There is now considerable research that has identified the main risk factors for the underling illnesses that lead to involuntary culling, and mathematical models have been developed to show the negative economic impact of these problems. Despite this, there are continued challenges with ensuring uptake of this knowledge by producers and implementation of best management practices known to improve cow health and welfare. We argue that having clear animal welfare standards that have been developed by the dairy industry itself, combined with greater use of benchmarking to allow producers to compare themselves with their peers, will help overcome these challenges.
(This article belongs to the proceedings from the Cow Longevity Conference 2013 that took place at Hamra farm, Sweden in August 2013)
1. High rates of involuntary culling on a dairy farm because of illness or reproductive problems occur because of poor cow welfare and reduce the profitability of dairy farms. Removing the main causes of involuntary culling will lead to improved animal welfare and improved farm profits.
2. Although world-wide rates of involuntary culling are high, some producers have a low rate showing that they have adopted housing and management practices that control the prevalence of the underlying health problems.
3. Calf illness and mortality is also a contributor to reduced longevity and poor calf management can have a negative impact on the cow’s later productivity.
4. Research has identified some of the main risk factors in indoor housing that lead to lameness, injury, and illness in dairy cattle but a challenge is getting this information implemented by producers.
5. Having clear, industry-led animal welfare standards, and greater use of benchmarking of farm performance, holds promise as a way of increasing the uptake of knowledge by dairy producers to improve the welfare of their animals and reduce involuntary culling
While cattle have the potential to live 20 years or longer, on most modern dairy farms few dairy cattle will live longer than 6 years. In Canada, between 30-40% of the cows are culled from the herd each year (Government of Canada, 2012) at an average age of between 5 and 6 years. It is expected that there will be some turnover of cows on a dairy farm as farmers remove cows because of low milk production or to sell. However, as we explain in this article, much of the low longevity of dairy cows results from involuntary culling of cows because of poor health or problems with fertility. These high rates of involuntary culling on a farm are a sign of poor animal welfare and cost producers considerable amounts of money. But many producers do have good cow longevity and low rates of involuntary culling. Implementation of what science has shown are best management practices can result in significant improvements in animal welfare and farm profitability.
Why is it important to reduce involuntary culling?
Culling because of poor animal health and welfare
One very important reason to reduce involuntary culling is that the majority of cows are now culled because of poor health and welfare. This can be seen when we look at the reasons the producers give for culling the cows. In most intensive dairy production systems in Europe and North America, dairy cows are culled mainly because of reproductive problems, mastitis or poor udder health, lameness and problems with feet and legs, and other forms of illness or injury (Figure. 1).
Figure 1. Percent of cullings that were reported as due to reproductive problems, mastitis, high somatic cell count of udder health problems, lameness or problems with feet and legs, or due to other illness or injury in Canada (Government of Canada, 2012), USA (USDA, 2007), UK (GB Cattle Health and Welfare Group, 2012), Sweden (Ahlman et al., 2011) and Poland (Pytlewski et al., 2010).
Unfortunately, we have much less data on causes of culling outside of North America and Europe, but what data we do have suggest a similar pattern (Figure 2).
Figure 2. Percent of cullings that were reported as due to reproductive problems, mastitis, high somatic cell count of udder health problems, lameness or problems with feet and legs, or due to other illness or injury in China (Wu et al., 2012), Iran (Ansari lari et al., 2012), Turkey (Erdogan et al., 2004),and Mexico (Vitela et al., 2004).
In the modern, intensive dairies developing in China, we see even higher percentages of involuntary culling due to the big three issues of reproductive problems, udder problems and feet and leg problems, while in Turkey and Mexico, other forms of illness or injury predominate (Figure 2). These main causes of culling are widely recognized as signs that the welfare of the animals is poor. Sickness and injury are obvious welfare problems, and lameness is widely recognized as the most serious welfare problem for dairy cows (Rushen et al., 2008; von Keyserlingk et al., 2009). Recently, the European Food Safety Authority concluded that reproductive failure, especially if this is due to health problems, can also be an indicator of poor welfare (European Food Safety Authority, 2009). Thus it is likely that high rates of involuntary culling on a farm indicate a poor level of animal welfare.
Furthermore, involuntary culling for these health/welfare problems is only the tip of the iceberg because the actual prevalence of the underlying health and welfare problems can be considerably higher than the rate of culling. For example, in Canada about 2% of cows are culled because of feet and leg problems (Government of Canada, 2011), but the actual prevalence of lameness among dairy cows is above 20% (Ito et al., 2010; de Passillé et al., 2012), while the prevalence of hoof lesions is even higher, averaging 46% in cows kept in free stalls (Cramer et al., 2008). Similarly, in the USA, an average of 20%-55% of dairy cows are lame at any one time (depending on the region) (Espejo et al., 2006; von Keyserlingk et al., 2013), even though only about 4% of dairy cows are actually culled for lameness (USDA, 2007). The same pattern can be found for mastitis: in Canada about 4% of cows are culled because of mastitis, high SSC or poor udder health (Government of Canada, 2011), but estimates of mastitis incidence are around 23 cases / 100 cow years (Riekerink et al., 2008). Thus, a high rate of involuntary culling on a farm is simply the most visible symptom of a larger problem of poor cow health and welfare.
Dairy cows that are culled for animal welfare reasons are also one of the main sources of bad publicity for dairy and beef production, when, for example, they are transported to feed-lots. Thus, many animal welfare standards for dairy cows now focus upon reducing the occurrence (and having better treatment for) culled dairy cows (e.g. Dairy Farmers of Canada, 2009). Although the overall aim is to reduce involuntary culling, this should be done by reducing the underlying causes, not by discouraging producers from culling animals that are obviously ill. When overall culling rates are high, a farm may have difficulty producing enough replacement heifers to maintain herd size (Orpin and Esslemont, 2010), so that many chronically diseased animals, which should be euthanized on humane grounds, remain in the herd, which is itself an animal welfare problem (Langford and Stott, 2012).
Culling and lost revenue
A high rate of involuntary culling can have a marked, negative economic impact on the farm (Orpin and Esslemont, 2010). Higher yielding cows tend to be more likely to suffer from the problems that lead them to be culled (Hadley et al., 2006) and high rates of involuntary culling may mean that other lower producing cows remain in the herd to maintain replacement rates (Langford and Stott, 2012). Furthermore, the types of health and welfare problems that lead producers to cull their animals are a very important source of lost revenue for producers. A number of studies have tried to estimate the amount of money that is lost as a result of diseases such as lameness or mastitis, and most conclude a cost of several hundred dollars per case is not unreasonable (e.g. Kossaibati and Esslemont, 1997; Guard, 1999). For example, a producer in the north east of the USA with an average prevalence of lameness of 55% (von Keyserlingk et al., 2013) may be losing over $10,000 per hundred cows (Guard, 1999). Based on one estimate of the cost of lameness of $308/case (www.gov.mb.ca), we estimated that Canadian dairy farmers on average were losing over $6,000 / 100 cows /year, while the farms with the highest lameness prevalence were losing over $20,000. Thus it is important for financial reasons that the dairy industry reduces the incidence of the health and welfare problems that lead to a high involuntary culling rate. Although some financial investment must be made to reduce the prevalence of the main causes of involuntary culling, the long term financial rewards have been shown to outweigh these costs (Langford and Stott, 2012).
Culling and environmental impact
In addition, the environmental impact of dairy production is reduced when the longevity of each cow and the efficiency of milk production are high (Gill et al. 2010). Diseases such as lameness that are responsible for involuntary culling have a negative impact on the cow’s productivity and longevity, thereby reducing the overall efficiency of dairy production and increasing the environmental footprint of the dairy industry.
Differences between farms in the causes of involuntary culling
The good news, however, is that while the average rate of involuntary culling is high across the world, there are large differences between individual farms within countries in the extent that older cows are retained in the herd (Figure 3), and in the prevalence of the main welfare problems that lead to involuntary culling.
Figure 3. The percent of cows that are of 3rd lactation or higher (a measure of cow longevity) on individual Canadian dairy farms.
For example, Orpin and Esslemont (2010) report that an average of 3% of cows studied in a sample of 843dairy herds in the UK were culled because of lameness. However, the best quartile of farms culled less than 1%, while the worst quartile of farms culled more than 5.7% of cows because of lameness. Thus, some farmers are able to keep the occurrence of these health and welfare problems down to a low level.
A large difference between farms is also apparent when we look at the main underlying causes of involuntary culling. For example, Figure 4 shows the prevalence of lameness on 100 tie-stall farms in Canada (Charlton et al., 2012).
Figure 4. The prevalence of lameness among dairy cows on 100 tie-stall dairy farms in Canada (data taken from Charlton et al., 2012).
Thus, while the average prevalence of lameness (20%) is high, a number of farms are clearly able to attain a low prevalence (less than 10%). Similar variability is found in the prevalence of lameness on free-stall farms (Ito et al., 2010). A similar situation is found with other causes of involuntary culling, such as mastitis and injuries. The average rate of mastitis on Canadian dairy farms is 23 cases / 100 cow years, but the rate varies across farms from 1% to 97%! (Riekerink et al., 2008). Lesions and injuries to the hock are surprising common on North American dairy farms and are associated with high rates of involuntary culling. Lame cows also tend to have a higher risk of having such an injury (Zaffino, 2012). In our recent survey of Canadian dairy farms, we found that the prevalence of severe hock injuries on individual dairy farms ranged from 0% to 82% (Zaffino, 2012). The large variability from farm to farm in these causes of involuntary culling suggests that certain housing or management methods may be risk factors and that some good producers have found ways to manage and house their animals so as to have only a low incidence of these problems.
Over the last few years, a considerable amount of research has identified some of the main risk factors for the underlying causes of involuntary culling. We do not have the space here to summarize all of this research but give some examples. A recent report of the European Food Safety Authority provides a good summary of the research (European Food Safety Authority, 2009).
One of the main developments in intensive dairy production over the last few decades is that fewer and fewer dairy cows now go to pasture. While there may be good production and environmental reasons for this, the majority of research has shown that this zero grazing increases the risk of lameness in particular (Haskell et al., 2006; von Keyserlingk et al., 2013), but also of other health and welfare problems such as metritis (Bruun et al., 2002) and hock injuries (Barrientos et al., 2013). An effective method of treating lame cows is to put them on pasture (Hernandez-Mendo et al., 2007). Some people conclude from this that it is necessary to allow cows access to pasture in order to ensure their well-being, but an equally plausible interpretation is that we simply have not yet designed ideal indoor-housing for lactating cows. Certainly it is not necessary to put cows out to pasture to have a low level of lameness: in our recent survey we found that nearly a quarter of Canadian dairy farms had a prevalence of lameness less that 10%, which is lower than what some find on pasture-based dairies. However, zero-grazing increases the risk of cows going lame and so places greater responsibility upon the producer to adopt housing and management techniques that allow cows to be kept indoors without going lame. Research has identified a number of the important risk factors in indoor housing that need to be controlled. For example, any factors that increase the time that cows spend standing, especially on wet, concrete floors (Bell et al., 2009) or which reduce the comfort of the lying stalls (Dippel et al. 2009) appear to increase the risk of lameness. Many of the changes needed to reduce the causes of involuntary culling need not require major changes to housing. Sand bedding, in particular, has been repeatedly shown to be associated with lower prevalence of lameness (Espejo and Endres, 2007).
Another cause for concern are the injuries or lesions that are often seen on the hocks, knees and necks of indoor-housed cattle (Figure 5).
Figure 5. An example of a severe injury to the hock (left) and knee (right) of a dairy cow. Based on the method described in Gibbons et al. (2011).
Although hock lesions often do not figure as the reason why producers cull cows, a high prevalence of hock injuries on a farm is associated with a high rate of involuntary culling (Fulwider et al., 2007), and lameness due to severe hock injury is a major cause of lost milk (Bareille et al., 2003). Some dairy farms in North America have a high prevalence with up to 80% of cows having a severe hock lesion (Zaffino, 2012). Furthermore, there is an association between having a hock lesion and being lame: cows that are lame are nearly 1.5 times as likely to have a severe hock lesion as cows that are not lame (Zaffino, 2012). This association may reflect that hock injuries and lameness have similar risk factors: for example use of sand bedding is associated both with a lower prevalence of hock injuries (Figure 6; Zaffino, 2012) and reduced lameness. While switching to sand bedding does require significant changes to the buildings, simply adding more straw or sawdust bedding can go a long way to reduce hock injuries (Zaffino, 2012). For example, on farms with mattresses and no use of bedding, we found that nearly 80% of the cows suffer from hock lesions, whereas on farms with more than 2cms of bedding the prevalence of hock lesions was 31% (de Passillé et al., 2012). Indeed, insufficient bedding is one of the major risk factors for hock injuries, especially when geotextile mattresses are used (Barrientos et al. 2013), which are associated with an increased risk of hock lesions (Zaffino, 2012; Figure 6).
Figure 6. The relative chance of a cow having a severe hock or knee injury as a function of the type of lying stall base (data taken from Zaffino 2012).
Although mattresses are associated with a high prevalence of hock lesions, adding mattresses to concrete based stalls can greatly reduce injuries to the knees (Figure 6, Zaffino, 2012). Other injuries can also be reduced by relatively simple changes in housing: increasing the height of the feed rail at the feed bunk to above 140cms from the floor greatly reduces the risk of neck injuries (Zaffino, 2012). Thus, significant reductions in cow injury rates can be achieved without major changes to the dairy buildings.
This, by identifying the main risk factors that are associated with a high prevalence of the main causes of involuntary culling, we should, in theory, be able to reduce the occurrence of involuntary culling.
Replacement calves and heifers
Low cow longevity is often considered the result of involuntary culling that occurs during or after the first lactation. However, many “cows” die or are removed from the herd while they are still calves, with most of this occurring during the milk-feeding period. In North America, pre-weaning calf mortality rates are high (Vasseur et al., 2012). In some countries of Europe, lower mortality rates have been reported (Svennson et al., 2006, Vasseur et al., 2012), and in Canada there are large differences between farms in calf mortality rates (Vasseur et al., 2012). Furthermore, there are substantial differences between farms in the extent that calf mortality is recognized and considered a problem. For example, Vasseur et al. (2010a) reports that some farms with mortality rates above 19% did not consider calf mortality to be a problem. Unfortunately, dairy producers, at least in North America, generally do not keep good records of calf mortality (Vasseur et al., 2012), and so it is difficult to estimate the true extent of this problem. Calf and heifer rearing is costly and many farmers underestimate the cost of this (Vasseur et al., 2010a). Nor is there a large body of evidence on the risk factors associated with calf mortality. Individual housing does not appear to solve the problem, at least when compared to small groups, but larger groups ( more than 7-10 animals) do appear to be associated with higher mortality (Losinger and Henrichs, 1997; Svensson et al., 2006). Calf illness is obviously an animal welfare concern but it is fair to say that dairy producers tend to underestimate the importance of calf mortality or morbidity, partly because producers do not have good information on how the performance of calves affects their later performance as lactating cows. Recent research showing the impact of pre-weaning growth rates on later milk yields (e.g. Soberon et al., 2012), for example, may help bring increased attention to the importance of maintaining good calf and heifer health and welfare. However, a failure on many farms to implement well-known and researched best management practices (Vasseur et al., 2010a) contributes to the continuing high levels of calf mortality on many farms. For this reason, we have developed an extension tool that helps dairy producers adopt management practices that will increase calf health and welfare (Vasseur et al., 2010b).
Conclusion: How to get the information to the farmer
There is now considerable research that has identified the main risk factors for the underling illnesses that lead to involuntary culling, and mathematical models have been developed to show the negative economic impact of these problems (Langford and Stott, 2012). Despite this, there are continued challenges with ensuring uptake of this knowledge by producers and implementation of best management practices known to improve cow health and welfare (Bell et al., 2009). We argue that having clear animal welfare standards that have been developed by the dairy industry itself (e.g. Dairy Farmers of Canada, 2009), combined with greater use of benchmarking to allow producers to compare themselves with their peers (de Passille et al., 2012; von Keyserlingk, et al., 2013), will help overcome these challenges.
1. Ahlman, T., Berglund, B., Rydhmer, L., Strandberg, E., 2011. Culling reasons in organic and conventional dairy herds and genotype by environment interaction for longevity. Journal of Dairy Science 94, 1568-1575.
2. Ansari-Lari, M, Mohebbi-Fani, M., Rowshan-Ghasrodashti. A. 2012. Causes of culling in dairy cows and its relation to age at culling and interval from calving in Shiraz, Southern Iran. Veterinary Research Forum. 2012; 3 : 233 – 237
3. Bareille, N., Beaudeau, F., Billon, S., Robert, A., Faverdin, P., 2003. Effects of health disorders on feed intake and milk production in dairy cows. Livestock Production Science 83, 53-62.
4. Barrientos, A.K., Chapinal, N., Weary, D.M., Galo, E., von Keyserlingk, M.A.G., 2013. Herd-level risk factors for hock injuries in freestall-housed dairy cows in the northeastern United States and California. Journal of Dairy Science.
5. Bell, N.J., Bell, M.J., Knowles, T.G., Whay, H.R., Main, D.J., Webster, A.J.F., 2009. The development, implementation and testing of a lameness control programme based on HACCP principles and designed for heifers on dairy farms. Veterinary Journal 180, 178-188.
6. Bruun, J., Ersbøll, A.K., Alban, L., 2002. Risk factors for metritis in Danish dairy cows. Preventive Veterinary Medicine 54, 179-190.
7. Charlton, G., Bouffard, V., Bécotte, F., Haley, D. B., Rushen, J., and de Passille, A. M. 2012. Lying times of dairy cattle on tie-stall farms. Dairy Cattle Welfare Symposium, Guelph, ON, Oct 24-26, 2012.
8. Cramer, G., Lissemore, K.D., Guard, C.L., Leslie, K.E., Kelton, D.F., 2008. Herd- and Cow-Level Prevalence of Foot Lesions in Ontario Dairy Cattle. J. Dairy Sci. 91, 3888-3895.
9. Dairy Farmers of Canada. 2009. Code of Practice for the Care and Handling of Dairy Cattle. Dairy Farmers of Canada, Ottawa, ON. Canada
10. de Passillé, A. M., J. Rushen, D. Vora, J. Gibbons, E. Vasseur, and G. Charlton 2012. Measuring up: Research project into improving cow longevity also benchmarks key aspects of adherence to Code of Practice recommendations. The Milk Producer. October 2012. Pp. 34-36.
11. Dippel, S., Dolezal, M., Brenninkmeyer, C., Brinkmann, J., March, S., Knierim, U., Winckler, C., 2009. Risk factors for lameness in cubicle housed Austrian Simmental dairy cows. Preventive Veterinary Medicine 90, 102-112.
12. Erdoǧan, H.M., Güneş, V., Çitil, M., Ünver, A., 2004. Dairy cattle farming in Kars District, Turkey: II. Health status. Turkish Journal of Veterinary and Animal Sciences 28, 745-752.
13. Espejo, L.A., Endres, M.I., Salfer, J.A., 2006. Prevalence of Lameness in High-Producing Holstein Cows Housed in Freestall Barns in Minnesota. J. Dairy Sci. 89, 3052-3058.
14. Espejo, L.A., Endres, M.I., 2007. Herd-Level Risk Factors for Lameness in High-Producing Holstein Cows Housed in Freestall Barns. J. Dairy Sci. 90, 306-314.
15. European Food Safety Authority Panel on Animal Health and Welfare (2009). Scientific Report on the Effects of farming systems on dairy cow welfare and disease. EFSA-Q-2006-113.
16. Fulwider, W.K., Grandin, T., Garrick, D.J., Engle, T.E., Lamm, W.D., Dalsted, N.L., Rollin, B.E., 2007. Influence of free-stall base on tarsal joint lesions and hygiene in dairy cows. Journal of Dairy Science 90, 3559-3566.
17. GB Cattle Health and Welfare Group. 2012. First Annual report. www.chawg.org.uk
18. Gibbons, J., Vasseur, E., Rushen, J., De Passillé, A.M., 2012. A training programme to ensure high repeatability of injury scoring of dairy cows. Animal Welfare 21, 379-388.
19. Government of Canada 2012. Culling and replacement rates in dairy herds in Canada. Ottawa, On. www.dairyinfo.gc.ca
20. Guard, C. 1999. Control programs for digital dermatitis. Advances in Dairy Technology. 11: 235-241
21. Hadley, G.L., Wolf, C.A., Harsh, S.B., 2006. Dairy cattle culling patterns, explanations, and implications. Journal of Dairy Science 89, 2286-2296.
22. Haskell, M.J., Rennie, L.J., Bowell, V.A., Bell, M.J., Lawrence, A.B., 2006. Housing system, milk production, and zero-grazing effects on lameness and leg injury in dairy cows. Journal of Dairy Science 89, 4259-4266.
23. Hernandez-Mendo, O., von Keyserlingk, M.A.G., Veira, D.M., Weary, D.M., 2007. Effects of Pasture on Lameness in Dairy Cows. J. Dairy Sci. 90, 1209-1214.
24. Ito, K., von Keyserlingk, M.A.G., LeBlanc, S.J., Weary, D.M., 2010. Lying behavior as an indicator of lameness in dairy cows. Journal of Dairy Science 93, 3553-3560.
25. Kossaibati, M.A., Esslemont, R.J., 1997. The costs of production diseases in dairy herds in England. Veterinary Journal 154, 41-51.
26. Losinger, W.C., Heinrichs, A.J., 1997. Management practices associated with high mortality among preweaned dairy heifers. Journal of Dairy Research 64, 1-11.
27. Orpin, P.G., Esslemont, R.J., 2010. Culling and wastage in dairy herds: An update on incidence and economic impact in dairy herds in the UK. Cattle Practice 18, 163-172.
28. Pytlewski, J., Antkowiak, I., Staniek, M., Skrzypek, R., 2010. Intensity and causes of culling in polish black-and-white holstein-friesian cows. Annals of Animal Science 10, 477-487.
29. Riekerink, R.G.M.O., Barkema, H.W., Kelton, D.F., Scholl, D.T., 2008. Incidence rate of clinical mastitis on Canadian dairy farms. Journal of Dairy Science 91, 1366-1377.
30. Rushen, J., de Passillé, A. M., von Keyserlingk, M., Weary, D. M. (2008). The Welfare of Cattle. Springer, Dordrecht, The Netherlands. pp303.
31. Rushen, J., and de Passillé, A. M. 2012. Assessment and Guidelines for Dairy Cattle Welfare. In: Proceedings of the 1st Dairy Cattle Welfare Symposium. Campbell centre for Animal welfare, University of Guelph, Guelph, ON. Pp: 42-49.
32. Soberon, F., Raffrenato, E., Everett, R.W., Van Amburgh, M.E., 2012. Preweaning milk replacer intake and effects on long-term productivity of dairy calves. Journal of Dairy Science 95, 783-793.
33. Svensson, C., Linder, A., Olsson, S.O., 2006. Mortality in Swedish Dairy Calves and Replacement Heifers. J. Dairy Sci. 89, 4769-4777.
34. USDA. 2007. Dairy 2007, Part I: Reference of Dairy Cattle Health and Management Practices in the United States, 2007 USDA-APHIS-VS, CEAH. Fort Collins, CO, USA
35. Vasseur, E., Borderas, F., Cue, R.I., Lefebvre, D., Pellerin, D., Rushen, J., Wade, K.M., de Passillé, A.M., 2010a. A survey of dairy calf management practices in Canada that affect animal welfare. Journal of Dairy Science 93, 1307-1315.
36. Vasseur, E., Rushen, J., de Passillé, A.M., Lefebvre, D., Pellerin, D., 2010b. An advisory tool to improve management practices affecting calf and heifer welfare on dairy farms. Journal of Dairy Science 93, 4414-4426.
37. Vasseur, E., Pellerin, D., de, P., M., A., Winckler, C., Lensink, B.J., Knierim, U., Rushen, J., 2012. Assessing the welfare of dairy calves: outcome-based measures of calf health versus input-based measures of the use of risky management practices. Animal Welfare 21, 77-86.
38. Vitela, I. M., Cruz-Vazquez, C., Parra, M. R. 2004. Identification of culling reasons in five dairy farms of Aguascalientes, Mexico. Técnica Pecuaria en México. 42: 437-444.
39. von Keyserlingk, M.A.G., Rushen, J., de Passillé, A.M., Weary, D.M., 2009. Invited review: The welfare of dairy cattle-key concepts and the role of science. Journal of Dairy Science 92, 4101-4111.
40. von Keyserlingk, M.A.G., Barrientos, A., Ito, K., Galo, E., Weary, D.M., 2012. Benchmarking cow comfort on North American freestall dairies: Lameness, leg injuries, lying time, facility design, and management for high-producing Holstein dairy cows. Journal of Dairy Science 95, 7399-7408.
41. Wu, J.J., Wathes, D.C., Brickell, J.S., Yang, L.G., Cheng, Z., Zhao, H.Q., Xu, Y.J., Zhang, S.J., 2012. Reproductive performance and survival of Chinese Holstein dairy cows in central China. Animal Production Science 52, 11-19.
42. Zaffino, J. C. 2012. An evaluation of hock, knee, and neck injuries on dairy cattle in Canada . M Sc Thesis, University of Guelph.