Variation in dry-matter yield at second harvests was studied in a long-term comparison of wheel traffic systems and soil compaction in grassland for silage in Scotland. Yields were obtained from compacted soil subjected to conventional traffic (C), from less compacted soil in a reduced ground-pressure traffic system (R) and from non-compacted soil in a zero-traffic system (Z). Relationships between the ratios of second-harvest yields, C2/Z2 and C2/R2, and a number of soil, rainfall and crop parameters were tested by correlation analysis. The yield ratios increased significantly with the number of days after mowing before 2 mm of rain fell in 1 d (r= 0·923*** and 0·715*, for C2/Z2 and C2/R2 respectively), and C2/Z2, but not C2/R2, decreased with increasing amount of rainfall in the 14 d after first mowing (r= 0·787*). It was concluded that yield from compacted soil was greater than that from non-compacted soil because the former depended less on rainfall in the weeks after first mowing. The degree of soil compaction in the reduced ground-pressure traffic system, although maintaining first-harvest yield benefit, reduced the risk of significantly diminished yield at second harvest in dry summers.