Developmental orthopaedic disease (DOD) is a syndrome including a variety of lesions in young horses (Figure 1). Within DOD, osteochondrosis is one of the most important skeletal problems encountered; and it has been studied worldwide for decades (Knight et al., 1985 and Gabel et al., 1987, in Ohio and Kentucky; O’Donohue et al., 1992 in Ireland; Pearce et al., 1998 in New Zealand; Denoix et al., 1996 in France, Coenen et al., 2004 in Germany). This interest is mainly due to the economic impact of DOD in the horse industry (Jeffcott, 1996). An 11.3% incidence of clinical DOD was reported in an Irish study involving more than 10% of the foals registered in Ireland in 1986, 1987 and 1988 (O’Donohue et al., 1992). Of the foals found to be suffering from clinical DOD, around 48% had reduced sale value. The prevalence of DOD varied from 10 to 50% depending on the study and the method of detection. The clinical significance and the future impact on the adult horse of lesions detected in foals are still unclear, but the radiograph method remains the best way to detect lesions in live horses.
Despite the large number of scientific publications on DOD, its pathogenesis is still unclear. All authors recognise it as multifactorial (Figure 1), and when studying DOD a large number of parameters must be included to avoid biases that would lead to misinterpretation of the results. Very few publications (Van Oldruitenborgh-Ooste et al., 1999) have considered all the parameters involved in the same study. For this reason a longitudinal study in France was undertaken beginning in 1997 in the main horse breeding region of Normandy. Data on nutrition, genetics, management, blood biochemistry, haematology profiles and X-rays of foals at 6 and 18 months of age were recorded from 1997 to 1999 and from 2002 to 2004. The study involved several research teams focusing on topics including genetics, haematology and biochemistry, biomechanics, nutrition, breeding and management. About 30 professional horse farms and three breeds of horses, Thoroughbred, Standardbred (French Trotter) and Selle Français, were included. Mares were followed from the 8th month of pregnancy to weaning and the foals from birth to 18 months of age.
Figure 1. The multifactorial nature of developmental orthopaedic disease.
Nutritional factors recognised as being involved in DOD are overfeeding, mainly due to energy excess (Glade et al., 1986; Savage et al., 1993), which exacerbates rapid growth rate and probably individual predisposition to DOD. Toxins are implicated (Gunson et al., 1982), however the key dietary factor is mineral nutrition, especially calcium (Ca), phosphorus (P), copper (Cu) and zinc (Zn).
Mineral nutrition is critical to skeletal growth and development in horses. Of the total mineral in the animal, bone contains 99% of the Ca, 81% of the P, 62% of the Mg, 47% of the Na, 30% of the Mn and 28% of total Zn (Grace et al., 1999). Mineral content of bones increases markedly between birth and 2 years of age, with for example, mineral content of canon bone approximately 2 g/cm3 at birth and 10 g/cm3 at 2 years of age (Jeffcott et al., 1986).
Bone growth and development require the conversion of cartilage to bone by endochondral ossification. This process involves enzymatic and endocrinological processes, protein synthesis, and the input of minerals in a balance between accretion and resorption. Copper is a co-factor of lysyl oxidase, an enzyme implicated in the initiation and regulation of linkages between collagen and elastin. Furthermore, copper has a stimulatory role in osteoblast function, without acting on osteoclasts. Zinc is a co-factor for alkaline phosphatase, with magnesium serving as a second co-factor. In the presence of excess zinc, the activity of alkaline phosphatase is inhibited through displacement of magnesium. In addition, bone cellular proliferation and differentiation require zinc.
In the digestive tract, copper and zinc absorption depend on numerous factors. Zinc absorption is reduced by calcium; and the competition between copper and zinc for metalloprotein has been observed in other species (Hintz, 1993; Bohanon, 1995). Unfortunately, very few studies on minerals are available in horses, either on copper or zinc absorption nor concerning the form of trace elements provided in the diet. A few studies have been published involving copper deficiency and zinc excess in foals (Bridges and Harris, 1988; Bridges and Moffitt, 1990) and in mares (Pearce et al., 1998; Knight et al., 1990).
The optimum dietary level of macrominerals (calcium, phosphorus) and trace elements (zinc and copper) in mares and foals of horse breeds characterised by rapid growth is still unknown precisely, however published recommendations are well known to prevent deficiencies (Tables 1 and 2). The global epidemiologic approach taken by various research groups will hopefully lead to more precise recommendations for such horses in the future.
From the study underway in Normandy, preliminary data from the first part of the survey (1997-1999) are available, however many questions have been raised.
Table 1. Trace element recommendations (mg/kg DM) for mares.
Table 2. Some trace element recommendations (mg/kg DM) for foals.
Material and methods
For the first three years of the survey (1997-1999), the data on diet composition are summarized for 14 farms, representing 79 foals and the corresponding mares. The study was conducted in western France (Normandy). Foals of three breeds, Thoroughbred, French Trotter and Selle Français, were weighed and measured from birth to 2 years of age. X-ray scoring of the lesions (Denoix et al., 2000) was conducted on yearlings in order to estimate the influence of the diet on DOD. As an initial approach, radiographic score (RS), which is the summation of indices of severity of all findings in all anatomic sites, were determined. Horses were grouped in three categories of DOD severity based on radiographic scores: 0-1 = least DOD, 2-10 = medium DOD, and scores >10 indicated highest DOD. Scores above 10 represented mainly lame horses.
The dietary nutrient composition of mares during pregnancy and lactation and of the foals from 6 to 10 months old, within the RS categories, will be subjected to ANOVA (SAS system) and statistically compared.
Some preliminary findings: diet zinc and copper content
While analysis and correlation with RS scoring is not complete, some aspects of the nutritional findings should be pointed out. For example, analysed zinc and copper levels are below recommendations both in pregnant and lactating mares; and the Ca:Zn ratio in mare diets, despite a wide disparity among farms, was much higher than recommended. Nutritional data must be carefully evaluated before interpretation given the differences in dry matter intake among mares and farms.
Evaluation of radiographic scoring made it clear that the X-ray scoring was not sufficient for classification of DOD status. The data must be examined in terms of the type of lesion and evolution of lesions between 6 and 18 months of age. This part of the study began in 2002.
COPPER AND ZINC CONTENT OF PASTURE AND FEED INGREDIENTS
The complete results of the second survey (2002-2004) are not yet available. However since an economic goal of every breeder farm is to maximise use of pasture in feeding mares and foals, presented here is the composition of the pasture and roughage sources sampled in Normandy over the three year period (Figure 2). The relatively low levels of copper and zinc in roughage are in accordance with other published data in Europe (Coenen et al., 2004), and justify supplementation to reach the levels associated with a lower incidence of DOD in rapidly growing foals. These results, together with the others studied, should emphasise to breeders, veterinarians, and the feed industry the importance of mineral nutrition and supplementation of both mare and foal diets in prevention of DOD.
Commercial concentrates and feed ingredients used in formulation for mares and foals had a very wide range of copper and zinc contents (Table 3). Given that complete diets for horses are not in use in France, clearly the so-called traditional diets, i.e. roughage + cereals with or without alfalfa hay, soybean, soybean meal etc., do not provide the required levels of copper and zinc. The introduction of commercial supplements, either minerals or complementary diets, appears a necessity if balanced feeds are to be provided.
Table 3. Zinc and copper ranges in concentrates and supplements for mares and foals.
Figure 2. Copper and zinc content in pasture and roughage sampled in Normandy.
In studies of DOD in horses related to nutrition, these questions appear:
- Are NRC and INRA recommendations adequate for foals of rapid growth breeds?
- What are optimum levels and ratios between different dietary minerals and trace elements?
- What is the best form of dietary trace elements for horses?
The feed compositional data obtained in France as part of the DOD study indicate that low levels of copper and zinc are common in pasture and roughage sources. The data also reflect wide variations in mineral content of supplements and complementary feeds. Acknowledgements Studies herein were conducted with the financial support of Région Basse Normandie.
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Author: GÉRALDINE BLANCHARD
National Veterinary School of Alfort, Maisons Alfort, France