Add spaces: Thecropbiomasswasreducedby3%whenthenumberofweedharrowingswasincreasedfromtwotothree,andby6%atanincreasefromtwotofourtimesharrowing.
The crop biomass was reduced by 3% when the number of weed harrowings was increased from two to three, and by 6% at an increase from two to four times harrowing.