Perhaps you should try some spelling variations:
"improving maternal performance series." » "improving materials performance series.", "improving external performance series.", "improving national performance series.", "improving maternal performance studies.", "improving maternal performance size.", "improve maternal performance series.", "improving water performance series."
"improving animal performance series." » "improving animal performance studies.", "improving animal performance size.", "improving annual performance series.", "improving thermal performance series.", "improving fiscal performance series.", "improving initial performance series.", "improving optimal performance series.", "improving criminal performance series."

Try widening your search to All Fields.

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

• Performing a fuzzy search may retrieve terms with similar spellings: (("improving animal performance series.") or ("improving maternal performance series."))~.
• If you are trying to use Boolean operators, they must be ALL CAPS: (("improving animal performance series.") OR ("improving maternal performance series.")).
• Removing quotes may allow a broader search: (( improving animal performance series. ) or ( improving maternal performance series. )).
• Adding a wildcard symbol may retrieve word variants: (("improving animal performance series.") or ("improving maternal performance series."))*.