Presence, impact and retention of Synergistes jonesii in ‘problem’ herds grazing leucaena

Summary

A study of 8 commercial cattle herds grazing leucaena was undertaken to determine 

(a) the efficacy of in vitro Synergistes jonesii inoculum (produced in an anaerobic fermenter) in degrading DHP; and 

(b) the persistence of the inoculum following a period grazing non-leucaena pastures.

Cattle were introduced to the leucaena pastures for an initial period varying from 17 to 71 days. Fourteen to 15 animals were then sampled for (a) urine and blood plasma to determine toxicity status as indicated by concentration of DHP; (b) faeces for estimation of diet composition; and (c) rumen fluid for detection of S. jonesii by nested PCR analysis. After a further 42-56 days, animals were resampled as before to confirm toxicity status and inoculated with the in vitro S. jonesii inoculum; the herds were then sampled a third time (42-60 days after inoculation depending on grazier availability) to test the effectiveness of the inoculum in degrading DHP.
Five of the herds were then removed from leucaena pastures for periods ranging from 80-120 days and returned to leucaena pastures for 21 days to check persistence of the inoculum as indicated by retention of capacity to degrade DHP.
It was concluded that while most herds showed some capability to degrade DHP, either from inoculation or from residual capability from previous exposure, they did not achieve the same rapid and complete DHP degradation reported in the 1980s. At that time, inoculation with rumen fluid containing S. jonesii caused rapid degradation of DHP to very low levels within days of inoculation. In the present trials, there was: 

(a) a very slow rate of degradation of DHP isomers on some properties prior to inoculation; 

(b) frequent occurrence of high levels of 2,3-DHP in urine indicating partial toxin degradation, both before and after inoculation; 

(c) a low incidence of detection of S. jonesii in rumen fluid after inoculation based on PCR amplification of 16s rDNA sequence of the type strain 78-1 thus indicating low populations in rumen fluid; 

(d) no evidence of DHP degradation on one of two properties tested for efficacy of inoculation; and 

(e) loss of protection from subclinical toxicity on some properties after <4 months on non-leucaena pastures.>

Given the uncertainty, there is a need to assess the integrity of the supply chain for the in vitro source of S jonesii and to test if the in-vitro source has changed in composition and/or effectiveness relative to the original mixed inoculum. Nevertheless, it was concluded that the in vitro inoculum was at least partially effective and should continue to be used by graziers until improved sources of inoculum and/or inoculation methodologies are demonstrated.