Only takes one plant to affect yield
SURVEYS show that about half of all winter rape fields contain cleavers, albeit not always at very high levels. Its aggressiveness means that even small populations can cause a lot of trouble, and a single escaped plant can produce 500 seeds or more.
Rothamsted Experimental Stations Dr Peter Lutman has found that in early drilled winter rape a mere 0.5 cleavers/sq m can cut yield by 5%, worth about £24 for a 3t/ha (24cwt/acre) crop. A more typical infestation is 2-4/sq m, he says, although the weed tends to occur in fairly dense patches rather than spread throughout the crop. Those populations cut the crops value by £96 to £192/ha (£79/acre).
Apart from causing direct yield loss, the weed raises harvesting, cleaning and drying costs and reduces the crops market value. And because combines cannot effectively handle clumps of cleavers, the amount of rape it discharges with the trash increases.
Generally seed samples with more than 45% cleavers are refused by crushers until cleaned. This costs growers £15-20/t. But the smallest cleavers seed is difficult to remove. If it exceeds the 4% ceiling, the sample will be rejected.