One keeps in mind the Ghost is in armor, and the men are aware of the possibility of a military attack on Denmark.  Agricultural cliches appear in historical descriptions of lopsided battles, along the lines of "our army went through the enemy like a scythe through a wheat field," or more simply, "we mowed them down."  Horatio can be understood as meaning he feels "defeated" by the Ghost in that "agricultural" way, like he is a weed run over by a harrow.
 
One keeps in mind the Ghost is in armor, and the men are aware of the possibility of a military attack on Denmark.  Agricultural cliches appear in historical descriptions of lopsided battles, along the lines of "our army went through the enemy like a scythe through a wheat field," or more simply, "we mowed them down."  Horatio can be understood as meaning he feels "defeated" by the Ghost in that "agricultural" way, like he is a weed run over by a harrow.
  
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