contented calf browsing hedge after TB testing summer 07

Okay, here goes. The paper which landed on Robert’s desk the week of our TB test wasn’t a recent publication…in fact it was published in 2006. It never receive the coverage it should have at the time. Strangely its arrival had nothing whatsoever to do with the farm or our impending TB test. It was forwarded to Robert because of his involvement in all things hedge (being  chair of the national hedgerow Biodiversity Action Plan group, Hedglink and co-chair of the Devon Hedge Group).

Robert popped his head around my office door “You should take a look at this.”

“What?” I looked up at him

“A paper on bovine TB linked to hedges”

“Hedges?”I scratched the top of my head and twiddled the hair “What…hedges decrease, increase the incidences?” As always when another study is published on bTB my reactions are mixed…hope, doubt, excitement, negativity and a kind of destructive inevitability.

“Decrease apparently”

“Oh something went wrong around here then!” I said wryly “Us being one of the most heavily hedged landscapes in Britain with some of the worst incidences of TB.”I leant back in my chair “So how do you mean? Good thick boundaries between contiguous farms?” I went on pre-empting his explanation “I didn’t know this…but the spread of disease from cattle to cattle out in the field is apparently quite unusual. The vet said a TB infected animal has to have developed lesions in the head and lungs and to hawk violently onto an area of grass just before it’s ingested by another animal to get cross contamination. Interesting…I always thought it was a much more tenuous encounter” I stretched “So, what’s its claim then? The paper? Where was the research carried out?” adding tongue-in-cheek “The hedgeless, stockless arable wastes in the east of the country?”

“No. Here. In North Devon. And on the Hereford/Gloucestershire boarder!” He grinned “I’ll send you a link. Have a look for yourself!”

So I did.

The study covered 120 dairy farms, 60 with recently infected herds and 60 without, and looked at the influence of various factors on the incidence of bovine TB.  These factors were farmland habitat, topography (landform), badger density, proximity of farms with breakdowns, stocking density and herd size.

As expected bTB incidence was found to be linked to herd size and distance to nearest infected farms.  In line with some other studies but not all, incidence was not found to be closely linked to badger density.  It was also not closely linked to the extent or configuration of deciduous woodlands nor to the extent of pasture (both favoured badger habitats).

What was new was that hedgerow characteristics were found to be strong predictors of breakdown incidence.   Hedgerow abundance, the number of hedgerow gaps, and the number of ungrazed field boundaries were all important.  For example, a ‘hedge-poor’ farm (as defined in the paper) would be expected on average to have a 1.6 times greater risk of bTB than a ‘hedge-rich’ farm.  Considering just total hedgerow length, an increase of 1 km per 100 ha was associated with a decrease in the risk of breakdowns by about 12.5%:  in absolute terms this equates to the annual risk of bTB changing from 9.2% to 8.1% for herds in the West of England: an annual reduction of 251 infected herds (based on 2004 figures).  These figures were controlled for herd size and distance to next bTB case: it is not simply the case that farms with plenty of good hedgerows have a lower stocking density.

The authors conclude that habitat management appears important to a farm’s bTB risk. ‘Nature friendly’ management practices – the presence of ungrazed wildlife strips, and the greater availability, width and continuity of hedgerow – are all associated with reduced bTB incidence.

They could only speculate on the mechanism through which hedgerows may reduce incidence of bTB, but  suggest it could be due to one of two factors.  The first is that hedge-rich farms are managed differently – for example they have different crop rotations which reduce the likelihood of cattle eating contaminated grass.  The second (and more likely) is that the presence of hedges reduces badger-cattle transmission because a higher proportion of contaminated grass is kept out of the reach of cattle.  Badgers preferentially use hedgerows as movement corridors and for their latrines, so where cattle are excluded from these areas by either hedge growth or fences, contact with the bacteria is reduced.

Given the practical difficulties associated with badger culling, and the fact that to be effective it has to be carried out over large areas (because of the perturbation effect), the authors suggest that improving habitat features such as hedgerows and ungrazed wildlife margins might be a more cost effective strategy to reduce infection.

So there you have it. A piece of research I had no idea existed. And the one thing we have on this farm? Exceptional density of hedges…with very few gaps; in fact we  have many more hedges now than were present on the 1840 Tithe Map.

Could this be (and I hardly dare think it, let alone say it) a reason why we ‘continue’ (whispered very quietly) to go clear?! hedges are good for something other than browsing?