Individual study: A pass to allow pronghorn antelope Antilocapra americana through sheep fences, Wyoming, USA
Mapston R.D., Zobell R.S., Winter K.B. & Dooley W.D. (1970) A pass for antelope in sheep-tight fences. Journal of Range Management, 23, 457-459
In some North American rangelands fencing is considered a necessity to control livestock. Concern was raised regarding possible detrimental effects of sheep-fencing on pronghorn antelope Antilocapra americana by restricting their movements. A trial was undertaken in Wyoming (northwest USA) to develop an inexpensive pass structure that would facilitate antelope movement but retain livestock.
In 1965, a sheep-tight fence 6 miles (9.6 km) long was erected in the study area. Six locations in the fence were selected for placement of test structures which were modified over time to improve facilitation of pronghorn movement. Cattle and sheep grazed on one side of the fence and cattle on the other, thus the structures were also evaluated for their effectiveness in restraining these livestock. Pronghorn movements were determined by track counts, direct observations and counts by aeroplane.
Over the first six months, pronghorns were wary of the fence and pass structures and few attempts were made to use passes. Subsequently, the most successful design was a ‘cattle grid’ (cattleguard) comprising a 6 x 6 foot (183 cm) lightweight metal grill (13 parallel bars) raised on 10 inch (25 cm) timbers (to reduce problems of infilling by drifting sand or snow) over a 15 inch (38 cm) deep pit with earth ramps either end. The antelopes could easily jump the grid, livestock could not. Structures with a jump distance of less than 2 m did not confine all livestock.