You will have to provide more information on what sort of field (football field which is well levelled and grass is always short, or perhaps some other field which has loads of bumps and dips with long grass and random stones and hedgehogs) robot is going to operate in and what sort of items robot is supposed to recognise and pick up (is it any items general, is it items that have some distinct marks)?
From what You've given us so far, all I can say that arguably the most flexible object recognition and detection systems are implemented using cameras. By flexible I mean that it can be programmed or trained to classify very different objects.
Do You know how are You going to make robot to pick item of interest up? Using a brush and shovel mechanism? Using robotic arm? Some other means?
Have You thought how robot is going to recognise the correct way to pick these different items up?