The use of machinery in harvesting can cause waste, as harvesters may be unable to discern between ripe and immature crops, or collect only part of a crop. Economic factors, such as regulations and standards for quality and appearance, also cause food waste; farmers often harvest selectively, preferring to leave crops not to standard in the field (where they can be used as fertilizer or animal feed), since they would otherwise be discarded later. This method of removing undesirable produce from harvest collection, distribution sites and grocery stores is called culling. However, usually when culling occurs at the production, food processing, retail and consumption stages, it is to remove or dispose of produce with a strange or imperfect appearance rather than produce that is spoiled or unsafe to eat. In urban areas, fruit and nut trees often go unharvested because people either don't realize that the fruit is edible or they fear that it is contaminated, despite research which shows that urban fruit is safe to consume.