Articular/epiphyseal osteochondrosis in Thoroughbred foals at 5 months of age: Influences of growth of the foal and prenatal copper supplementation of the dam

Commercial equine production in New Zealand. 2. Growth and development of the equine athlete

During the past 20 years, there has been a contraction in the New Zealand Thoroughbred industry and, to a lesser extent, within the Standardbred industry. These changes have seen an increasing proportion of the market being associated with fewer larger commercial farms. Many of these farms manage their own mares, and the mares and foals of several clients. This, in turn, has increased the similarity of the management of breeding and young stock within New Zealand. The temperate climate allows the majority of the management of breeding and young stock to be pasture based. The predominant pasture is ryegrass–clover mix that has been demonstrated to provide adequate nutrition for growth and development. The temperate climate also permits management of horses at pasture year round, which is proposed to stimulate development of the musculoskeletal system. Apart from a brief period during weaning, most young stock remain at pasture from birth until the start of yearling preparation. Free access to pasture exercise provides the opportunity to stimulate the musculoskeletal system for the future challenges as a racehorse. The export focus of many of the equine industries heavily influences the management decisions. Despite the availability of good-quality pasture post-weaning, many foals receive up to 50% of the daily dietary energy requirement as concentrates, possibly reflecting the emphasis on early sales as yearlings and the drive to optimise growth. The observations of drench resistance of common internal parasites presents an emerging problem for pasture-based production systems. A large proportion of the Thoroughbred foal crop is exported as yearlings or ready to run 2-year-olds. There is an increasing trend for Standardbreds to be sold as yearlings and this has resulted in changes in the management of Standardbred young stock.

Management of the pregnant mare and long-term consequences on the offspring

The study of early developmental conditioning of health and disease in adulthood is particularly relevant in the horse, which is bred mainly to perform in demanding sport challenges. On the basis of this concept, the management of the broodmare could be considered an effective means to produce animals with the desired features. Knowledge on the Developmental Origins of Health and Disease in the equine species remains relatively scarce, with some experimental studies and one single epidemiologic study. Data highlight the determinant role of the maternal environment for postnatal body conformation, immune response, energy homeostasis, osteoarticular status and thyroidal, adrenocortical, and cardiovascular functions of the foal. Most research, however, focuses on the first months/years after birth. Long-term effects on the adult horse phenotype have not been investigated so far.

An Update on the Pathogenesis of Osteochondrosis

Osteochondrosis is defined as a focal disturbance in endochondral ossification. The cartilage superficial to an osteochondrosis lesion can fracture, giving rise to fragments in joints known as osteochondrosis dissecans (OCD). In pigs and horses, it has been confirmed that the disturbance in ossification is the result of failure of the blood supply to epiphyseal growth cartilage and associated ischemic chondronecrosis. The earliest lesion following vascular failure is an area of ischemic chondronecrosis at an intermediate depth of the growth cartilage (osteochondrosis latens) that is detectable ex vivo, indirectly using contrast-enhanced micro- and conventional computed tomography (CT) or directly using adiabatic T1ρ magnetic resonance imaging. More chronic lesions of ischemic chondronecrosis within the ossification front (osteochondrosis manifesta) are detectable by the same techniques and have also been followed longitudinally in pigs using plain CT. The results confirm that lesions sometimes undergo spontaneous resolution, and in combination, CT and histology observations indicate that this occurs by filling of radiolucent defects with bone from separate centers of endochondral ossification that form superficial to lesions and by phagocytosis and intramembranous ossification of granulation tissue that forms deep to lesions. Research is currently aimed at discovering the cause of the vascular failure in osteochondrosis, and studies of spontaneous lesions suggest that failure is associated with the process of incorporating blood vessels into the advancing ossification front during growth. Experimental studies also show that bacteremia can lead to vascular occlusion. Future challenges are to differentiate between causes of vascular failure and to discover the nature of the heritable predisposition for osteochondrosis.

A cross-sectional survey of Thoroughbred stud farm management in the North Island of New Zealand

AIM

To obtain initial baseline data on the management of Thoroughbred stud farms in the North Island of New Zealand.

METHODS

Data on the management of Thoroughbred stud farms were collected from a sample of 22 stud farms located in the south Auckland/Waikato region (n=15) and lower North Island (n=7) of New Zealand, using a face-to-face survey. The studmaster provided information on the size, scope and management of the farms during the 2004/2005 breeding season. Analysis was based on the location of the farm and size of the breeding operation (number of resident mares).

RESULTS

Effective farm size ranged from 20 to 526 ha and averaged 167 (standard error (SE) 36) and 88 (SE 49) ha in the south Auckland/Waikato and lower North Island areas, respectively. Some farms in the Auckland/Waikato region stood shuttle stallions. The median number of stallions per farm was three (range 0-9), and the median mare-to-stallion ratio was 43 (range 10-250). The farms had a mean of 50 (range 7-180) wet mares and 21 (range 0-100) dry mares. The number of mares per breeding stallion increased with increasing size of breeding operation (p=0.04), being 28 (95% confidence interval (CI) = 10-56) vs 40 (95% CI=16-74) vs 74 (95% CI=44-113) for moderate (<or=70 mares), medium (90-199 mares) and large (>or=200 mares in total) operations, respectively. Seventy-one percent of farms aimed to breed dry mares early in the breeding season, and used a combination of lights, hormone therapy, and rising plane of nutrition to achieve this. Foaling took place in foaling paddocks monitored using a night foaling attendant (17/22) or with foaling alarms (5/22). At birth, 17/22 studmasters routinely administered antibiotics, 14/22 administered tetanus antitoxin, 9/22 administered an enema to foals, and 2/22 did not routinely administer prophylactic treatments. Weaning occurred at 5 (range 3.7-7) months of age, and foals were confined to a box for 1-2 weeks on 16/22 farms. Weaned foals were drenched with anthelmintics every 7 (range 4-9) weeks, and were fed 2.9 (range 1-6) kg of concentrate feed while at pasture until intensive management associated with preparation of the horses for auction began 13 (range 6-20) weeks before the yearling sales. Eight farms weighed the weanlings, at least monthly, to monitor growth.

CONCLUSIONS AND CLINICAL RELEVANCE

The management of Thoroughbred horses was relatively consistent throughout the regions surveyed. Utilisation of breeding stallions tended to be more efficient on the larger stud farms in the south Auckland/Waikato region. Even though foals are grown at pasture they are often provided with large quantities of concentrate feed.

Growth curves from birth to weaning for Thoroughbred foals raised on pasture

AIM

To mathematically describe the growth of a population of Thoroughbred foals reared on pasture in New Zealand from birth to weaning.

METHODS

Twice-monthly liveweight data (including birthweight; BW) from 218 foals (98 colts and 120 fillies) born over a 6-year period, and reared on four different pasture types, were available for analysis. Data truncated to the time of weaning (107-217 days old) provided 3,200 data points. After testing for non-linearity of the relationship of age and liveweight (LW), five non-linear regression equations were tested for goodness of fit.

RESULTS

Colts and fillies had similar BW, weaning ages, weaning weights (WW) and average daily gain (ADG) between birth and weaning (ADG b-w). Weaning age explained 48% (p<0.001) of the variation in WW and 11% (p<0.001) of the variation in ADGb-w. After correction for weaning age, BW explained 16% (p<0.001) of the variation in WW and 6% (p<0.001) of the variation in ADG b-w. The best fit of age and LW was the polynomial equations based on Akaike's information criteria (AIC), residual variance (RV), residual standard deviation (RSD), and coefficient of determination (R2). The most accurate, biologically sound representation of growth was provided by Brody's type equation: LW = [b0- (b0-BW) x e(-b1 x age)], and an equation including BW: LW = [((b0 x BW x age) + BWb1)b2], that was developed in this study (b0 is an intercept, b1 and b2 are regression coefficients, and e is the base of natural logarithms). The advantage of the Brody's equation is that ADG at a given LW can simply be calculated as: ADG (kg/day) = (b0 x b1)-b1 x LW.

CONCLUSIONS AND CLINICAL RELEVANCE

The similarity of the present data with growth data from the Northern Hemisphere provides further recognition that foals reared on pasture can grow as well as foals reared in grain-based systems in the Northern Hemisphere. BW provides a logical set point for the explanation of growth of the foal up to weaning, rather than the use of polynomial equations, which do not fully describe the biology of growth, even though they provide an accurate mathematical description.

Enlargements of the distal third metacarpus and metatarsus in Thoroughbred foals at pasture from birth to 160 days of age

AIMS

To assess the relationship between the radiographic and microscopic appearance of the physeal regions of the distal third metacarpal (Mc3) and metatarsal (Mt3) bones of Thoroughbred foals at 160 days of age, and to clinically assess changes in contour of the distal Mc3 and Mt3 physeal regions from birth to 160 days of age. To assess relationships between maximum clinical physis scores and age, time of year, foal's sex, condition score, growth rate, copper (Cu) concentration in the liver of the foal, and supplementation of the dam with Cu in late gestation.

METHODS

Dams were given Cu (n=5 and n=11, in Years 1 and 2, respectively) or saline (n=5 and n=12, in Years 1 and 2, respectively) injections in late gestation. Liver biopsies were harvested from foals in the first week of life, and the whole liver homogenised after the animals were euthanised at around 160 days of age, to determine Cu concentrations. Pasture samples were collected every 4-8 weeks for analysis of mineral composition. During Year 1, 10 foals were weighed and examined every 2 weeks from birth to 160 days of age for evidence of pain and lameness in the distal Mc3/Mt3. In Year 2, 23 foals were weighed, condition-scored and examined for evidence of pain and lameness weekly for the first 5 weeks of life, then every 2 weeks from birth to 160 days of age, and a clinical physis score for the distal Mc3/Mt3 given. Cabinet radiographs of frontal slices of the physeal region of the distal Mc3/Mt3 at around 160 days of age were given a radiographic physis score. Physes were then examined histologically for evidence of abnormal endochondral ossification.

RESULTS

Gross enlargements of the distal Mc3 and Mt3 were observed in all foals in this study, but were not associated with lameness, pain or inflammation. The most severe clinical physis scores occurred over 2 months in late summer/autumn, and were not influenced by the foal's growth rate, sex, or Cu concentration in the liver, or treatment of dams with Cu in late gestation. The clinical physis score was highly correlated to radiographic evidence of shouldering in the forelimb and hindlimb (both p<0.001). Focal disturbances in endochondral ossification were evident radiographically and histologically in the some of the physes at 160 days of age. The mineral composition of pasture was similar in Years 1 and 2, and concentrations of Cu and zinc were below those currently recommended for growing horses.

CONCLUSIONS

Gross enlargements of the distal Mc3 and Mt3 were not consistent with previous descriptions of physitis. Results suggest that while many Thoroughbred foals at pasture will have visible boney enlargements of the distal Mc3/Mt3 in the first 5 months of life, few have physeal cartilage abnormalities or significant compromise of endochondral ossification. The importance of these clinical swellings may be overestimated, and they may more appropriately be called physiological enlargements associated with remodelling of bone.