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Abstract
AIM: To investigate the efficacy of intra-ruminal Zn boluses as aids in providing protection from natural Pithomyces chartarum challenge in calves.
METHODS: Sixty-two calves (mean weight 187 (SEM 3.25) kg) were divided into three groups. Commencing on Day 0, they received either a proprietary bolus containing 83% ZnO (ZnO group), a prototype Zn bolus containing 88% elemental Zn (Zn group), or remained untreated (Control group). Concentrations of Znin serum and faeces were measured weekly between Days 6 and 34, activities of gamma-glutamyl transferase (GGT)in serum on Days -8, 20, 34 and 41, and faecal spore counts between Days 13 and 41.
RESULTS: Between Days 6 and 34, mean concentrations of Zn in serum increased in both Zn treatment groups compared with Controls (p<0.001), but were lower in Zn than ZnO animals between Days 6 and 28 (p=0.05). Concentrations of Zn in faeces were increased in both groups following treatment with Zn. Mean concentrations of Zn in faeces in ZnO animals was higher than Zn animals on Days 13 and 20 (p<0.001), but decreased by Day 34, whereas for Zn animals they were still elevated on Day 34.
Faecal spore counts were 354,000 and 183,000 on Days 13 and 20, respectively, 610,500 on Day 28, and 115,500 spores/g on Day 34. There were no clinical signs of facial eczema, but based on the activities of GGT on Day 41, 17/20 (85%) Controls were moderately or severely affected (GGT >250 IU/L) compared with 1/21 (5%) ZnO and 3/21 (14%) Zn animals. Mean activities of GGT on Day 41 for ZnO and Zn animals were not different (35 (95% CI=23-54) and 54 (95% CI=32-92) IU/L, respectively; p=0.18), but were below those of the Controls (502 (95% CI 296, 850) IU/L), confirming efficacy of both bolus treatments. In both ZnO and Zn animals, there was no significant relationship between concentration of Zn in serum and activity of GGT, but in Zn animals there was a significant relationship between the concentration of Zn in faeces and activity of GGT 21 days later. The regression equation y=2.136- 0.002197x (where y is log10GGT activity, and x the concentration of Zn in faeces) provided an estimate of the threshold level of concentration of Zn in faeces.
CONCLUSION: The study confirmed efficacy of elemental Zn boluses in providing protection from natural P. chartarum challenge in young cattle.
Acknowledgements
Special thanks to the farmer Lance Morley, at Waiuku, who participated in the on-farm trial, and veterinarian, Douglas Death. Zn analyses and spore counts were carried out by Gribbles Veterinary Pathology, Dunedin, New Zealand.
