Perhaps you should try some spelling variations:
"crop introduction" » "chip introduction", "aop introduction", "erp introduction"
"drop introduction" » "drop5 introduction", "deep introduction", "aop introduction"
"crop production" » "crop reproduction"

Try widening your search to All Fields.

You may be able to get more results by adjusting your search query.

• If you are trying to use Boolean operators, they must be ALL CAPS: (("crop production") OR ((("crop introduction") OR ("drop introduction")))).
• The words AND, OR and NOT may confuse the search; try adding quotes: "((\"crop production\") or (((\"crop introduction\") OR (\"drop introduction\"))))".
• Removing quotes may allow a broader search: (( crop production ) or ((( crop introduction ) OR ( drop introduction )))).
• Adding a wildcard symbol may retrieve word variants: (("crop production") or ((("crop introduction") OR ("drop introduction"))))*.