Barb wire stretching...

if it were me i would put a few turnbuckles in there

1/4" X 4" Galvanized Eye & Eye Turnbuckle

now, im guessing there is a certain amount of slack per foot that needs to be taken out, for wich im sure there is a formula that i dont know

id find that out before purchasing the turnbuckles to ensure they will draw them far enough