropin and rumenundegradable protein on mammary growth of prepubertal dairy heifers and subsequent milk production. J Dairy Sci. 2004;87(11):3762e3769. 12. Moallem U, Dahl GE, Duffey EK, Capuco AV, Erdman RA. Bovine somatotropin and rumenundegradable protein effects on skeletal growth in prepubertal dairy heifers. J Dairy Sci. 2004;87(11):3881e3888. 13. Rius AG, Dahl GE. Exposure to long-day photoperiod prepubertally may increase milk yield in first-lactation cows. J Dairy Sci. 2006;89(6):2080e2083. 14. Collier RJ, Dahl GE, VanBaale MJ. Major advances associated with environmental effects on dairy cattle. J Dairy Sci. 2006;89(4):1244e1253. 15. Dahl GE, Tao S, Laporta J. Triennial lactation symposium/bolfa: late gestation heat stress of dairy cattle programs dam and daughter milk production. J Anim Sci. 2017;95(12):5701e5710. 16. Skibiel AL, Pe~ nagaricano F, Amorín R, et al. In utero heat stress alters the offspring epigenome. Sci Rep. 2018;8(1):14609. 17. van Lingen HJ, Fadel JG, Bannink A, et al. Multi-criteria evaluation of dairy cattle feed resources and animal characteristics for nutritive and environmental impacts. Animal. 2018;12(s2):s310es320. 18. Halmemies-Beauchet-Filleau A, Rinne M, Lamminen M, et al. Review: alternative and novel feeds for ruminants: nutritive value, product quality and environmental aspects. Animal. 2018;12(s2):s295es309.
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19. Bauman DE, Peel CJ, Steinhour WD, et al. Effect of bovine somatotropin on metabolism of lactating dairy cows: influence on rates of irreversible loss and oxidation of glucose and nonesterified fatty acids. J Nutr. 1988;118(8):1031e1040. 20. McArt JA, Nydam DV, Oetzel GR, Overton TR, Ospina PA. Elevated non-esterified fatty acids and b-hydroxybutyrate and their association with transition dairy cow performance. Vet J. 2013;198(3):560e570. 21. Dann HM, Litherland NB, Underwood JP, et al. Diets during far-off and close-up dry periods affect periparturient metabolism and lactation in multiparous cows. J Dairy Sci. 2006;89(9):3563e3577. 22. Hansen PJ. Current and future assisted reproductive technologies for mammalian farm animals. Adv Exp Med Biol. 2014:7521e7522. 23. García-Ruiz A, Cole JB, VanRaden PM, et al. Changes in genetic selection differentials and generation intervals in US Holstein dairy cattle as a result of genomic selection. Proc Natl Acad Sci USA. 2016;113(28):E3995eE4004. 24. Bisinotto RS, Ribeiro ES, Santos JE. Synchronisation of ovulation for management of reproduction in dairy cows. Animal. 2014;8(Suppl. 1):151e159. 25. Bruinje TC, Colazo MG, Ribeiro ES, Gobikrushanth M, Ambrose DJ. Using in-line milk progesterone data to characterize parameters of luteal activity and their association with fertility in Holstein cows. J Dairy Sci. 2019; 102(1):780e798. 26. Mayo LM, Silvia WJ, Ray DL, et al. Automated estrous detection using multiple commercial precision dairy monitoring technologies in synchronized dairy cows. J Dairy Sci. 2019;102(3):2645e2656. 27. De Vries A, Overton M, Fetrow J. Exploring the impact of sexed semen on the structure of the dairy industry. J Dairy Sci. 2008;91(2):847e856. 28. Gonz alez-Recio O, Toro MA, Bach A. Past, present, and future of epigenetics applied to livestock breeding. Front Genet. 2015;6:305. 29. Mrode R, Ojango JMK, Okeyo AM, Mwacharo JM. Genomic selection and use of molecular tools in breeding programs for indigenous and crossbred cattle in developing countries: current status and future prospects. Front Genet. 2019;9:694.
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30. Ruegg PLA. 100-Year Review: mastitis detection, management, and prevention. J Dairy Sci. 2017;100(12): 10381e10397. 31. Lago A, Godden SM. Use of rapid culture systems to guide clinical mastitis treatment decisions. Vet Clin North Am Food Anim Pract. 2018;34(3):389e412. 32. Petersson-Wolfe CS, Leslie KE, Schwarz TH. An update on the effect of clinical mastitis on the welfare of dairy cows and potential therapies. Vet Clin North Am Food Anim Pract. 2018;34(3):525e535. 33. Vanhoudt A, van Hees-Huijps K, van Knegsel ATM, et al. Effects of reduced intramammary antimicrobial use during the dry period on udder health in Dutch dairy herds. J Dairy Sci. 2018;101(4):3248e3260. 34. Boutinaud M, Isaka N, Gandemer E, et al. Inhibiting prolactin by cabergoline accelerates mammary gland remodeling during the early dry period in dairy cows. J Dairy Sci. 2017;100(12):9787e9798. 35. Fan W, Plaut K, Bramley AJ, et al. Persistency of adenoviral-mediated lysostaphin expression in goat mammary glands. J Dairy Sci. 2004;87(3):602e608. 36. Cardoso CS, von Keyserlingk MAG, H€ otzel MJ, Robbins J, Weary DM. Hot and bothered: public attitudes towards heat stress and outdoor access for dairy cows. PLoS One. 2018;13(10):e0205352. 37. Dahl GE, Tao S, Thompson IM. Lactation biology symposium: effects of photoperiod on mammary gland development and lactation. J Anim Sci. 2012;90(3): 755e760. 38. John AJ, Clark CE, Freeman MJ, et al. Review: milking robot utilization, a successful precision livestock farming evolution. Animal. 2016;10(9):1484e1492. 39. John AJ, Freeman MJ, Kerrisk KF, Garcia SC, Clark CEF. Robot utilisation of pasture-based dairy cows with varying levels of milking frequency. Animal. 2018;21:1e7. 40. Halachmi I, Guarino M, Bewley J, Pastell M. Smart animal agriculture: application of real-time sensors to improve animal well-being and production. Annu Rev Anim Biosci. 2018. https://doi.org/10.1146/annurevanimal-020518-114851.
II. Lactation and management of dairy cattle
7 Physiology of lactation in dairy cattledchallenges to sustainable production Geoffrey E. Dahl Department of Animal Sciences, University of Florida, Gainesville, FL, United States
O U T L I N E Current state of affairs
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Genetic innovations
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Mammary growth and function
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Animal health and well-being
126
Nutrition and metabolism
124
Housing and monitoring
127
Reproduction
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References
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Current state of affairs Milk is defined as a “fluid secreted by the mammary gland of females for the nourishment of their young.”1 Milk from ruminants, especially cows, provides a significant source of nutrient dense calories in the form of fluid milk and associated products such as cheese and yogurt. Indeed, a growing body of evidence suggests that including animal source foods in the diet overcomes nutrient deficiencies associated with their lack in the diet, particularly physical
Animal Agriculture https://doi.org/10.1016/B978-0-12-817052-6.00007-0
and cognitive stunting.2,3 And, milk protein is the most efficient of all animal proteins when compared on a production per unit of land basis, almost twice the efficiency of poultry meat and rivaling that of maize protein (Fig. 7.1).4 Dairy production ultimately depends on the rate of milk synthesis in the mammary gland, which can be affected by a number of factors. In the US and most developed countries, milk yield per cow has increased tremendously in the past 70 years due to improvements in genetic selection for yield and technical advances in feeding,
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Copyright © 2020 Elsevier Inc. All rights reserved.
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FIG. 7.1 Estimates of square meters of land required to produce 1 gm of human edible protein from various crops or production systems. Copied with permission from Britt et al.4
management and health that support expression of the higher genetic potential. In 2017, the average annual yield per cow was 10,404 kg, and the record yield is 35,451 kg.5 Thus, the modern dairy cow, properly managed, can produce a substantial volume of nutrient rich, human consumable food. And, the potential is more than three-fold greater than currently realized. In developing countries, however, milk yield hovers between 500 and 1,500 kg annually.6 In many countries native breeds serve as the base of the dairy herd, and genetic limitations are apparent in those animals. But, more important to explanation of low yields are management and feeding deficiencies that hamper the ability of even those animals with lower genetic potential to express greater productivity. With the demand for animal source foods increasing in many developing countries as incomes rise, and as awareness of the importance of animal source foods to prevent stunting increases, there is a push to improve the output of milk in total and on a per cow basis. Much of this increase
can be realized with improved feed availability, better management of reproduction and health, and improvements in animal and milk harvest hygiene. Over the next 30 years, continued improvements in milk output will be achieved in developed and developing countries. Rather than a review of mammary biology, the rest of this chapter considers broad areas related to mammary function and cow management that are likely to impact the continued rise in milk yields. Sustainably producing milk in 2050 is likely to be associated with continued increases in output on a per cow basis, but that rise in yield will only occur with improvements in genetic selection, nutrition and management of the herd worldwide. A variety of management interventions are discussed below and some of these factors are summarized in Fig. 7.2.
Mammary growth and function The mammary gland is a modified skin gland developmentally, and secretion of milk components is controlled by the endocrine system.7 Structurally, the secretory epithelial cells are organized into alveoli, which expel milk components into the open area apical to the cell orientation. With regard to structure, there is no difference in the basic secretory unit regardless of productive potential, rather, it is the number of secretory cells that are linked to productivity. Whereas udder size is not highly correlated with output on an animal-to-animal basis, per se, as lactation number advances and udder volume increases there is an increase in milk output. There is little reason to envision substantial increases in udder volume in the future, rather, more efficient output is the route to higher yields. That is not to say, however, that mammary epithelial cell number cannot be manipulated. There is constant loss and regeneration of cells during lactation, but cows typically experience a net loss of about
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FIG. 7.2 Depiction of the relationship of mammary cell number and milk yield throughout the lactation cycle of a cow, with emphasis on factors that can alter cell number and activity across the lactation and dry period. The lactation curve is characterized by a peak in cell number and then activity during the initial 6e8 weeks in milk, as depicted in the open curve. The solid black curve depicts the typical milk yield response observed in cows in many emerging and developing country systems, characterizing the lower peak and overall yield of milk, as well as the shorter duration of lactation. Early in lactation, cell number increases and that can be further stimulated by more frequent milking (IMF) in the first 3 weeks in milk. Following the peak of lactation, cell number and thus, yield, decline as lactation advances. A number of factors such as poor nutrition, sub-clinical mastitis as indicated by high SCC, heat stress and incomplete milk removal can limit the peak and accelerate the decline in yield. Conversely, management interventions such as bovine somatotropin (bST), increased milking frequency, and long day photoperiod can be used to slow the decline in even increase the yield of well-fed cows (illustrated as the shaded curve). The dry period is characterized by involution and some cell loss followed by regeneration of mammary cells as parturition approaches. Dry period interventions that increase mammary development and subsequent yield (i.e., the shaded curve) include short day photoperiod exposure and heat stress abatement. Therefore, many management factors can be manipulated throughout the lactation cycle to further enhance mammary gland output. These factors are discussed throughout this chapter. Adapted from Capuco and Ellis.8
50% of the secretory cells they begin lactation with as that lactation advances.8 Indeed, one area of particular interest is mammary stem cell biology and how that may be harnessed to increase the number and duration of activity of mammary epithelial cells in a given lactation. Mammary growth or mammogenesis begins in utero and development mirrors overall body growth until puberty. Estrogen and progesterone are the dominant hormones related to mammary growth, although adrenal steroids, somatotropin, prolactin and a number of local growth factors also play roles.9 At the onset of estrous cyclicity (or menstruation in primates) the waves of estrogen and progesterone during the cycle drive progressive increases in ductular
expansion into the mammary fat pad, as well as some alveolar development. With pregnancy, mammary growth accelerates under the synergistic effects of greater circulating concentrations of estrogen and progesterone. This is particularly evident with regard to secretory tissue development. But, growth in itself does not mean that milk secretion begins without the process of lactogenesis, which occurs around the time of parturition in response to the periparturient surge of prolactin. That increase in prolactin activates the cellular mechanisms responsible for lactose (and casein) production, and lactose secretion from the mammary epithelial cell results in fluid movement into the alveolar lumen, thus milk secretion. Maintenance of
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milk secretion, i.e., galactopoiesis, depends on continued milk removal from the gland, and endocrine system support via somatotropin, prolactin, thyroid hormones and cortisol; the gonadal steroids have limited effects after lactation is established. With regard to mammogenesis, there are a number of management factors that can be associated with reductions in ultimate productivity, likely because of impaired mammary growth and development. One of the most studied is that of excessive fat accumulation prepubertally and postpubertally, which limits the extent of parenchymal infiltration of the mammary fat pad and, ultimately, the volume of secretory tissue in the gland. Before puberty, when mammary growth proceeds in an allometric manner, nutrient supply appears to be critical to maximize growth. Specifically, when high volumes of whole milk are fed to developing heifers, mammary growth increases and subsequent first lactation yields are improved,10 but these effects appear to be independent of skeletal growth. After puberty, when mammary development returns to an isometric growth rate, neither nutrient supply nor stimulation of the somatotropic axis alters parenchymal accumulation, although overall body growth was improved.11,12 Thus, it appears that prior to puberty, nutrition limits maximal mammary growth whereas that influence recedes after puberty. Manipulation of photoperiod, or the duration of light that a heifer is exposed to each day will also impact mammary growth and eventually, production of milk. Calves that are raised under long days of 16 h of light and 8 h of darkness will have increased mammary parenchymal development when compared to herdmates housed under a short day photoperiod of 8 h of light and 16 h of darkness. Both prolactin and insulin-like growth factor-1 (IGF-1) are increased in heifers housed on long days, and they may be the drivers to increase parenchymal growth and lean body growth in general.13 When those heifers calve, their first lactation yields exceed
those of short-day heifers, further supporting the concept that parenchymal growth is related to milk yield, and that management factors can impact mammary development long before the first lactation begins. Heat stress is the other environmental factor that has significant impacts on mammogenesis and mammary function, both in mature cows and during development. While the effects of heat stress during an established lactation are well described,14 recent studies support the concept that heat stress in late gestation reduces mammary growth in the dry period which leads to lower milk yield in the subsequent lactation and poorer immune status through the transition into lactation.15 Of interest, the developing fetus also suffers negative effects of heat stress, wherein calves born to heat stressed dams subsequently produce less milk in their initial lactation, suggesting that in utero heat stress alters mammary development for life, likely through epigenetic alterations of the genome.16 Because the majority of cows producing milk reside in subtropical or tropical climates worldwide, these findings have significant relevance to future improvements in the efficiency of milk production around the globe.
Nutrition and metabolism The ruminant digestive system offers a significant advantage with regard to feed availability and byproduct utilization. This is particularly relevant when considering the conversion of non-human consumable feeds and byproducts into highly nutritious milk and milk products. For example, crop residues, oilseed meals and other byproducts of food manufacturing comprise at least 40% of dairy diets in developed countries, and are an ever-growing contribution to rations in developing countries.17 In addition, the list of novel alternative feeds is expanding, particularly in developing countries, where scarce land and water resources limit overall
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feed production capacity.18 This is a significant component of the sustainable nature of milk as a food for human consumption. Indeed, by converting non-human consumable byproducts and inedible highly fibrous plant material to milk, dairy cows fill a niche that would otherwise be a drag on waste streams. Further, those byproducts and other roughages would still produce methane as they decayed, so the net production of nutritious food per unit of greenhouse gas (GHG) is likely underestimated because no credit is assigned for the conversion. Despite their ability to convert a variety of forage and byproducts to milk, the volume of feed needed for dairy cows presents an ongoing challenge, one that is exacerbated early in lactation. The metabolic load of a high yielding cow in early lactation represents a significant challenge to maintain normal function while simultaneously secreting 1.5 kg of milk protein, 2.5 kg of lactose and 1.75 kg of milk fat each day, assuming a typical 50 L of milk daily yield. The output of energy dwarfs the maintenance requirements of the rest of the body, and it is perhaps most accurate to say that the cow is an appendage of the gland in early lactation, rather than the reverse. Cows are simply incapable of consuming enough dry matter to meet their energy requirements in early lactation and they enter a phase of negative energy balance, and must mobilize tissue energy reserves. There is a coordination of the somatotropic system such that the catabolic actions predominate to increase mobilization of fat stores,19 and this state continues for 6e8 weeks into lactation when energy intake from dry matter intake exceeds that of milk energy output and energy balance is restored. While substantial literature suggests that the consequences of negative energy balance, including elevated circulating concentrations non-esterified fatty acids and ketone bodies, predisposes the cow in early lactation to metabolic and pathogen-induced disease, it is a normal physiological process that can be monitored
and managed to limit negative effects.20 One of the primary issues is appropriate nutrition during the dry period to avoid excessive body condition at calving. Managing the nutrient intake of cows during the dry period to limit conditioning should be a priority. Significant bodyweight gains, or extended lengths of the dry period, have been associated with greater risk of metabolic disease post-partum. In contrast, dry periods of under 30 days can limit productivity in the next lactation. A number of studies have investigated nutritional management to reduce the nutrient density of prepartum diets to slow the accumulation of body condition in dry cows so that a dry period of 45e60 days can be managed effectively and without overconditioning the cow.21
Reproduction As the final phase of the reproductive process in mammals, normal lactation depends on successful reproduction, and that is particularly important in dairy cattle as milk yield wanes with advancing lactation. Therefore, maintaining annual calving results in the greatest overall milk output. Reproductive performance in heifers is typically quite robust in a well-managed herd. But a steady decline in daughter pregnancy rate (DPR) was associated with increased emphasis on selection for milk yield from 1960 to 2010, and reproductive performance suffered as yield climbed.22 Over the past decade, however, the DPR has increased primarily because of two factors. First, improved genetic selection for DPR has shown significant improvement in reproductive performance.23 Second, the advent of timed breeding protocols, i.e., Ov-synch, have dramatically improved the ability to achieve high levels of reproductive performance in lactating dairy cows.24 The improved ability to attain reproductive success without sacrificing continued milk yield increments, however, may not be limitless. Consumer pressure over the
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use of exogenous hormones for management may affect the ability to take advantage of systems to synchronize ovulation in the future. But, other technologies are being developed and perfected to replace older techniques that become obsolete or unavailable. For example, cow activity monitoring and in-line milk progesterone monitoring25,26 are currently being used to determine estrus in a non-invasive manner. But those approaches require significant financial investments and may not be suitable in many situations in developing countries. Selection for reproductive performance using genomic, and eventually epigenomic methods, may further improve our ability to realize highly fertility along with high milk yield. Sex sorted semen allows for gender selection of over 90%, a substantial boon to the dairy industry with regard to the production of heifers as potentially lactating animals. Obviously the number of females is increased, but with that selection comes a greater ability to improve the overall genetic value of the herd. Alternatively, with the advent of sexed semen, there has been a significant increase in heifer availability throughout the dairy industry in North America.27 As the number of available heifers increases, alternative approaches to breeding the entire herd are realized, specifically the selective mating of the bottom half of the herd to beef sires, in an effort increase the value of those offspring for the beef market. In countries where the dairy industry is less developed, the application of AI is useful to accelerate genetic progress. But, it is important to consider the management and feeding capacity of those countries in concert with genetic improvement, as forage quantity and quality in particular may mask expression of the genetic capacity for production.
Genetic innovations Genomic selection has had a tremendous impact on selection of dairy cattle for superior
traits, but this has been especially true for health and well-being traits relative to production traits.23 As improvements are made in identification of markers for disease resistance and reproductive performance, and even for geographic or management specific traits, it should be possible to select for more resilient animals that are best suited for that production system (e.g., pasture vs. freestall). In addition, better understanding of the epigenomic28 effects on mammary gland function and metabolism will enhance our ability to identify highly productive cows that function well within specific production systems. Indeed, advances in genomic and even epigenomic selection may be of huge impact in developing countries to improve yield in indigenous breeds without sacrificing adaptive traits.29 Selection of superior indigenous bulls may then allow for wider propagation of those improved genetics through coordinated breeding programs using AI and other advanced reproductive technologies. However, it is critical that performance estimates be confirmed under local conditions of adequate management, or improvements will not be realized at the farm level.
Animal health and well-being Udder health is a key component of successful lactation. Mastitis, or inflammation of the gland, is most commonly caused by an invasion of bacterial pathogens, and can cause substantial reductions in yield and quality of milk, and even death of the cow depending on the pathogen and associated complications.30 Despite improvements in milk harvest and sanitation, mastitis continues to be a problem in the dairy industry. Antibiotics are routinely used as a therapy, but a growing body of evidence suggests that many infections will clear-up spontaneously without antimicrobial therapy and the pathogen profile is shifting away from microorganisms that are sensitive to those products available for use in food producing animals.31 Indeed,
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improvements in on-farm identification of particular mastitis pathogens to more effectively target responsive pathogens is likely to realize a reduction in antibiotic use and greater cure rates overall relative to the less discriminate treatment strategies of the past. Moreover, supportive therapies, including fluids and nonsteroidal anti-inflammatory drugs (NSAID’s), to deal with the broader effects of infection with gram negative pathogens such as E. Coli are an important consideration to recovery of the animal and a functional mammary gland.32 Stem cell biology may be useful in overcoming the loss of productivity associated with mastitis, particularly in instances where mammary tissue loss occurs.8 In many cases of severe mastitis, local inflammation of the gland results in loss of secretory tissue and even functionality of individual quarters. Typically, milk secretion from those quarters never recovers, which suggests a loss of regenerative capacity of the mammary epithelial cells. With greater understanding of stem cell biology, it may be possible to develop therapies that overcome the loss of secretory capacity and allow for recovery of milk production from an infected quarter of the mammary gland. Blanket antibiotic therapy for dry cows has yielded great improvements in reducing chronic infections in cows, but is an area of substantial research emphasis currently. There is the potential to dramatically limit the use of antibiotics in the dairy industry. That is due to the fact that many cows may have no infection at the end of lactation, yet all quarters of the mammary gland are treated regardless of infection status. Selecting specific cows or quarters to receive dry cow antibiotic therapy will require improved screening methods to ensure pathogen identification, and a higher level of individual cow management.33 Practices such as gradually reducing milk removal and pharmaceutical interventions34 to rapidly limit milk secretion may sufficiently limit residual milk, even in high producing cows at dry off. This would decrease the potential
for new intramammary infections during the dry period, but field based evidence is lacking to confirm the utility of those approaches. While judicious use of antibiotics will continue to be important for managing mastitis in dairy cattle, consumers and regulators are exerting pressure to minimize the amount and range of antibiotic therapies. Therefore, improvements in host ability to resist pathogen infiltration and establishment of infection offers a potential approach to reduce the volume and necessity of antibiotic use related to dairy management. Transgenic approaches have been used to achieve this goal, but with limited success.35 In addition, these methods would likely face regulatory and consumer acceptance burdens that would limit their rapid translation into industry practice.
Housing and monitoring Evidence continues to emerge that housing affects cow productivity and welfare, and represents an area for further management interventions. Free-stall barns that allow for expression of individual cow behavior for feeding, grooming and mobility are now used widely in developed countries. That will likely continue as the housing method of choice, while tie-stalls and other barn designs that limit animal movement will decline in use. Pasture, as a housing system (vs. nutrition), is an area of interest, as some perceive this as a more “natural” choice for cattle and promote movement toward having all cows having the option for all cows to have outdoor access. However, it is important to consider the effects of heat stress on pastured cattle as there are significant limitations to cooling systems in the pasture setting compared with cows housed in conventional barns.36 As previously mentioned, photoperiod and heat stress can impact mammogenesis and thus alter mammary output of milk. However, there are also direct effects of both environmental factors on milk production. Manipulation of
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photoperiod improves the production efficiency during lactation and the potential for milk yield during the dry period.37 Therefore, appropriate management of the lighting system to which cows are exposed in modern confinement operations can yield significant improvements in efficiency. Heat stress is an example of an environmental insult to productive efficiency across the life cycle of the dairy cow. Lactating cows decrease dry matter intake under conditions above a temperature:humidity index (THI) of 72, and experience a corresponding reduction in milk yield.14 The lower milk yield, however, is more severe than expected from the reduced energy intake, thus the cows are dramatically less efficient. Robotics is another example of an emerging technology that will potentially improve lactation and overall cow management.38 Automated milking systems allow a cow to select the appropriate number of milkings each day, and there is evidence that cows alter the frequency of milking as lactation advances. This is a good example of a technology that will allow animals to express natural behaviors with fewer constraints imposed by the management system itself.39 That, in turn, should improve animal welfare. Individual monitoring of cows is streamlined in automated milking systems and thus “personalized” cow management can be practiced with ease with that technology.40 As monitoring and data reduction become more pervasive in the dairy industry, new opportunities will arise for decision support tools that can be used to optimize individual management of cows regardless of their point in the lactation cycle.
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