BONANZA PLT LOSES SIGHT OF GRASS STRIP DURING OVERHEAD TFC PATTERN AND LANDS IN PASTURE.

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4 BONANZA PLTS MET AT ZZZ FOR FORMATION PRACTICE TO CONCLUDE BY LNDG AT A PRIVATE GRASS STRIP. ALL PLTS WERE QUALIFIED FORMATION PLTS WITH 3 HOLDING FF1 LEAD CARDS. THE FLT WAS BRIEFED ON STANDARD FORMATION FORMS MODELED AFTER THE T-34 MANUAL. THE FLT DEPARTED ZZZ IN ELEMENTS OF TWO AND JOINED IN FINGER RIGHT. THE FLT THEN PRACTICED RWY PASSES AT ZZZ OBSERVING ALL LCL PATTERN ALTS. AFTER 3 PASSES THE FLT HEADED TO ZZZ1 AT 1500 FT MSL AND MADE 2 PASSES PRIOR TO SINGLE SHIP BREAK FROM R ECHELON. I WAS FLYING THE #4 POS. LEAD AND #2 HAD BEEN INTO THE GRASS STRIP SEVERAL DAYS PRIOR AND GAVE THE DESCRIPTION OF THE FIELD; STRESSING AN OFFSET THRESHOLD MARKED BY 2 WHITE MARKERS WITH A HIGH LINE ON THE APCH. A 6 SECOND BREAK TIME WAS USED BECAUSE OF A GRASS RWY AND TO LEAVE PLENTY OF SPACE BTWN PLANES SO EACH PLANE COULD BE AT THE END OF THE RWY PRIOR TO ANOTHER TOUCHING DOWN. AS MY TOTAL ATTN WAS FOCUSED ON MY LEAD WHEN IN WING POS AND ALL PLANES WHEN IN THE SLOT I DID NOT GET A VISUAL OF THE STRIP; AS A GRASS STRIP DOES NOT SHOW UP IN PERIPHERY VISION AS A REGULAR RWY DOES. DUE TO THE DISTANCE BTWN PLANES; #3 WAS DIFFICULT TO KEEP IN VIEW AFTER THE BREAK. I HAD GOOD VISUAL CONTACT WHEN #3 PUT WHEELS DOWN AND TURNED BASE AND ENOUGH CONTACT TO TURN FINAL BEHIND #3; AND THEN I LOST VISUAL. WITH GOOD GND VISUAL I LOOKED FOR THE GRASS STRIP; HIGH LINE; AND WHITE MARKERS. I SAW A HIGH LINE; 2 LARGE WHITE MARKERS; AND A CLR GRASS STRIP IN PERFECT ALIGNMENT WITH THE INTENDED LNDG FIELD. I THEN COMMITTED TO LAND WITH THE SLOWEST SAFE SPD BECAUSE OF THE SHORTER THAN NORMAL LNDG STRIP. AFTER BEING FULLY COMMITTED AND TOO LATE TO GO AROUND SAFELY; I REALIZED I WAS LANDING IN A PASTURE. WHAT I TOOK FOR WHITE MARKERS WERE 2 LARGE WHITE WATERING TROUGHS. THE PLANE COULD NOT BE FLOWN OUT OF THE PASTURE AND HAD TO BE MOVED TO THE GRASS STRIP THROUGH OTHER PASTURES BY A PROFESSIONAL PLANE RETRIEVER. THE PLANE WILL BE CHKED BY AN A&P INSPECTOR AND FERRY PERMIT OBTAINED PRIOR TO FLYING OUT OF THE STRIP. PRIOR TO KNOWING THERE WERE NO INJURES; THE PASTURE OWNER CALLED 911; EMER VEHICLES WERE DISPATCHED; AND THE FAA NOTIFIED. I FAXED INFO OF THE INCIDENT TO THE FAA SAFETY OFFICER. SITUATIONAL AWARENESS ON MY PART COULD HAVE PREVENTED THIS EVENT. AS I BEGAN TO LOSE VISUAL CONTACT WITH #3; I SHOULD HAVE ANNOUNCED THIS ON OUR FORMATION FREQ AND USED THE TOOLS I HAD AVAILABLE TO LOCATE THE GRASS STRIP AND DO A GAR IF NECESSARY. INSTEAD; I HAD A MINDSET OF VISUAL CONTACT AND THAT THE 2 WHITE MARKERS AND HIGH LINE WERE THE CUES TO SEE; IN MY MIND; BUT I WAS NOT SITUATIONALLY AWARE OF ALL THE OTHER CUES THAT INDICATED THIS WAS NOT THE GRASS STRIP AT WHICH I INTENDED TO LAND.