American black walnut is one of the most favored species, the world over. With rich brown lustrous heartwood, good machining and strength properties, and good availability (although somewhat expensive compared to other species) black walnut simply has no substitute. 

In addition, highly figured black walnut burl and crotch wood is available. As a result, it is used for furniture, cabinets, millwork, gunstocks, novelties and other applications by both the traditional industrial woodworking industry and woodworking enthusiasts.

But unlike most hardwoods, black walnut should not be used for animal bedding.

From the horses’ viewpoint, bedding down on walnut shavings or sawdust could literally result in death. If only a small percent of bedding is walnut, it could result in laminitis within 24 to 48 hours.  Symptoms frequently begin with mild laminitis and swelling of the legs and can progress to extremely severe laminitis, swelling, and edema of all four limbs; and pitting edema of the ventral abdomen; and colic. Horses vary in their reaction to walnut contamination; therefore, horses should never be bedded on wood shavings containing walnut.

Walnut ranges from the Great Plains to the East Coast and from the Central Lake States to the Gulf Coastal Plain. It has been widely planted outside of its native range and especially in the Pacific Northwest. It is also common as an urban tree. Traditional manufacturers are mostly aware of the laminitis issue and take precautions to prevent walnut shavings or sawdust from being used as bedding. 

In the last several years scaled down sawmills and wood working equipment have made it possible for many small operators to enter the industry. Because walnut is a prized species of higher value, nontraditional and new lumber producers and users may process one to a few trees and they may not be aware of the potential for laminitis. These individuals may pass their sawdust and shavings along for another use and actually feel they are doing a good deed.

Another possible source of contamination is where a third party gathers sawdust and shavings from different sources and processes it for mulch and or bedding. Since producers do not know what the end use is, they are not likely to be concerned about walnut contamination. Material intended for mulch should not be used as horse bedding. During cold winter weather, sawdust and shavings can become scarce as larger companies often use this material for fuel. This in turn can result in smaller users being accessed for material.

Because of the characteristic color of walnut heartwood and the pore size, most woodworkers should be able to identify even a few shavings when mixed into the sample. Fine sawdust is much harder to identify and could require the use of a microscope.

If you are processing walnut lumber or finished products be sure to dispose of the residues in such a way that they will not be used for bedding or even for mulch that might also be misused as bedding. If you happen to have a hoarse be sure to check the source of bedding and what it contains.

Additional information is at http://www.extension.purdue.edu/extmedia/FNR/FNR-254.pdf.

Daniel Cassens is Professor of Wood Products, Department of Forestry and Natural Resources at Purdue University. Christina R. Wilson, Ph.D., is Clinical Associate Professor of Toxicology, College of Veterinary Medicine at Purdue University.