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Publication Date

1981

Description

Grazing sheep may affect the association between grass and clover in a mixed sward by defoliation, treading, and the circula­tion of nitrogen (N) via excreta. Our objective was to examine the effect of treading and excreta on a perennial ryegrass (Lolium perenne)-white clover (Trifolium repens)sward defoliated by sheep at high and low stocking rates. Established swards of perennial ryegrass and white clover were continuously stocked from May to October with yearling wether sheep at 25 and';5() head/ha. Sward production and species composition were measured inside graze-through cages that allowed defoliation without treading and excreta return, and outside, where sheep grazed either fitted with harness to prevent excreta return ·or unharness to allow normal excreta return. Herbage pr'oauction from the defoliated sward was estimated at 5.42 and 8.99 metric tons (t) dry matter (DM)/ha at high and ow stocking rates, respectively. Treading reduced these figures by 10% at high stocking rate (P < 0.05) and by 4% at low stocking rate (P < 0.05). Excreta return increased herbage production by 53% at high stocking rate and 26% at low stocking rate.

At low stocking rate, treading had little effect on botanical composition, Without the return of excreta, clover content in­ creased from about 58 % at the start of the experiment in spring to about 71 % in autumn. Excreta return at this stocking rate reduced clover to 45 % in autumn. The quantity of clover (kg/ha) present at the end of the experiment was higher than at the beginning in the treatments both with and without excreta return; the reduction in clover percentage where excreta were re­turned was primarily the result of an increase in the quantity of ryegrass present. Measurement of clover stolon changes showed that excreta return resulted in little change in stolon length, but without excreta return there was a substantial increase in stolon length. At the high stocking rate, the percentage of clover present in autumn was 31 % in the defoliated sward, 25 % where this sward was trodden, and 18% where excreta were returned. Clover stolon length declined during the grazing season in all treatments, with the decline being greatest where excreta were returned. In conclusion, at a stocking rate where treading by continuously grazing sheep had a negligible effect, N transfer via excreta significantly increased herbage yield and reduced the proportion of clover in a grass-clover sward. At a higher stocking rate, herbage yields were depressed, and the benefits of increased N transfer via excreta were outweighed by the detrimental effects of increased defoliation and treading.

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Effects of Treading and the Return of Excreta on a Perennial Ryegrass - White Clover Sward Defoliated by Continuously Grazing Sheep

Grazing sheep may affect the association between grass and clover in a mixed sward by defoliation, treading, and the circula­tion of nitrogen (N) via excreta. Our objective was to examine the effect of treading and excreta on a perennial ryegrass (Lolium perenne)-white clover (Trifolium repens)sward defoliated by sheep at high and low stocking rates. Established swards of perennial ryegrass and white clover were continuously stocked from May to October with yearling wether sheep at 25 and';5() head/ha. Sward production and species composition were measured inside graze-through cages that allowed defoliation without treading and excreta return, and outside, where sheep grazed either fitted with harness to prevent excreta return ·or unharness to allow normal excreta return. Herbage pr'oauction from the defoliated sward was estimated at 5.42 and 8.99 metric tons (t) dry matter (DM)/ha at high and ow stocking rates, respectively. Treading reduced these figures by 10% at high stocking rate (P < 0.05) and by 4% at low stocking rate (P < 0.05). Excreta return increased herbage production by 53% at high stocking rate and 26% at low stocking rate.

At low stocking rate, treading had little effect on botanical composition, Without the return of excreta, clover content in­ creased from about 58 % at the start of the experiment in spring to about 71 % in autumn. Excreta return at this stocking rate reduced clover to 45 % in autumn. The quantity of clover (kg/ha) present at the end of the experiment was higher than at the beginning in the treatments both with and without excreta return; the reduction in clover percentage where excreta were re­turned was primarily the result of an increase in the quantity of ryegrass present. Measurement of clover stolon changes showed that excreta return resulted in little change in stolon length, but without excreta return there was a substantial increase in stolon length. At the high stocking rate, the percentage of clover present in autumn was 31 % in the defoliated sward, 25 % where this sward was trodden, and 18% where excreta were returned. Clover stolon length declined during the grazing season in all treatments, with the decline being greatest where excreta were returned. In conclusion, at a stocking rate where treading by continuously grazing sheep had a negligible effect, N transfer via excreta significantly increased herbage yield and reduced the proportion of clover in a grass-clover sward. At a higher stocking rate, herbage yields were depressed, and the benefits of increased N transfer via excreta were outweighed by the detrimental effects of increased defoliation and treading.