A study was conducted in New Zealand to evaluate the degree of soil damage caused by grazing animals on (very) steep slopes (47-75%). Breeding cows (average weight - 1,000 lbs) were grazed in the winter on wet soils in perennial ryegrass sod. Sixteen to 28 cows/Acre grazed for 2-3 days to generate 43%, 53%, and 72% soil disturbance. Cattle impact varied across the field: It was worst on animal tracks and camps, intermediate on easy contoured ridges and gullies and least on very steep inter-tracks (>47% slope). This was explained by cattle spending most time in the tracks/camps, and least on the steep inter-track areas. Five different forms of soil damage were observed:
- compaction - impact from a hoof but no broken surface
- cleft - where hoof indentation was observed
- skid - where animals skid downslope and ripped away 0.5-1 inch of soil
- ridge - where soil was pushed up higher than surrounding area
- puddling - where soil was left in a muddy state.
These levels of damage are in increasing order of severity, that is, it takes longer for them to disappear. Compaction and cleft damage was highest on the more level tracks/camps and the easy contoured areas, while skidding and ridging were more prevalent on the steep inter-track areas. Puddling was significant on highly damaged tracks and camps.
The damage affected forage production in the spring, but if the animals were kept off the field, the fields 'repaired themselves' and by summer the effects of this one-time compaction event were mostly gone. The researchers observed that if heifers were grazed in early spring recovery of the land was slower than if sheep were grazed. This time of year would be a good time to observe your pastures and determine if you see any of these signs of soil degradation. Although compaction and cleft damage can be accepted as normal, the effects of grazing, skidding, ridging and puddling are very undesirable.
The latter types of soil degradation can be avoided by not grazing animals when soils are too wet. If a pasture was damaged, it is important to keep the animals out until later in the spring so the sod can restore itself. Processes that repair soil are partly physical (wetting-drying, freezing-thawing cycles), but mostly biological (root growth, earthworm activity, mycorrhizae, detrital fungi hyphae, bacteria, protozoa, etc.). Stimulating vigorous root growth and high biological activity helps make your soil resist compaction and realize a quicker come-back from its effects.
From the article by G.W Sheath and W.T. Carlson. 1998. Impact of cattle treading on hill land: 1. Soil damage patterns and pasture status. New Zealand Journal of Agricultural Research 41:271-278