Selective grazing by dairy cows in the presence of dung and the defoliation of tall grass dung patches

Two studies investigated the effect of contaminated pasture on selective grazing overall grazing behaviour and the process of defoliation of dung patches through experiments targeted at four major questions: (a) how does relative utilization of tall and short grass change as the sward is grazed down? (b) what effect does herbage mass and sward height have on the relative utilization of shout and tall grass? (c) how are tall grass patches actually utilized by cattle? and (d) how is overall grazing behaviour influenced by contamination of the sward? Experiments were conducted in mid to late season using Friesian dairy cattle. In experiment I, two pasture types (topped sward (T) v. grazed-only sward (G)) were used. The distribution of bites on tall grass from both pasture types indicated that the grazing animals tended initially to graze short grass when they met a new standard, and then select tall grass as the swards were progressively grazed down. This switch happened earlier in the defoliation process in the topped sward. In experiment 2 observations were conducted on previously grazed and previously ungrazed swards. The distribution of bites on tall grass showed a similar trend to that found in experiment 1 and as the sward was gradually grazed, biting rate significantly declined. There was also a significantly higher total grazing time on the previously ungrazed sward (no contamination by dung). Comparing data based on a consistent biting rate (calculated as the time for 20 consistent bites) and natural biting rate (calculated as the total time for 20 bites) suggested that the grazing animals had increased difficulty in handling tall grass which may explain the declining biting rate as the swards were being grazed down and move bites were directed at tall grass. The defoliation of tall grass dung patches appeared to be concentrated around the edges of the patch. The average area of sward affected by a single dung pat was 1.04 m(2) measured at the pre-grazing stage and was markedly reduced to 0.51 m(2) at the post-grazing stage. In conclusion, selective grazing is likely to exist due to the presence of dung and conditioned by dung distribution and sward type and this in turn modifies biting rate during grazing down of a sward.