Davis - Weed Science Program General Information Weeds compete with crops for water, nutrients, light, and space. If weeds are left uncontrolled, they can reduce crops yields and weaken or even destroy the stand. Weeds also reduce the quality and value of crops because most weeds are less palatable and less nutritious than the crop. Others, such as fiddleneck and yellow starthistle, are poisonous and, if present in sufficient quantities, make the forage unsuitable of livestock consumption. Weed control can be particularly challenging.
Weeds invade the open areas that often occur in older depleted stands.
Weed control is problematic when fields remain in the same crop for many years and where few rotation crops are grown, two conditions that are common.