Animal diseases are responsible for a significant production loss in dairy farms. Diseases are seldom planned for. Some cannot be eradicated, some are zoonotic, and hence they present a fatal risk to the whole herd and public health. Furthermore, some animal diseases are too costly to treat, ^{11 , 32} hence are further regarded as indirect causes of poor milk yield because sick animals lose appetite and are mostly depressed, and as such cannot yield the expected quantity of milk. ^{11 , 33} Important considerations are made before a decision to cull is taken, such as the physiological status of the cow, genetic variability and adaptability, and milk production potential. ³⁴ Diseases directly influence on the culling and mortality rates of dairy cows. ¹¹ Some of these diseases include bovine neosporosis caused by Neospora caninum, which affects intensive dairy production. In addition, other common diseases or disorders that play an important role in culling decisions include milk fever, lameness, dystocia, udder deformities, ketosis, metritis, and mastitis.

Pathogenesis of milk fever

Hypocalcaemia (milk fever) is a metabolic disorder which may induce reproductive failures if not properly controlled. ³⁹ It is a condition whereby the cow’s calcium (Ca) requirements exceed the supplied calcium induced by homeorhetic processes (sudden increased loss of blood Ca into the milk). Cows obtain calcium from bloodstream, rumen and bones. Briefly, when blood calcium levels are low, Parathyroid Hormone (PTH) is released into the circulatory and endocrine system thus enhancing Ca mobilisation by the bone. The higher the PTH levels, the higher the body calcium levels. The Ca absorption and resorption process is facilitated by the presence of vitamin D, a product generated from vitamin D2 from plants. ³⁹ This process of Ca mobilisation is largely influenced by the amount of Ca in the cow’s diet, high levels reduce the process efficiency, but constant supply of Ca is necessary. Therefore, any poor reaction by the rumen and intestines to the variation in dietary Ca levels results in hypocalcaemia. It is this technicality (balancing dietary Ca levels) that may contribute to the continued prevalence of milk fever amongst dairy cows as feed formulation expertise become critical during calving period. Ca mobilisation process is also influenced by the age of the dairy cow i.e., older cows have slower rate compared to young cows thus increasing the risk of milk fever amongst older cows. This is because metabolism rate deteriorates with age.

Pathophysiology of ketosis

In dairy, ketosis is a metabolic disorder caused by an imbalance in demand and supply of energy in the form of glucose and glycogen in dairy cows characterised by high levels of ketone bodies in blood and urine. ⁴⁰ This means that, it is a rapid loss of ketone bodies during lactation caused by high milk production. ⁴¹ Dairy cows derive glucose and glycogen from metabolic and digestive activities. Over the years, there has been varying views about pathophysiology of ketosis whether it is largely biochemically or hormonally influenced.

Briefly, ketone bodies are compounds made up of acetone, acetoacetate and beta-hydroxybutyrate that serve as energy substrates to ruminants. Ketone bodies concentration in dairy cows relies on the energy content of the diet especially during early lactation. An increased energy demand induced by heavy milk production stimulates fat mobilisation by the adipose tissue while mitochondria of liver cells assimilate non-esterified fats (NEFA). ^{40, 41} The absorption of NEFA in large quantities facilitates the formation of lipids and ketogenesis. However, if there is poor supply of dietary energy during early lactation, the process of fat mobilisation becomes inefficient thus limiting the process of ketogenesis resulting in low ketone bodies in the blood and body resulting in a disorder known as ketosis.

Culling and mortality

During early lactation, the first 30 days, there is a high culling rate due to milk fever. ¹¹ It has also been reported that milk fever contributes to lactation culling, especially late lactation after 240 days. ¹¹ The other disease of interest that has been identified as a causative agent for culling and mortality of dairy cows is ketosis. Ketosis refers to an energy imbalance in the cow in terms of the demand and available energy for the cow. ^{11 , 32} This disease requires a high level of management as it has resulted in the culling of many cows during late lactation. ¹¹ Cows with ketosis are culled because they tend to disrupt the breeding process and, in some cases, results in infertility. ¹¹ However, the effect of ketosis on culling of dairy cows has not been concluded as different studies have contradicting reports in terms of the risk level. Contrary to earlier studies, it has been reported that ketosis can be easily managed and has no effect on milk yield after 30 days, thus invalidating ketosis as a major factor responsible for culling. ³⁵

Mastitis

Mastitis is the most significant disease of economic importance in the dairy industry. In clinical mastitis cases, the udder is characterised by inflammation, pain (the cow refuses to be milked), discolouration of milk, watery and bloody milk, flakes or clots to purulent exudate, and reddish discolouration of the skin and swelling of the udder. ^{11 , 36 – 38} The disease has been responsible for the culling of many dairy cows during the first and second lactation with a risk ratio of 1.9. ¹¹ Evidence from many recent studies underpins finding in previous studies that mastitis increases the risk of culling in dairy cows. ^{11 , 39 , 40} The prevalence of mastitis varies with the cow’s stage of lactation; heifers are more susceptible to clinical mastitis immediately after calving. ^{21 , 41} Furthermore, cows that exhibit clinical mastitis recurrence are more likely to be culled than first-time casualties as they become a liability to the farm. ⁴²

At any milk production stage, mastitis in dairy cows may be induced by one pathogen or a combination of many pathogens. ²⁰ Mastitis cases are classified in terms of the origin of the pathogen, i.e. contagious pathogens and environmental pathogens. ⁴³ Examples of contagious pathogens are Staphylococcus aureus, Streptococcus agalactiae, Corynebacterium bovis and Mycoplasma species. ^{37, 38, 43, 44} Contagious pathogens are mainly found in the cow’s intra-mammary glands. They are transmitted from one cow to another through milking machines, flies, and milking operators’ hands. ⁴³ Environmental pathogens originate from the cow’s surroundings and are transmitted into the cow through the teat during contact with contaminated water, contaminated bedding, and contaminated soil. ^{43 , 44} Environmental pathogens are Escherichia coli, Streptococcus uberis, Streptococcus dysgalactiae and Klebsiella species.

The severity of infection caused by these pathogens varies and depends on the stage of lactation, lactation number, age of the cow, and the breed type. ²⁰ These mechanical dynamics present a risk of lesions or alteration of the teat end, inducing subclinical mastitis. ⁴⁴ Gram-negative pathogens are accountable for clinical mastitis in high-producing multiparous cows. ⁴⁵

In terms of cost, mastitis is a disease of economic importance in dairy farms as it directly influences the milk quality and quantity, ^{42 , 45} which, when compromised, have significant financial implications. ^{41 , 42} Mastitis-related economic losses in a dairy farm include costs of medication, veterinarian costs, labour costs, loss of quality (drop in butterfat content), discarded milk, culling, and occurrence of other diseases. ⁴⁶ These economic losses depend on the type of production system; an intensive system incurs more costs due to the high-input production methods, whereas an extensive system incurs the least costs. The economic losses also vary on the type of mastitis: clinical mastitis (associated with acute cases) and subclinical cases (chronic cases). In recent studies, economic losses due to mastitis have ranged from 2369.72 Rand to 2765 Rand/cow/year. ^{46 , 47} In a study conducted in the Netherlands, approximately 107 kg and 336 kg of milk per lactating cow were lost due to subclinical mastitis and clinical mastitis, respectively. ⁴⁷ It is evident that clinical mastitis results in severe milk production losses even though it is associated with acute cases. Milk production losses also include discarded milk due to contamination, whether thrown away or fed to the calves. ⁴⁸ In the first 30 days of a lactation study, discarded milk accounted for 5.7% of direct costs incurred in a dairy farm per case of clinical mastitis, approximately R450.00 per case. ⁴⁸ In the same study, veterinary costs accounted for only 1% of the total direct costs due to mastitis, the report was in line with previous studies. ^{46 – 48}

Mastitis further causes significant losses through reproductive disruptions which, when they occur, means that the farmer is most likely forced to cull the cow. ^{11 , 20 , 49} Replacement heifers’ susceptibility and high incident rate to clinical mastitis peri-partum presents the farm with substantial financial losses in terms of medical costs and drop in milk yield. ²¹

Lameness is a paramount indicator of animal welfare; this is because lameness is a condition that is commonly associated with pain. ^{50 – 52} In the United States, lameness is noted as a significant contributor to the culling of dairy cows. ⁵³ For dairy farmers, lameness is an important subject because it imposes serious financial losses, as such culling becomes an alternative. Lameness is also important to dairy farmers as it may be associated with the farm’s poor detection of locomotion defects. ⁵⁰ In the United Kingdom, one study reported the prevalence of lameness as 36.5% in 205 dairy farms. ⁵⁰ The study further emphasised that lameness cases vary with the seasons and environment in which the cows are exposed. There are more cases of lameness in spring. ⁵⁴ Lameness is closely associated with an intensive milk production system, in which the cows are subjected to standing for long periods on a concrete floor and imbalanced diets that are solely formulated to enhance milk production yet predispose the cow to lameness. ⁵²

A study conducted in New Zealand reported that lameness is closely correlated to reproductive failures. In cases where lameness affects reproductive performance, such cows have increased chances of being culled. ³⁰ This further buttress that reproductive failures are mainly responsible for the culling decision in many dairy farms. ^{9 , 23 , 25} Lameness is most common in multiparous cows, especially cows that are in third or higher parity. Therefore, lameness coincides with peak production of the dairy cows, which is tricky for a farmer as replacement heifers would not immediately meet production levels required. ^{8 , 30} Lameness can be in the form of claw ulcers closely associated with milk yield or production performance. ³¹ Lameness is considered as a disease of economic importance as, upon occurrence, it affects the whole lactation period of the cow. As a result, culling due to lameness is a rationally taken decision in most cases. Lameness is a disease of economic importance in dairy farms; it significantly reduces milk yield. ^{50 , 59} Milk yield is the currency of a dairy farm. Thus, any loss in milk yield has a monetary value attached to it. Consequently, a cow with a persisting lameness case is at high risk of being culled. Lameness has also been closely linked to poor fertility amongst dairy cows. ⁵³ Poor fertility is a major factor responsible for culling. ²⁵ One of the effects of lameness is delayed ovarian cycles, which disrupt the breeding cycle ⁵³ and eventually induce poor reproductive performance. ³¹ Dairy cows with a reduced reproductive performance are immediately culled from the dairy farm. ¹⁵ There is a positive correlation between delayed ovarian cycle and body condition losses, ketosis, and puerperal disturbances. Also, veterinary costs associated with the treatment and management of lameness increase the cost of managing and running dairy farms. ³⁰ Furthermore, sub-optimal treatment of lameness results in high lameness recurrence cases. ³⁰

The factors that induce lameness are environment, management, the cow (breed, stage of lactation, age), and nutrition. ⁵¹ Furthermore, environmental factors such as grazing pastures, type of housing, and concrete surfaces are other predisposing factors of lameness. ⁵⁰ Specifically, concrete surface is closely associated with hoof lesions. ^{50 , 55} There are more lameness (claw disorders) incidences amongst dairy cows housed indoors (concrete floors) than those under pasture-based systems, whereby they are frequently susceptible to small rocks, wire and metal. ⁵² A damaged hoof and joint stiffness impair a dairy cow’s locomotion, ³¹ directly affecting the cow’s feeding patterns. For this reason, farmers must pay close attention to claw health. Impaired locomotion is more evident amongst Holstein-Friesland because of their high milk yield pedigree. ⁵⁶ Bony changes in the pedal bone accounted for the increased vulnerability to the lameness of old dairy cows. ³⁰

The nutritional disorder closely associated with lameness is clinical and subclinical ruminal acidosis. ⁵¹ Discrepancies in herd management often lead to nutrition disorders such as systemic or metabolic acidosis. ³¹ A concentrate diet fed with limited functional fibres results in reduced chewing, so the cow produces a limited quantity of buffering saliva, resulting in a drop of the rumen pH. ⁵⁷ Hence the blood becomes acidic from too much acid being absorbed from the rumen into the blood, leading to acidosis.

Acidic blood cannot carry as much oxygen, and cow’s feet, at the farthest points of the cow’s body, receive the least oxygen. ^{57 , 58} The lack of blood supply causes the release of histamine, a vasodilator and arterial constrictor. The sensitive tissues lining the outside wall of the hoof and coffin bone cause ulcers, pain, and haemorrhages associated with laminitis. ^{57 , 58}

Several studies have reported a very close association between reproductive failures and high culling rates. ^{9 , 15 , 23} Reproductive failures have been closely linked to the age at first calving. Early age at calving improves reproductive performance. ¹⁷ Conception status is considered the critical influencer in culling a cow. ¹¹ Reproduction failures, specifically missing conception or failure to deliver the calf, are a leading global factor that causes the culling of dairy animals. ^{9 , 13 , 23 , 25} Cows that generally fail to conceive from a single service tend to have lengthy calving intervals. ²⁹ Such cows that are costly to the farm, as a result, have a high probability of being culled than the cows that readily conceive.

Heat stress

Heat stress negatively interferes with production and reproduction dynamics. Hence it is of economic importance in the dairy production industry. ^{60 – 62} The oestrus cycle and the conception rate of dairy cows are key factors that affect breeding outcomes in a particular breeding season. During periods of heat stress, cows are seldom likely to show signs of oestrus or heat, which further indicates decreased amounts of reproductive hormones in the blood. ⁶³ Without signs of oestrus, the farmer does not know that a cow should be artificially inseminated or be bred, and when a cow does not get inseminated, the cow cannot conceive. Besides, heat stress disrupts the embryo's development by significantly reducing the cow’s dry matter intake, which is a critical component of gestating dairy cows. ⁶⁴ It further distorts embryo development by inhibiting the functioning of dominant follicles and oocytes, thus rendering them unable to breed successfully.

Infertility

The goal of any breeding program should be to have 90-95% of cows bred in a 65-day breeding season. ⁶⁵ Often, this is not the case due to nutritional and non-nutritional factors. Factors causing infertility and animals showing a diminished or absent capacity to produce viable offspring are removed from the breed stock. ⁶⁵ In a study conducted in France, infertility (26.1%) was the most frequent cause for culling than low milk yield. ²⁵ In the same study, more than 50% of other reasons were health-related such as lameness. ⁵⁰ Poor fertility is a global dairy production challenge that has caused the culling of many dairy cows. For instance, in one study investigating culling in 50 dairy herds in England, poor fertility was responsible for 37% of culling. ²⁸ A review conducted in Southern Africa amongst smallholder dairy farms mentioned that factors contributing to infertility include inadequate housing, poor nutrition and improper health management systems. ⁶⁶ Infertility results in poor herd growth, a decrease in replacement of heifers, and generally low farm productivity ⁶⁶ ; hence it is logical to cull infertile cows as they become a liability to the farm. The unavailability of replacement heifers limits voluntary culling in which the farmer has the prerogative to cull dairy cows. ^{17 , 18} It further results in the retention of older cows even when the milk yield is declining.

Parity

Previous studies have shown a relationship between parity and the culling rate of cows. ^{67 , 68} An increase in parity is often associated with a continuous drop in culling frequency, with the highest culling rate at parity 1 to 3. ⁶⁷ Animals in parity 1 to 3 produce more milk than animals in the other parities. Hence, culling dairy cows between first parity and third parity is a costly exercise for dairy farms because it is a stage whereby cows are producing more milk. ^{18 , 67} Udder morphological disorder is one of the leading predisposing factors for culling across parities. ^{8 , 23 , 25} However, culling due to udder problems is more evident during the fourth to sixth parity, where it accounts for almost 50% as a reason to cull. In contrast, reproductive failures are the main factors that lead to culling in second and third parities. ²⁹ Parity presents a significant risk for culling, especially to Holstein-Friesian, more than other dairy breeds. ²⁹ The first and second parity are two vulnerable stages in the cow’s production life as it is exposed to cases of mastitis and ketosis. ¹¹ It is in these stages that culling due to ketosis is frequently reported.

Culling based on age varies with farm management styles/plans, herd size, and milk yield prowess. ¹⁷ There is an association between herd size and age at culling, in which the smallest herd size had the oldest age at culling. ¹⁷ Several studies have reported fewer incidences of culling due to age. Therefore, this low incidence of culling due to age is hypothetically an indicator of a lack of potential for longevity and lengthy productivity amongst dairy cows. The availability of replacement heifers and mean milk yield by the seasoned cow strongly dictates the age of culling. ^{17 , 69}

In a commercial dairy farm, it is expected that a cow not yielding the anticipated quantities of milk is culled. Hence, it could be assumed that low-producing cows are at high risk of culling as milk is the primary product. However, previous studies have reported that high producing cows are more likely to be culled than low producing cows due to the complications associated with high production levels. ^{18 , 42} Complications such as recurrent milk fever and abortion are common reasons high milk producers are often culled. ^{12 , 18 , 67} Poor milk yield has been attributed to age, clinical mastitis, recurrent mastitis, and breed types. On the other hand, a rise in the culling rate of cows has often been associated with an increase in milk yield. ⁴² There is no clear scientific explanation for this correlation between culling and high-producing cows. However, several reports have highlighted that high milk yield is a contributing factor to the culling of dairy cows. ^{12 , 13 , 24 , 25} In addition to the ambiguity of why high milk producers are culled, there is a negative correlation between mastitis (a common reason for culling dairy cows) and high milk production. ¹² The closest reasoning or influence to the culling of high-producing cows is abortion cases, especially in Holstein breeds. Another reason for culling high-producing cows is a significant drop in milk production, the drop in milk yield is influenced by age, nutrition, and diseases. ^{12 , 29}

A systematic recording system of health records may help minimise misdiagnosis and premature culling of dairy cows. An earlier study suggested that an improved recording system should provide an epidemiological baseline for each health factor that may tamper with dairy cows’ production and reproduction performance. ¹⁶ This is essential considering that the decision to cull dairy cows voluntarily is the farmer’s prerogative. ^{12 , 75} The reliance on farmers' context to voluntarily cull a dairy cow may be subjective; hence, a systematic recording system is necessary. Systematic recording promotes the evaluation of regular health disorders. Also, data derived from a systematic recording system should be the basis of breeding plans to improve the longevity of dairy cows. ¹⁶

The performance of dairy cows is mainly influenced by genetics and the environment, particularly nutrition and management. Genetic evaluation and prediction of cow survival should form part of management strategies to lower culling and mortality rates. ⁷⁶ However, genetic evaluation and selection should not only revolve around the milk production potential of the cow. ⁶⁸ Genetic evaluation throughout the heifer rearing stage should also be prioritised to help predict cow survival. Genetic evaluation at the heifer level may limit premature culling and eliminate important traits as some functional traits are cumulative despite low heritability. ⁷⁶

Over the years, selection for mainly milk production has proven unsustainable, especially with high-producing dairy cows being reported as highly susceptible to mastitis, lameness and reduced length of reproductive life. ³⁰ Also, when semen is selected, physical characteristics, such as good leg and feet, and condition of the genitals, precedes other considerations such as cow survival. ⁷⁰

Furthermore, the influence of heat stress on dairy cows varies with breeds and stages of production because there are genetic traits of heat tolerance that vary with breeds. ⁶³ Previous attempts to cross-breed indigenous cattle breeds with the exotic breed to mitigate heat stress have, however, resulted in reduced milk yield. ^{3 , 61} However, the level of reduction of milk yield due to cross-breeding is not extensively documented. As such, it is unclear whether this drop is of any significance.

High ambient temperatures directly adversely influence the productivity of the cows to the extent that they can even contribute to the mortality of the cows. ^{19 , 77} This is because animals respond to high temperatures by reducing feed intake, altering respiration rate, and increasing water intake to facilitate cooling. This might have a limited impact over a single day or two during a hot summer’s day. However, under prolonged periods of high temperatures, the drop in feed intake becomes highly unsustainable, resulting in a decrease in milk yield volumes. ^{3 , 61} Hence, establishing shelters and shades around the farm would provide significant relief to dairy cows and reduce culling and mortality rates.