Animal health and disease therapy on organic dairy farms in the Republic of Ireland

ORGANIC livestock farming differs from conventional farming in many aspects, including pasture management, animal nutrition, housing, animal health maintenance and animal disease management (Rahmann 2001), and any actual effect of organic husbandry practices on the animal health remains unclear (Athanasiadou and others 2001, Sundrum 2001, Hovi and Kossaibati 2002). Maintenance of the health of organic livestock represents a challenging and expanding area for veterinary medicine. This short communication describes a study designed to establish baseline animal health data on Irish organic dairy farms. Monitoring programmes were established on six organic dairy farms representing most of the fully converted organic dairy herds actively producing milk in Ireland in 2001. Monitoring took place at the institutional organic dairy herd kept by Teagasc, the Irish agricultural research and development authority, at Johnstown, County Wexford, for two years: 2000 and 2001, and on five commercial organic dairy farms throughout the Republic of Ireland for one year, 2001. The herds had been managed organically between one and 10 years (mean five years) at the outset of the study. The mean herd size was 43 cows per herd, with a range from 15 to 60. Most cows were of the HolsteinFriesian breed, and the mean milk yield was approximately 4300 litres per cow per year. Two of the six herds had calving peaks in both spring and autumn, while the other four herds had a spring-calving pattern. Two of the herds had loose straw-based winter housing, and four had cubiclebased housing. Animal health events were recorded by the farmer. It is difficult to completely validate data gathered in this manner; however, the results are likely to be at least as representative of organic animal health as data based upon a retrospective recollection of events with questionnaire studies (Krutzinna and others 1996, Magerison and others 2001), or data derived solely from records of veterinary interventions (Hardeng and Edge 2001, Hamilton and others 2002), or extrapolation from occasional discrete defined visits (Vaarst and others 1993, Huxley and others 2004). Data from the present study should be directly comparable with previous farmer-reporting studies of organic dairy cow health (Offerhaus and others 1994, Weller and Bowling 2000). Clinical mastitis was the most frequently reported animal health problem on all herds, with a mean incidence for all study cows of 50·4 cases per 100 cows per year (Table 1). This is higher than previously reported mean incidences for organic cows of 36·4 and 42·0 (Hovi and Roderick 2000, Sato and others 2002) and also greater than the 36·0 to 43·3 cases for dairy cows maintained in conventional herds in the UK (Whitaker and others 2004). There was substantial variation among herds in the present study, with incidences ranging from 8·8 to 84·4 cases per 100 cows per year. The median (interquartile range) herd incidence of clinical mastitis was 61·3 (48·3) cases per 100 cows per year. Microbiological data from three of the herds implicated Staphylococcus aureus as being the most prevalent mastitic pathogen, with some involvement from Streptococcus uberis. In the present study, antimicrobials were used in only 30·2 per cent of clinical mastitis cases, and dry-cow therapy was used on a small subset (16·6 per cent) of defined problem cows. This is similar to the low antimicrobial usage in organic dairy cows previously reported by Zwald and others (2004). The potential for intramammary infections that have been treated incorrectly to become subclinical sources of recurrent infection may have contributed to the high level of mastitis seen in the present study. The high number of cases per affected cow (mean of 1·64 case per affected cow), and the high prevalence of presumptive subclinical mastitis in three herds (35·3 per cent of 1290 cow milk samples ≥200,000 somatic cells/ml), would also be consistent with this hypothesis. Previous studies have also reported a high prevalence of subclinical mastitis in organic dairy cows (von Weber and others 1993, Busato and others 2000, Hardeng and Edge 2001). The mean incidence of clinical mastitis in cows from herds using loose straw-based winter housing (73·5 cases per 100 cows per year) was significantly higher than the mean incidence in cows from herds with cubicle-based winter housing (26·7 cases per 100 cows per year) (P<0·001). This agrees with previously published organic studies (Weller and Bowling 2000) and with conventional studies (Berry 1998, Peeler and others 2000, Whitaker and others 2004), and may be a result of the potentially increased exposure of teat ends to high pathogen levels with straw bedding systems (Hogan and others 1989). The overall mean incidence of lameness in the study was 9·6 cases per 100 cows per year (Table 1). Most of the lameness cases occurred in one herd and were associated with upper limb problems rather than foot lameness. These findings should be interpreted with caution, as the incidence of lameness has previously been identified as being underestimated when relying on farmer records (Wells and others 1993, Mill and Ward 1994, Whay and others 2003). The low incidence of clinical lameness in organic cows may be due to the restricted use of concentrate supplementation in organic farming. The use of high-starch, high-protein concentrates is implicated in horn quality degradation, laminitis and sole ulceration (Hoblet and Weiss 2001). However, rare reports of higher lameness incidences in organic dairy cows involve infectious conditions such as digital dermatitis (Smolders 2001), which may be associated with the potential of straw bedding to increase exposure to infectious agents. Conversely, straw bedding also provides greater comfort and hence contributes to reduced lameness (Leonard and others 1994, Whitaker and others 2004). Only one case each of clinical hypocalcaemia and hypomagnesaemic tetany were reported from the 288 calvings in the study. These low incidences are in agreement with previous studies (Weller and Bowling 2000, Hardeng and Edge 2001, Hamilton and others 2002). Lower peak milk yields and the reduced use of synthetic high-potassium fertilisers are aspects of organic husbandry with potential positive benefits in this context. Veterinary Record (2006) 159, 680-682