Discussions of animal welfare often stress the disagreements that exist between scientists and farmers or industry groups or between different scientists. In contrast, I suggest that there exists considerable agreement as to what does constitute poor welfare particularly when we focus on welfare problems that result from health problems. With the research emphasis that has been placed on the behavioural aspects of good welfare, we risk overlooking the fact that for many high producing animals health problems represent one of the major threats to their welfare. It is on these that I will focus my attention.
The High Producing Animal
As an example of a high producing animal, I shall take the dairy cow. Fig 1. shows data from a number of countries that illustrate some of the changes that have occurred over the last few decades in the production and husbandry of dairy cows.
First, and most striking, is the large increase in average milk production for each individual cow. Based on figures from North Carolina (NC Dept of Agriculture and Consumer Services 1999), each cow produces nearly four times as much milk as she did in the 1950's. Milk production per cow in the US in 1910 was reported to be 1320 kg per year (Peterson 1950), suggesting that relatively little change occurred before 1950. Data from the US as a whole (USDA, National Agricultural Statistics Service 2000) shows that milk production per cow has increased by 20% over the last ten years alone. Similar increases have also been reported in most developed countries (USDA, Foreign Agriculture Service 2000).
A second trend is the apparent reduction in genetic diversity within the dairy herds. At the most obvious level, this trend is apparent in the enormous dominance in many countries of Holstein cows, which now account for over 95% of dairy cows in the US (USDA, Animal and Plant Health Inspection Service 1996). By contrast, in 1948, according to figures in Peterson (1950), Holstein cows accounted for no more than 48% of all registered milk cows in the US. Even within the Holstein breed, there appears to have been a reduction in the genetic diversity. For example, Thompson et al. (2000) reports a fourfold increase in the inbreeding coefficient of dairy cows in the US from 1970 to 1998 (Fig. 1). Unfortunately, there is little data to judge whether a similar situation exists in other countries.
Figure 1. Historical trends in mean milk production per cow (NC Department of Agriculture and Consumer Affairs 1999), coefficient of inbreeding (Thompson et al., 2000) and herd size (NZ Dairy Board 1999). Data expressed as a percent of 1998 values.
Third, is a change in the way animals are housed. There has been a marked increase in the number of cows on each farm. For example, the data from New Zealand shows that average herd size nearly doubled in 20 years (Fig 1; NZ Dairy Board 1999). Similar increases in herd size are likely to be widespread in developed countries. In some countries, particularly the US and Canada, this increase in herd size has occurred alongside a change in the type of housing used for dairy cows. In particular, the development of free-stall housing (cubicles) has resulted in a reduction in the number of cows having access to pasture. For example, in the US 50% of cows now have no access to pasture, while 12% have no access at all to an outside area (USDA, Animal and Plant Health Inspection Service 1996).
As well as these changes in the production and management of dairy cattle, there has been an increase in the incidence of production related diseases. This is most obvious in the reported incidence of lameness in dairy herds of the UK. Surveys done before 1980 report the incidence of less than 10% (e.g. Russell et al. 1982), whereas surveys done since 1980 report the mean incidence of more than 20% (Whitaker et al. 1983; Clarkson et al. 1996; Kossaibati and Esslemeont 1999; Whitaker et al., 2000). A high incidence of lameness is now reported in dairy cattle in many countries (Harris et al. 1988; Wells et al 1993, Philipot et al. 1994; Barkema et al 1994). Data from the milk recording program in Quebec, suggests that the incidence of other production diseases has also increased (Dürr et al. 1997). For example, in 1980 over 16% of cows culled were the result of "voluntary" culling for low milk production. However, in 1994 the number of cows culled "voluntarily" was reduced to only 4.5%, mainly because of a large increase in involuntary culling for problems with reproduction (8%), mastitis (5%), lameness (4%) or because of an unspecified illness (4.5%). Similar rates of culling for these production diseases have been reported from countries as diverse as the UK (Whitaker et al 2000), the US
(Ruegg et al. 1998), Finland (Rajala-Schultz and Gröhn 1999), France (Seegers et al. 1998) and Australia (Stevenson and Lean 1998), again suggesting a widespread trend. These production disease are now considered to be the most important welfare problems facing dairy cattle (Farm Animal Welfare Council 1997), and represent a major financial loss (Enting et al. 1997; Kossaibati et al. 1997; Bennett et al. 1999).
Is intensification the problem?
An obvious question is whether the trends shown in Fig 1 i.e. an increase in milk produced per cow, larger herds with less access to pasture and reduced genetic diversity
are the cause of the production diseases. Critics of modern animal agriculture often point to these aspects of intensification as a cause of reduced animal welfare of dairy cattle (e.g.Adcock and Finelli 1995). Certainly the incidence of lameness appears higher in countries such as the UK (Whitaker et al., 2000), the US (Wells et al. 1993 1995) and the Netherlands (Barkema et al. 1994) with more intensive dairy production (that is: higher production per cow, larger farms, more indoor housing) than in Switzerland (Frei et al. 1997) which has much smaller farms or Australia (Harris et al. 1988) which has much lower production and much less use of indoor housing. In a number of European countries there has been a great increase in the sales of "organic" milk which try to meet the growing consumer demand for milk from less intensive agricultural systems, and which claim to pay greater attention to animal welfare issues. However, surveys to date show that such organic dairy farms do no better than the conventional ones in terms of the incidence of these production diseases (Vaarst et al. 1998; Weller and Bowling 2000)
Farm surveys of the incidence of such maladies always document considerable variation between farms. For example, Whitaker et al (2000) reported that the incidence of culling for reproductive problems, mastitis and lameness in the highest quartile of UK dairy farms was 26 % but was less than 2% in the lowest quartile. Such differences may reflect differences between farms in various factors related to the genetics and housing of the animals. In the following sections, I look at the evidence that relates the incidence of various production disease, with a particular emphasis on lameness, to levels of production per se, genetics, herd size and type of housing. Although both management and nutrition factors are important, I do not discuss these.
Effects of increased production and genetic contributions.
A number of studies have investigated whether or not the occurrence of such maladies among dairy cows is associated with a high level of milk production. Unfortunately, such results are not easy to interpret: the occurrence of these maladies can themselves result in lower milk production (Deluyker et al. 1991) or a correlation between high production and a high treatment rate of diseases may reflect a higher level of vigilance and concern for disease among better herd managers (Emanuelson and Oltenacu 1998). High levels of milk production are widely associated with high levels of mastitis (Faye et al. 1997; Emanuelson and Oltenacu 1998; Waage et al. 1998). However, such a relationship is not as obvious for lameness. Early studies did report higher levels of lameness in herds with higher levels of milk production (Deluyker et al. 1991; Enevildsen and Gröhn 1991a,b), but more recent studies have not reported such a relationship (Vaarst et al 1998; Whitaker et al 2000).
To understand the correlation between levels of milk production and lameness (or indeed of all production diseases) it is necessary to separate the effects of genetics and the effects of management in bringing about the level of production. Rauw et al (1998) reviewed a number of studies that separately calculated the genetic correlations and the environmental correlations between milk production levels and the incidence of various diseases. Environmental correlations were generally absent. In contrast, there were small but important positive genetic correlations between the level of milk production and the incidence of ketosis, mastitis and lameness. This has been confirmed in more recent studies for lameness (Van Dorp et al. 1998). These results suggest that management differences between farms that result in differing levels of production need not necessarily result in changes in the incidence of mastitis or lameness. In contrast, continued breeding and selection of animals purely on the basis of high milk production could likely result in an increased incidence of such production diseases. Rauw et al. (1998) suggested that strong artificial selection for a trait such as milk production leads to the animal using its biological resources to the maximum leaving few resources left to respond to other demands or to various stressors.
The finding that many of these maladies are heritable (even if heritabilities are relatively low) has renewed interest in including such health parameters in selection indices for dairy cattle (Boettcher et al. 1998). However, it remains to be seen whether the genetic correlations between high production and health problems can be uncoupled, or whether they are inevitable, as Rauw et al.'s (1998) argument suggests.
The role of genetic factors in production diseases is emphasised by the fact that Holstein cattle suffer from a higher incidence of both mastitis (Washburn et al. 1998) and lameness (Harris et al. 1988; Alban 1995) than Jersey cattle. In conventional and organic dairy herds in Denmark, a higher incidence of acute sole haemorrhages were found among Danish Holsteins than among other dual purpose breeds (Vaarst et al. 1998). The increasing incidence of inbreeding that has been noted on US farms seems likely to have lead to reduced survival of dairy cattle (Thompson et al., 2000) but there is no evidence available to judge whether or not inbreeding leads to an increased incidence of mastitis or lameness. However, there are increasing calls for the practice of cross breeding in dairy cattle (Hansen 1999).
Concerning the effect of breeding on animal welfare, the UK Farm Animal Welfare Council recommends that: "Achievement of good welfare should be of paramount importance in breeding programmes. Breeding companies should devote their efforts primarily to selection for health traits so as to reduce current levels of lameness, mastitis and infertility; selection for higher milk yield should follow only once these health issues have been addressed" (1997 p66).
Effect of housing and environment.
Considerable research supports the idea that the housing can affect the incidence of lameness and other production diseases. Removing or limiting access to pasture has been found to increase incidence of mastitis (Washburn et al. 1998; Waage et al. 1998; Barkema et al. 1999) and digital dermatitis (Wells et al., 2000). Although free stalls may seem preferable to tie stalls in terms of greater freedom of movement, the use of free stalls has been found to be associated with a higher incidence of lameness (Rowlands et al 1983; Ingvartsen and Anderson 1993; Whitaker et al., 2000). Although loose housing on deep straw has been found to reduce the incidence of lameness compared to free stalls (Whitaker et al, 2000), the reverse relationship has been found with mastitis (Faye et al. 1997; Whitaker et al, 2000). Even considering hoof problems alone, the effects of housing can be complex. For example, compared to straw yards, free stalls have been found to increase sole haemorrhages but decrease heel horn erosion (Livesey et al. 1998), while housing heifers indoors compared to giving them access to dry lots increases haemorrhaging on some parts of the horn but decreases it on other parts of the horn (Vermunt and Greenough 1996). The fact that different housing systems affect different welfare problems in different ways is a common finding in research on animal welfare (Rushen and de Passillé 1993) and makes it difficult to make generalizations regarding the effect of housing on animal welfare.
The effect of type of housing on welfare can depend to a great extent upon the details of the particular system. For example, cows housed in tie-stalls with concrete floors had a higher incidence of sole haemorrhages than cows housed in tie-stalls with rubber mats (Bergsten and Frank 1996). Stalls that are too small or are designed in a way that make it difficult for the cow to get up and lie down have also been found to be associated with an increased incidence of lameness or sole disorders (Philipot et al. 1994; Faull et al. 1996).
Most commentators believe that the main walking surfaces for cows play a major role in affecting the incidence of lameness, with concrete surfaces being seen as a particular problem. Philipot et al. (1994) assessed risk factors for chronic and subacute laminitis as well as heel horn erosion and found that conditions such as high steps were risk factors linked to subacute laminitis. Inappropriate flooring can increase the incidence of lameness by causing excessive and uneven wear of the hoof, by direct damage as a result of uneven surfaces or protrusions, or by causing skin breaks which increase the risk of infectious diseases such as footrot. An epidemiological survey of farms concluded that lameness was prevalent where walking surfaces were smooth concrete (Faull et al. 1996). Cow walking surface has also been related to the incidence of apparently infectious foot diseases: the incidence of papillomatous digital dermatitis (foot warts) was found to be substantially higher on farms where cows walked on grooved concrete compared to farms where cows walked on dirt, pasture or smooth concrete (Wells et al. 1999) perhaps because the abrasive properties of the flooring caused damage to the hooves.
There is not strong evidence that herd size per se increases the incidence of various production disorders. A survey of a large number of farms in the US found no evidence that culling rates for lameness, mastitis or reproductive problems (USDA, Animal and Plant Health Inspection Service 1996) were higher on larger farms . However, in other studies large herd size has been found associated with an increased risk of digital dermatitis (Wells et al. 1999), lameness (Alban 1995) and mastitis in heifers (Waage et al. 1998). It is understandable that larger herds, leading to increased contact between a larger number of animals, would increase the risk for infectious diseases. However, it is clear that large herd size alone should not be assumed to be associated with poorer levels of welfare in general. In contrast, crowding does clearly lead to a higher incidence of production diseases. A reduced number of feeding spaces per cow has been found associated with an increased incidence of mastitis (Barkema et al. 1999) while too few cubicles per cow leads to higher incidence of hoof lesions (Leonard et al. 1996).
The role of behaviour
Although I have concentrated on health related welfare problems, the behaviour of the cattle can be an important mediating factor in influencing how housing systems lead to lameness.
In early research, Colam-Ainsworth et al (1989) found that the incidence of lameness was high in one group of heifers that showed reduced time lying down, presumably because the stalls were uncomfortable. In more systematic research, Leonard et al (1994; 1996) found that when cows were kept either in uncomfortable stalls, or with too few stalls per cow, both the time spent lying down was reduced and the incidence of hoof lesions increased. Furthermore, there were significant negative correlations across individual cows between lying time and the incidence of hoof lesions. This correlation was likely due to reduced lying time leading to increased hoof problems, rather than the reverse, because at pasture lame cows lie down longer than healthy cows (Hassall et al. 1993). This correlation between reduced lying time and increased hoof problems has recently been confirmed (Chaplin et al., 2000).
Rest is clearly an important behaviour for dairy cattle (e.g. Munksgaard and Løvendahl 1993) but can be greatly affected by the design of stalls (e.g. Haley et al., 2000). In a survey of UK dairy herds, Faull et al (1996) noted a widespread use of stalls that were either too small or poorly designed. It seems possible that by reducing resting time in cattle the use of such stalls is a contributing factor to the high incidence of lameness.
Although this review has focussed on the dairy cow, many of the factors leading to the welfare problems discussed are typical of other high producing animals such as pigs and poultry (Rauw et al. 1998). Production diseases such as lameness and mastitis represent the most serious welfare problems for dairy cattle. The incidence of these are clearly affected by a combination of genetics and housing, which is perhaps no surprise. The mounting evidence of genetic correlations between high milk production and the incidence of these maladies, combined with the worrying trend to higher levels of inbreeding in dairy cattle herds, suggest that some thought be given to selecting animals primarily on the basis of health related traits and considering greater use of breeds other than the Holstein. However, the housing environment is clearly important, and the fact the cattle genetically selected for high milk production are more susceptible to these maladies, emphasises the importance of ensuring good housing conditions for high producing animals. However, the relationship between housing and the incidence of these maladies is not a simple one, and few generalizations can be made regarding types of housing such as tie-stalls, cubicles etc. Much depends upon the particular details of each type of housing, such as the number of stalls available and their size and design.
Adcock M, Finelli M (1995) The dairy cow: America's "foster mother" [Web Page]. Located at: www.hsus.org/programs/farm/mcarthur-foster101299.html. Accessed 2000 Aug 1.
Alban L (1995) Lameness in Danish dairy cows: frequency and possible risk factors. Preventive Veterinary Medicine 22, 213-225.
Barkema HW, Westrik JD, van Keulen KAS, Schukken YH, Brand A (1994) The effects of lameness on reproductive performance, milk production and culling in Dutch dairy herds. Preventive Veterinary Medicine 20, 249-259.
Bennett RM, Christiansen K, Clifton-Hadley RS (1999) Estimating the costs associated with endemic diseases of dairy cattle. Journal of Dairy Research 66, 455-459.
Bergsten C, Frank B (1996) Sole haemorrhages in tied primiparous cows as an indicator of periparturient laminitis: effects of diet, flooring and season. Acta Veterinaria Scandinavia 37, 383-394.
Boettcher PJ, Dekkers JCM, Warnick LD, Wells SJ (1998) Genetic analysis of clinical lameness in dairy cattle. Journal of Dairy Science 81, 1148-1156.
Chaplin SJ, Ternent HE, Offer JE, Logue DN, Knight CH (2000) A comparison of hoof lesions and behaviour in pregnant and earlylactation heifers at housing. Veterinary Journal 159, 147-153.
Clarkson MJ, Downham DY, Faull JW, Manson FJ, Merritt JB, Murray RD, Russell WB, Sutherst JE, Ward WR (1996) Incidence and prevalence of lameness in cattle. The Veterinary Record 138, 563-567.
Deluyker HA, Gay JM, Weaver LD, Azari AS (1991) Change of milk yield with clinical diseases for a high producing dairy herd. Journal of Dairy Science 74, 436-445.
Dürr JW, Monardes HG, Cue RI, Philipot JC (1997) Culling in Quebec holstein herds. 2. Study of phenotypic trends in reasons for disposal. Canadian Journal of Animal Science 77, 601-608.
Emanuelson U, Oltenacu PA (1998) Incidences and effects of diseases on the performance of Swedish dairy herds stratified by production. Journal of Dairy Science 81, 2376-2382.
Enevoldsen C, Gröhn YT (1991) Heel erosion and other interdigital disorders in dairy cows: associations with season, cow characteristics, disease and production. Journal of Dairy Science 74, 1299-1309.
Enevoldsen C, Gröhn YT (1991) Sole ulcers in dairy cattle: associations with season, cow characteristics, disease, and production. Journal of Dairy Science 74, 1284-1298.
Enting H, Kooij D, Dijkhuizen AA, Huirne RBM, Noordhuizen-Stassen EN (1997) Economic losses due to clinical lameness in dairy cattle. Livestock Production Science 49, 259-267.
Farm Animal Welfare Council (1997) Report on the Welfare of Dairy Cattle. Ministry of Agriculture, Fisheries and Food, Surbiton, Surrey.
Faull WB, Hughes JW, Clarkson MJ, Downham DY, Manson FJ, Merritt JB, Murray RD, Russell WB, Sutherst JE, Ward WR (1996) Epidemiology of lameness in dairy cattle: the influence of cubicles and indoor and outdoor walking surfaces. Veterinary Record 139, 130-136.
Faye B, Lescourret F, Dorr N, Tillard E, MacDermott B, McDermott J (1997) Interrelationships between herd management practices and udder health status using canonical correspondance analysis. Preventive Veterinary Medicine 32, 171-192.
Frei C, Frei PP, Stark DC, Pfeiffer DU, Kihm U (1997) The production system and disease incidence in a national random longitudinal study of Swiss dairy herds. Preventive Veterinary Medicine 32, 1-21.
Haley DB, Rushen J, de Passillé AM (2000) Behavioural indicators of cow comfort: activity and resting behaviour of dairy cows in two types of housing. Canadian Journal of Animal Science 80, 257-263.
Hansen L (1999) Increased inbreeding and relationships of Holsteins- How much further should we go? In: Kennelly, J. J., Editor. The Tools for Success in the new Millenium: Advances in Dairy TechnologyRed Deer. Alberta. Edmonton: University of Alberta. p 1-12.
Harris DJ, Hibburt CD, Anderson GA, Younis PJ, Fitspatrick DH, Dunn AC, Parsons JW, McBeath NR (1988) The incidence, cost and factors associated with foot lameness in dairy cattle in south-western Victoria. Australian Veterinary Journal 65, 171-176.
Hassall SA, Ward WR, Murray RD (1993) Effects of lameness on the behaviour of cows during the summer. Veterinary Record 132, 578-580.
Ingvartsen KL, Andersen HR (1993) Space allowance and type of housing for growing cattle. Acta Agriculture Scandanvica: Animal Science 43, 65-80.
Kossaibati MA, Esslemont RJ (1997) The costs of production diseases in dairy herds in England. Veterinary Journal 154, 41-51.
Kossaibati MA, Esslemont RJ (1999) The incidence of lameness in a group of dairy herds in England. In. Proceedings of the British Society of Animal Science 1999; 1999 Mar; Scarborough, UK. British Society of Animal Science.
Leonard FC, O'Connell JM, O'Farrell KJ (1996) Effect of overcrowding on claw health in first-calved Friesian heifers. British Veterinary Journal 152, 459-472.
Leonard FC, O'Connell J, O'Farrell K (1994) Effect of different housing conditions on behaviour and foot lesions in Friesian heifers. The Veterinary Record 134, 490-494.
Livesey CT, Harrington T, Johnston AM, May SA, Metcalf JA (1998) The effect of diet and housing on the development of sole haemorrhages, white line haemorrhages and heel erosions in Holstein hiefers. Animal Science 67, 9-16.
Munksgaard L, Løvendahl P (1993) Effects of social and physical stressors on growth hormone levels in dairy cows. Canadian Journal of Animal Science 73, 847-853.
New Zealand Dairy Board (1999) The New Zealand Dairy Industry [Web Page]. Located at: www.nzmilk.co.nz/facts/figures-industry.html. Accessed 2000 Aug 1.
North Carolina Department of Agriculture and Consumer Services: Agricultural Statistics Division (1999) Milk cow numbers and milk production [Web Page]. Located at: www.ncagr.com/stats/dairy/drymchtv.htm. Accessed 2000 Aug 8.
Petersen WE (1950) Dairy Science. 2nd Edition. J. B. Lippincott Co., Chicago.
Philipot JM, Pluvinage P, Cimarosti I, Sulpice P, Bugnard F (1994) Risk factors of dairy cow lameness associated with housing conditions. Veterinary Research 25, 244-248.
Rajala-Schultz PJ, Gröhn YT (1999) Culling of dairy cows. Part II. Effects of diseases and reproductive performance on culling in Finnish Ayrshire cows. Preventive Veterinary Medicine 41, 279-294.
Rauw WM, Kanis E, Noordhuizen-Stassen EN, Grommers FJ (1998) Undesirable side effects of selection for high production efficiency in farm animals: a review. Livestock Production Science 56, 15-33.
Rowlands GJ, Russell AM, Williams LA (1983) Effects of season, herd size, management system and veterinary practice on the lameness incidence in dairy cattle. Veterinary Record 113, 441-445.
Ruegg PL, Fabellar A, Hintz RL (1998) Effect of the use of bovine somatotropin on culling practices in thirty-two dairy herds in Indiana, Michigan, and Ohio. Journal of Dairy Science 81, 1262-1266.
Rushen J, de Passillé AMB (1992) The scientific assessment of the impact of housing on animal welfare: a critical review. Canadian Journal of Animal Science 72, 721-743.
Russell AM, Rowlands GJ, Shaw SR, Weaver AD (1982) Survey of lameness in British dairy cattle. Veterinary Record 111, 155-160.
Seegers H, Beaudeau F, Fourichon C, Bareille N (1998) Reasons for culling in French Holstein cows. Preventive Veterinary Medicine 36, 257-271.
Stevenson MA, Lean IJ (1998) Descriptive epidemiological study on culling and deaths in eight dairy herds. Australian Veterinary Journal 76, 482-488.
Thompson JR, Everett RW, Hammerschmidt NL (2000) Effects of inbreeding on production and survival in Holsteins. Journal of Dairy Science 83, 1856-1864.
USDA Animal and Plant Health Inspection Service (1996) Dairy 96. Washington: USDA.
USDA Foreign Agricultural Service (2000) Dairy: World Markets and Trade [Web Page]. Located at: www.fas.usda.gov/dlp/circular/2000/00-07Dairy/toc.htm. Accessed 2000 Aug 8.
USDA National Agricultural Statistics Service (2000) Milk per cow, 1990-1999 [Web Page]. Located at: www.usda.gov/nass/aggraphs/cowrates.htm. Accessed 2000 Aug 8.
Vaarst M, Hindhede J, Enevoldsen C (1998) Sole disorders in conventionally managed and organic dairy herds using different housing systems. Journal of Dairy Research 65:175-86.
Van Dorp TE, Dekkers JCM, Martin SW, Noordhuizen JPTM (1998) Genetic parameters of health disorders, and relationships with 305-day milk yield and conformation traits of registered Holstein cows. Journal of Dairy Science 81, 2264-2270.
Vermunt JJ, Greenough PR (1996) Sole haemorrhages in dairy heifers managed under different underfoot and environmental conditions. British Veterinary Journal 152, 57-73.
Waage S, Sviland S, Ødegard SA (1998) Identification of risk factors for clinical mastitis in dairy heifers. Journal of Dairy Science 81, 1275-1284.
Washburn SP, White SL, Green JT, Benson GA (1998) Reproduction, udder health and body condition scores among spring and fall calving dairy cows in pasture or confinement systems. Journal of Dairy Science 81 Supplement 1, 265.
Weller RF, Bowling PJ (2000) Health status of dairy herds in organic farming. The Veterinary Record 146, 80-81.
Wells SJ, Garber LP, Wagner BA (1999) Papillomatous digital dermatitis and associated risk factors in US dairy herds. Preventive Veterinary Medicine 38, 11-24.
Wells SJ, Trent AM, Marsh WE, McGovern, PG, Robinson RA (1993) Individual cow risk factors for clinical lameness in dairy cows. Preventive Veterinary Medicine 17, 95.
Wells SJ, Trent AM, Marsh WE, Williamson NB, Robinson RA (1995) Some risk factors associated with clinical lameness in dairy herds in Minnesota and Wisconsin. Veterinary Record 136, 537-540.
Whitaker DA, Kelly JM, Smith EJ (1983) Incidence of lameness in dairy cows. The Veterinary Record 113, 60-62.
Whitaker DA, Kelly JM, Smith S (2000) Disposal and disease rates in 340 British dairy herds. Veterinary Record 146, 363-367.