Why do you think increasing yields greatly benefits farmers? If the pencil industry doubles its output, do you imagine their profits will increase twofold?
The phrase "better yields" is exactly synonymous with "increased yields". A crop's yield is its volume, measured in bushels. There is literally no other way to interpret this.
Citation: I grew up on a farm, and my family has farmed for at least 200 years.
EDIT: This is a particularly weird reason to downvote me...
And that'd be pedantic. Better can be more cost-effective, cheaper in time or money or equipment or quality of land required. I grew up on a farm, live on one now, my family has farmed for 300 years, so I guess I win?
Better could mean anything from maintaining current output with less costs to increasing the quality of your crop.