Sheep on spring pastures may scour excessively due to the low levels of neutral detergent fibre. Scouring leads to dag formation, which predisposes sheep to flystrike, particularly in the breech area, posing a considerable challenge to sheep producers. This trial was undertaken to test whether supplementing sheep on spring pasture with fibre would reduce diarrhoea and thereby reduce dag formation and flystrike. Four hundred first-lambing Merino ewes were randomly allocated to control (no fibre supplementation) and fibre treatment (ad libitum cereal hay or wheat straw supplementation) groups in a 2-paddock crossover trial, which was conducted from August through the spring flush to crutching in October. The bulk fecal egg counts of each group were no different at the start of the trial but the fibre group had a higher (P<0.05) fecal egg count than the control at crutching. Fecal egg count was not correlated with dag score. Fecal dry matter percentage was higher (P<0.05) in the fibre group but there was no difference in dag wet weight or dry matter percentage between the treatment groups. Dag scores of a sample of 100 sheep from each group were significantly (P<0.01) lower at crutching. There was no difference between the groups in the incidence of flystrike, however, there were significantly (P<0.001) more sheep with high dag scores (>3) in the control group than the fibre treatment group. These results indicate that the supplementation of spring pasture with low quality hay or straw can reduce dag formation by improving the consistency of feces and, thereby, reduce the number of sheep with dag scores greater than 3.