A Holstein cow with prominent udder and less muscle than is typical of beef breeds

Dairy cattle (also called dairy cows) are cattle bred for the ability to produce large quantities of milk, from which dairy products are made. Dairy cattle generally are of the species Bos taurus.[1]

Historically, little distinction was made between dairy cattle and beef cattle, with the same stock often being used for both meat and milk production. Today, the bovine industry is more specialized and most dairy cattle have been bred to produce large volumes of milk.

Management

Cows on a dairy farm in Maryland, U.S.

Dairy cows may be found either in herds or dairy farms, where dairy farmers own, manage, care for, and collect milk from them, or on commercial farms. Herd sizes vary around the world depending on landholding culture and social structure. The United States has an estimated 9 million cows in around 75,000 dairy herds, with an average herd size of 120 cows. The number of small herds is falling rapidly with the 3,100 herds with over 500 cows producing 51% of U.S. milk in 2007.[2] The United Kingdom dairy herd overall has nearly 1.5 million cows, with about 100 head reported on an average farm.[3] In New Zealand, the average herd has more than 375 cows, while in Australia, there are approximately 220 cows in the average herd.[4][5]

The United States dairy herd produced 84.2 billion kilograms (185.7 billion pounds) of milk in 2007,[6] up from 52.9 billion kilograms (116.6 billion pounds) in 1950,[7] yet there were only about 9 million cows on U.S. dairy farms—about 13 million fewer than there were in 1950.[7] The top breed of dairy cow within Canada's national herd category is Holstein, taking up 93% of the dairy cow population, have an annual production rate of 10,257 kilograms (22,613 pounds) of milk per cow that contains 3.9% butter fat and 3.2% protein.[8]

Dairy farming, like many other livestock-rearing methods, can be split into intensive and extensive management systems.[9]

Intensive systems focus towards maximum production per cow in the herd. This involves formulating their diet to provide ideal nutrition and housing the cows in a confinement system such as free stall or tie stall. These cows are housed indoors throughout their lactation and may be put to pasture during their 60-day dry period before ideally calving again. Free stall-style barns involve cattle loosely housed where they can have free access to feed, water, and stalls, but are moved to another part of the barn to be milked multiple times a day. In a tie-stall system, the milking units are brought to the cows during each milking. These cattle are tethered within their stalls with free access to water and feed provided. In extensive systems, cattle are mainly outside on pasture for most of their lives. These cattle are generally lower in milk production and are herded multiple times daily to be milked. The systems used greatly depend on the climate and available land of the region in which the farm is situated.[9]

Cow caring for her newborn calf
A cow caring for her newborn calf

To maintain lactation, a dairy cow must be bred and produce calves.[10] Depending on market conditions, the cow may be bred with a "dairy bull" or a "beef bull." Female calves (heifers) with dairy breeding may be kept as replacement cows for the dairy herd. If a replacement cow turns out to be a substandard producer of milk, she then goes to market and can be slaughtered for beef. Male calves can either be used later as a breeding bull or sold and used for veal or beef. Dairy farmers usually begin breeding or artificially inseminating heifers around 13 months of age.[11] A cow's gestation period is about nine months.[12] Newborn calves are separated from their mothers quickly, usually within three days, as the mother/calf bond intensifies over time and delayed separation can cause extreme stress on both cow and calf.[13]

Domestic cows can live beyond 20 years;[12] however, those raised for dairy rarely live that long, as the average cow is removed from the dairy herd around age six and marketed for beef.[14] In 2014, roughly 9.5% of the cattle slaughtered in the U.S. were culled dairy cows - cows that can no longer be seen as an economic asset to the dairy farm.[15] These animals may be sold due to reproductive problems or common diseases of milk cows such as mastitis and lameness.[14]

Calf

Most heifers (female calves) are kept on farm to be raised as replacement heifers, bred to enter the production cycle. Market calves are generally sold at two weeks of age and bull calves may fetch a premium over heifers due to their size, either current or potential. Calves may be sold for veal, or for one of several types of beef production, depending on available local crops and markets. Such bull calves may be castrated if turnout onto pastures is envisaged, to make them less aggressive. Purebred bulls from elite cows may be put into progeny testing schemes to find out whether they might become superior sires for breeding. Such animals can become extremely valuable.

Most dairy farms separate calves from their mothers within a day of birth to reduce transmission of disease and simplify management of milking cows. Studies have been done allowing calves to remain with their mothers for 1, 4, 7 or 14 days after birth. Cows whose calves were removed longer than one day after birth showed increased searching, sniffing and vocalizations. However, calves allowed to remain with their mothers for longer periods showed weight gains at three times the rate of early removals as well as more searching behavior and better social relationships with other calves.[16][17]

After separation, some young dairy calves subsist on commercial milk replacer, a feed based on dried milk powder. Milk replacer is an economical alternative to feeding whole milk because it is cheaper, can be bought at varying fat and protein percentages, and is typically less contaminated than whole milk when handled properly. Some farms pasteurize and feed calves milk from the cows in the herd instead of using replacer. A day-old calf consumes around 5 liters of milk per day.[18]

Cattle are social animals; their ancestors tended to live in matriarchal groups of mothers and offspring. The formation of "friendships" between two cows is common and long lasting. Traditionally individual housing systems were used in calf rearing, to reduce the risk of disease spread and provide specific care. However, due to their social behaviour the grouping of offspring may be better for the calves' overall welfare. Social interaction between the calves can have a positive effect on their growth. It has been seen that calves housed in grouped penning were found to eat more feed than those in single pens,[19] suggesting social facilitation of feeding behaviour in the calves. Play behaviour in pre-weaned dairy calves has also been suggested to help build social skills for later in life. It has been seen that those reared in grouped housing are more likely to become the dominant cattle in a new combination of animals.[20] These dominant animals have a priority choice of feed or lying areas and are generally stronger animals. For these reasons, it has become common practice to group or pair calves in their housing. It has become common in Canada to see paired or grouped housing in outdoor hutches or in an indoor pack penning.[21]

Bull

A bull calf with high genetic potential may be reared for breeding purposes. It may be kept by a dairy farm as a herd bull, to provide natural breeding for the herd of cows. A bull may service up to 50 or 60 cows during a breeding season. Any more and the sperm count declines, leading to cows "returning to service" (needing to be bred again). A herd bull may only stay for one season, as when most bulls reach over two years old their temperament becomes too unpredictable.

Bull calves intended for breeding are commonly bred on specialized dairy breeding farms, not production farms. These farms are the major source of stocks for artificial insemination.

Milk production levels

Dairy cattle in Mangskog, Sweden, 1911.
Dairy cattle, Collins Center, New York, USA, 1999

The dairy cow produces large amounts of milk in its lifetime. Production levels peak at around 40 to 60 days after calving. Production declines steadily afterwards until milking is stopped at about 10 months. The cow is "dried off" for about sixty days before calving again. Within a 12 to 14-month inter-calving cycle, the milking period is about 305 days or 10 months long.[22][23][24] Among many variables, certain breeds produce more milk than others within a range of around 6,800 to 17,000 kg (15,000 to 37,500 lb) of milk per year.[25]

The Holstein Friesian is the main breed of dairy cattle in Australia, and said to have the "world's highest" productivity, at 10,000 litres (2,200 imp gal; 2,600 US gal) of milk per year.[26] The average for a single dairy cow in the US in 2007 was 9,164 kg (20,204 lb) per year, excluding milk consumed by her calves,[6] whereas the same average value for a single cow in Israel was reported in the Philippine press to be 12,240 kg (26,980 lb) in 2009.[27] High production cows are more difficult to breed at a two-year interval. Many farms take the view that 24 or even 36 month cycles are more appropriate for this type of cow.[28][29]

Dairy cows may continue to be economically productive for many lactation cycles. In theory a longevity of 10 lactations is possible. The chances of problems arising which may lead to a cow being culled are high, however; the average herd life of US Holstein is today fewer than 3 lactations. This requires more herd replacements to be reared or purchased. Over 90% of all cows are slaughtered for 4 main reasons:

  • Infertility – failure to conceive and reduced milk production.
Cows are at their most fertile between 60 and 80 days after calving. Cows remaining "open" (not with calf) after this period become increasingly difficult to breed, which may be due to poor health. Failure to expel the afterbirth from a previous pregnancy, luteal cysts, or metritis, an infection of the uterus, are common causes of infertility
Mastitis is recognized by a reddening and swelling of the infected quarter of the udder and the presence of whitish clots or pus in the milk. Treatment is possible with long-acting antibiotics but milk from such cows is not marketable until drug residues have left the cow's system, also called withdrawal period.
  • Lameness – persistent foot infection or leg problems causing infertility and loss of production.
High feed levels of highly digestible carbohydrate cause acidic conditions in the cow's rumen. This leads to laminitis and subsequent lameness, leaving the cow vulnerable to other foot infections and problems which may be exacerbated by standing in faeces or water soaked areas.
  • Production – some animals fail to produce economic levels of milk to justify their feed costs.
Production below 12 to 15 L (2.6 to 3.3 imp gal; 3.2 to 4.0 US gal) of milk per day is not economically viable.[30]

Cow longevity is strongly correlated with production levels.[31] Lower production cows live longer than high production cows, but may be less profitable. Cows no longer wanted for milk production are sent to slaughter. Their meat is of relatively low value and is generally used for processed meat. Another factor affecting milk production is the stress the cow is faced with. Psychologists at the University of Leicester, UK, analyzed the musical preference of milk cows and found out that music actually influences the dairy cow's lactation. Calming music can improve milk yield, probably because it reduces stress and relaxes the cows in much the same way as it relaxes humans.

Cow comfort and its effects on milk production

Certain behaviors such as eating, ruminating, and lying down can be related to the health of the cow and cow comfort. These behaviors can also be related to the productivity of the cows. Likewise, stress, disease, and discomfort negatively affect milk productivity. Therefore, it can be said that it is in the best interest of the farmer to increase eating, rumination, and lying down and decrease stress, disease, and discomfort to achieve the maximum productivity possible.[32] Also, estrous behaviors such as mounting can be a sign of cow comfort, because if a cow is lame, nutritionally deficient, or housed in an over crowded barn, its estrous behaviors are altered.[33]

Feeding behaviors are important for the dairy cow, as feeding is how the cow ingests dry matter. However, the cow must ruminate to fully digest the feed and utilize the nutrients in the feed.[34] Dairy cows with good rumen health are likely to be more profitable than cows with poor rumen health—as a healthy rumen aids in digestion of nutrients. An increase in the time a cow spends ruminating is associated with the increase in health and an increase in milk production.[32] The productivity of dairy cattle is most efficient when the cattle have a full rumen.[35] Also, the standing action while feeding after milking has been suggested to enhance udder health. The delivery of fresh feed while the cattle are away for milking stimulates the cattle to feed upon return, potentially reducing the prevalence of mastitis as the sphincters have time to close while standing.[36] This makes the pattern of feeding directly after being milked an ideal method of increasing the efficiency of the herd.

Cows have a high motivation to lie down.[34] They should lie down for at least five to six hours after every meal to ruminate well.[37] When the lactating dairy cow lies down, blood flow is increased to the mammary gland which in return results in a higher milk yield.[38] When they stand too long, cows become stressed, lose weight, get sore feet, and produce less milk.[37]

To ensure that the dairy cows lie down as much as needed, the stalls must be comfortable.[37] A stall should have a rubber mat and bedding, and be large enough for the cow to lie down and get up comfortably. Signs that the stalls may not be comfortable enough for the cows are the cows are standing, either ruminating or not, instead of lying down, or perching, which is when the cow has its front end in the stall and their back end out of the stall.[39] Dried manure, almond shells, straw, sand, or waterbeds are used for cow bedding.[37]

There are two types of housing systems in dairy production, free style housing and tie stall. Free style housing is where the cow is free to walk around and interact with its environment and other members of the herd. Tie stall housing is when the cow is chained to a stanchion stall with the milking units and feed coming to them.[40]

Artificial light and daylight inlets have an impact on milk production and cow behavior.[41] For cows in lactation a light program of 16 hours light and 8 hours of darkness is recommended,[42] while for non-lactating pregnant cows 8 hours of light and 16 hours of darkness seem to be a better fit.[43]

By-products and processing

Pasteurization is the process of heating milk to a high enough temperature for a short period of time to kill the microbes in the milk and increase keep time and decrease spoilage time. By killing the microbes, decreasing the transmission of infection, and elimination of enzymes the quality of the milk and the shelf life increases. Pasteurization is either completed at 63 °C (145 °F) for thirty minutes or a flash pasteurization is completed for 15 seconds at 72 °C (162 °F).[44] By-products of milk include butterfat, cream, curds, and whey. Butterfat is the main lipid in milk. The cream contains 18–40% butterfat. The industry can be divided into 2 market territories; fluid milk and industrialized milk such as yogurt, cheeses, and ice cream.[45]

Whey protein makes up about 20% of milk's protein composition and is separated from the casein (80% of milk's protein make up) during the process of curdling cheese. This protein is commonly used in protein bars, beverages and concentrated powder, due to its high quality amino acid profile. It contains levels of both essential amino acids as well as branched that are above those of soy, meat, and wheat.[46] "Diafiltered" milk is a process of ultrafiltration of the fluid milk to separate lactose and water from the casein and whey proteins. This process allows for more efficiency in cheese making and gives the potential to produce low-carb dairy products.[47]

Reproduction

Since the 1950s, artificial insemination (AI) is used at most dairy farms; these farms may keep no bull. Artificial insemination uses estrus synchronization to indicate when the cow is going through ovulation and is susceptible to fertilization. Advantages of using AI include its low cost and ease compared to maintaining a bull, ability to select from a large number of bulls, elimination of diseases in the dairy industry, improved genetics and improved animal welfare.[48] Rather than a large bull jumping on a smaller heifer or weaker cow, AI allows the farmer to complete the breeding procedure within 5 minutes with minimum stress placed on the individual female's body.[49]

Dairy cattle are polyestrous, meaning they cycle continuously throughout the year. They tend to be on a 21 day estrus cycle. However for management purposes, some operations use synthetic hormones to synchronize their cows or heifers to have them breed and calve at the ideal times. These hormones are short term and only used when necessary. For example, one common protocol for synchronization involves an injection of GnRH (gonadotrophin releasing hormone). which increases the levels of follicle stimulating hormone and luteinizing hormone in the body. Then, seven days later prostaglandin F2-alpha is injected, followed by another GnRH injection 48 hours later. This protocol causes the animal to ovulate 24 hours later.[50]

Estrus is often called standing heat in cattle and refers to the time in their cycle where the female is receptive towards the male. Estrus behaviour can be detected by an experienced stockman. These behaviours can include standing to be mounted, mounting other cows, restlessness, decreased milk production, and decreased feed intake.[51]

More recently, embryo transfer has been used to enable the multiplication of progeny from elite cows. Such cows are given hormone treatments to produce multiple embryos. These are then 'flushed' from the cow's uterus. 7–12 embryos are consequently removed from these donor cows and transferred into other cows who serve as surrogate mothers. This results in between three and six calves instead of the normal single or (rarely) twins.

Hormone use

Farmers in some countries sometimes administer hormone treatments to dairy cows to increase milk production and reproduction.

About 17% of dairy cows in the United States are injected with Bovine somatotropin, also called recombinant bovine somatotropin (rBST), recombinant bovine growth hormone (rBGH), or artificial growth hormone.[52] The use of this hormone increases milk production by 11%–25%. The U.S. Food and Drug Administration (FDA) has ruled that rBST is harmless to people. The use of rBST is banned in Canada, parts of the European Union, as well as Australia and New Zealand.

In the United States the Pasteurized Milk Ordinance requires a milk sample is taken from every farm and from every load of milk delivered to a processing plant.[53] These samples are then tested for antibiotic and any milk testing positive is discarded and farm identified. Traceback to the dairy is undertaken by the FDA with further consequences including the possibility revocation of ability to sell milk.[54]

Nutrition

Dairy cattle at feeding time

Nutrition plays an important role in keeping cattle healthy and strong. Implementing an adequate nutrition program can also improve milk production and reproductive performance. Nutrient requirements may not be the same depending on the animal's age and stage of production. Diets are formulated to meet the dairy cow's energy and amino acid requirements for lactation, growth, and/or reproduction.[55]

Forages, which refer especially to anything grown in the field such as hay, straw, corn silage, or grass silage, are the most common type of feed used. The base of most lactating dairy cattle diets is high quality forage. Cereal grains, as the main contributors of starch to diets, are important in helping to meet the energy needs of dairy cattle. Barley is an excellent source of balanced amounts of protein, energy, and fiber.[56]

Ensuring adequate body fat reserves is essential for cattle to produce milk and also to keep reproductive efficiency. However, if cattle get excessively fat or too thin, they run the risk of developing metabolic problems and may have problems with calving.[57] Scientists have found that a variety of fat supplements can benefit conception rates of lactating dairy cows. Some of these different fats include oleic acids, found in canola oil, animal tallow, and yellow grease; palmitic acid found in granular fats and dry fats; and linolenic acids which are found in cottonseed, safflower, sunflower, and soybean.[58]

Diets can additionally be formulated to strategically reduce methane emissions. Ruminants such as the cow have microbes in their rumen called methanogens which are capable of digesting down plant material so it can be utilized for energy, but also generates methane as a byproduct which is then released into the atmosphere by belching. Diets that include feed additives and supplements such as 3-nitrooxypropanol,[59] and the red seaweed (Asparagopsis taxiformis) [60] have been found to reduce enteric methane emissions. Some of these feed additives have already been approved for farmer usage[61] while others continue to be studied for safety and efficacy.

Breeds

Braunvieh with a unique milk quality (Switzerland)

According to the Purebred Dairy Cattle Association, PDCA, there are 7 major dairy breeds in the United States. These are: Holstein Black/White and Red/White, Brown Swiss, Guernsey, Ayrshire, Jersey, and Milking Shorthorn.[62]

Holstein cows originate from the Netherlands and have distinct black and white or more rarely red and white markings. Holstein cows are the biggest of all dairy breeds. A full mature Holstein cow usually weighs around 700 kilograms (1,500 lb) and is 147 centimetres (58 in) tall at the shoulder. They are known for their outstanding milk production among the main breeds of dairy cattle. An average Holstein cow produces around 10,000 kilograms (23,000 lb) of milk each lactation. Of the 9 million dairy cows in the U.S., approximately 90% of them are of the Holstein descent.[63] The top breed of dairy cow within Canada's national herd category is Holstein, taking up 93% of the dairy cow population, have a production rate of 10,257 kilograms (22,613 lb) of milk per cow that contains 3.9% butter fat and 3.2% protein[8]

Brown Swiss cows are widely accepted as the oldest dairy cattle breed, originally coming from a part of northeastern Switzerland. Some experts think that the modern Brown Swiss skeleton is similar to one found that looks to be from around the year 4000 BC Also, there is evidence that monks started breeding these cows about 1000 years ago.[64]

The Ayrshire breed first originated in the County of Ayr in Scotland. It became regarded as a well established breed in 1812. The different breeds that were crossed to form the Ayrshire are not exactly known. However, there is evidence that several breeds were crossed with the native cattle to create the breed.[65]

Guernsey cows originated just off the coast of France on the small Isle of Guernsey. The breed was first known as a separate breed around 1700. Guernseys are known for their ability to produce very high quality milk from grass. Also, the term "Golden Guernsey" is very common as Guernsey cattle produce rich, yellow milk rather than the standard white milk other cow breeds produce.[66]

The Jersey originates on the island of Jersey in the Channel Islands. Cows usually weigh some 350–400 kg (800–900 lb). The milk is rich and has a yellowish tinge; the fat content may exceed 6%.[67]: 212  American Jerseys have been selectively bred for higher milk yield, and are often larger and coarser than the island stock.[67]: 212 

Animal welfare

Mobile milking parlor for grazing cattle

Animal welfare refers to both the physical and mental state of an animal, and how it is coping with its situation. An animal is considered in a good state of welfare if it is able to express its innate behaviour, comfortable, healthy, safe, well nourished, and is not suffering from harmful states such as distress, fear and pain. Good animal welfare requires disease prevention and veterinary treatment, appropriate shelter, management, nutrition, and humane handling. If the animal is slaughtered then it is no longer "good animal welfare".[68] It is the human responsibility of the animals' wellbeing in all husbandry and management practices including humane euthanasia.

Proper animal handling, or stockmanship, is crucial to dairy animals' welfare as well as the safety of their handlers. Improper handling techniques can stress cattle leading to impaired production and health, such as increased slipping injuries. Additionally, the majority of nonfatal worker injuries on a dairy farm are from interactions with cattle. Dairy animals are handled on a daily basis for a wide variety of purposes including health-related management practices and movement from freestalls to the milking parlor. Due to the prevalence of human-animal interactions on dairy farms, researchers, veterinarians, and farmers alike have focused on furthering our understanding of stockmanship and educating agriculture workers. Stockmanship is a complex concept that involves the timing, positioning, speed, direction of movement, and sounds and touch of the handler.[69]

A recent survey of Minnesota dairy farms revealed that 42.6% of workers learned stockmanship techniques from family members, and 29.9% had participated in stockmanship training. However, as the growing U.S. dairy industry increasingly relies on an immigrant workforce, stockmanship training and education resources become more pertinent. Clearly communicating and managing a large culturally diverse workforce brings new challenges such as language barriers and time limitations.[70] Organizations like the Upper Midwest Agriculture Safety and Health Center offer resources such as bilingual training videos, fact sheets, and informational posters for dairy worker training. Additionally the Beef Quality Assurance Program offer seminars, live demonstrations, and online resources for stockmanship training.[71]

For cows to reach high performance in milk yields and reproduction, they must be in great condition and comfortable in the system. Once an individual's welfare is reduced, so does her efficiency and production. This creates more cost and time on the operation, therefore most farmers strive to create a healthy, hygienic, atmosphere for their cattle. As well as provide quality nutrition that keep the cows yield high.[72]

The production of milk requires that the cow be in lactation, which is a result of the cow having given birth to a calf. The cycle of insemination, pregnancy, parturition, and lactation is followed by a "dry" period of about two months before calving, which allows udder tissue to regenerate. A dry period that falls outside this time frames can result in decreased milk production in subsequent lactation.[73] Dairy operations therefore include both the production of milk and the production of calves. Bull calves are either castrated and raised as steers for beef production or used for veal.

The practice of dairy production has been criticized by animal rights proponents. Some of the ethical reasons regarding dairy production cited include how often the dairy cattle are impregnated, the separation of calves from their mothers, the fact that dairy cows are considered "spent" and culled at a relatively young age, that most male calves are of no use to the dairy industry and are sold for beef or veal, as well as environmental concerns regarding any cattle production.[74]

Impacts of climate change

This section is an excerpt from Effects of climate change on livestock § Cattle.[edit]
Various pathologies which can be caused by heat stress, many specific to cattle.[75]

As of 2009, there were 1.2 billion cattle in the world, with around 82% in the developing countries;[76] the totals only increased since then, with the 2021 figure at 1.53 billion.[77] As of 2020, it was found that in the current Eastern Mediterranean climate, cattle experience mild heat stress inside unadapted stalls for nearly half a year (159 days), while moderate heat stress is felt indoors and outdoors during May, June, July, August, September, and October. Additionally, June and August are the months where cattle are exposed to severe heat stress outside, which is mitigated to moderate heat stress indoors.[78] Even mild heat stress can reduce the yield of cow milk: research in Sweden found that average daily temperatures of 20–25 °C (68–77 °F) reduce daily milk yield per cow by 200 g (0.44 lb), with the loss reaching 540 g (1.19 lb) for 25–30 °C (77–86 °F).[79] Research in a humid tropical climate describes a more linear relationship, with every unit of heat stress reducing yield by 2.13%.[80] In the intensive farming systems, daily milk yield per cow declines by 1.8 kg (4.0 lb) during severe heat stress. In organic farming systems, the effect of heat stress on milk yields is limited, but milk quality suffers substantially, with lower fat and protein content.[81] In China, daily milk production per cow is already lower than the average by between 0.7 and 4 kg (1.5 and 8.8 lb) in July (the hottest month of the year), and by 2070, it may decline by up to 50% (or 7.2 kg (16 lb)) due to climate change.[82] Some researchers suggest that the already recorded stagnation of dairy production in both China and West Africa can attributed to persistent increases in heat stress.[83]: 747 

Heatwaves can also reduce milk yield, with particularly acute impacts if the heatwave lasts for four or more days, as at that point the cow's thermoregulation capacity is usually exhausted, and its core body temperature starts to increase.[84] At worst, heatwaves can lead to mass mortality: in July 1995, over 4,000 cattle in the mid-central United States heatwave, and in 1999, over 5,000 cattle died during a heatwave in northeastern Nebraska.[85] Studies suggest that Brahman cattle and its cross-breeds are more resistant to heat stress than the regular bos taurus breeds,[76] but it is considered unlikely that even more heat-resistant cattle can be bred at a sufficient rate to keep up with the expected warming.[86] Further, both male and female cattle can have their reproduction impaired by heat stress. In males, severe heat can affect both spermatogenesis and the stored spermatozoa. It may take up to eight weeks for sperm to become viable again. In females, heat stress negatively affects conception rates as it impairs corpus luteum and thus ovarian function and oocyte quality. Even after conception, a pregnancy is less likely to be carried to term due to reduced endometrial function and uterine blood flow, leading to increased embryonic mortality and early fetal loss.[85] Calves born to heat-stressed cows typically have a below-average weight, and their weight and height remains below average even by the time they reach their first year, due to permanent changes in their metabolism.[87] Heat-stressed cattle have also displayed reduced albumin secretion and liver enzyme activity. This is attributed to accelerated breakdown of adipose tissue by the liver, causing lipidosis.[75]

Serous exudate from udder in E. coli mastitis in cow (left), in comparison to normal milk (right).

Cattle are suspectible to some specific heat stress risks, such as ruminal acidosis. Cattle eat less when they experience acute heat stress during hottest parts of the day, only to compensate when it is cooler, and this disbalance soon causes acidosis, which can lead to laminitis. Additionally, one of the ways cattle can attempt to deal with higher temperatures is by panting more often, which rapidly decreases carbon dioxide concentrations and increases pH. To avoid respiratory alkalosis, cattle are forced to shed bicarbonate through urination, and this comes at the expense of rumen buffering. These two pathologies can both develop into lameness, defined as "any foot abnormality that causes an animal to change the way that it walks". This effect can occur "weeks to months" after severe heat stress exposure, alongside sore ulcers and white line disease.[75] Another specific risk is mastitis, normally caused by either an injury to cow's udder, or "immune response to bacterial invasion of the teat canal."[75] Bovine neutrophil function is impaired at higher temperatures, leaving mammary glands more vulnerable to infection,[88] and mastitis is already known to be more prevalent during the summer months, so there is an expectation this would worsen with continued climate change.[75]

One of the vectors of bacteria which cause mastitis are Calliphora blowflies, whose numbers are predicted to increase with continued warming, especially in the temperate countries like the United Kingdom.[89] Rhipicephalus microplus, a tick which primarily parasitises cattle, could become established in the currently temperate countries once their autumns and winters become warmer by about 2–2.75 °C (3.60–4.95 °F).[90] On the other hand, the brown stomach worm, Ostertagia ostertagi, is predicted to become much less prevalent in cattle as the warming progresses.[91]

References

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