My sense of the word 'farm' is that it implies that vegetative crops are grown too. The word 'ranch' means primarily larger animals (sheep and bigger, not chickens) are what's grown. Black baldies are cows.
I just Googled and found this about leafy spurge - as usual, Annie Proulx says a surprising amount with just a couple of words:
Leafy spurge (Euphorbia esula L.) is a creeping, herbaceous perennial weed of foreign origin that reproduces from seed and vegetative root buds. It can reduce rangeland cattle carrying capacity by 50 to 75 percent. About half of this loss is from decreased grass production. Cattle won't graze in dense leafy spurge stands and these areas are a 100 percent loss to producers. (Entire article here: http://www.ext.colostate.edu/pubs/natres/03107.html)So the two words "leafy spurge" confirm the sense of run-down, neglect, and poverty of the Twist ranch. No one has licked that leafy spurge into shape. It helps us understand Mr. Twist's bitterness at the promised help that never come to pass.
Here's a poignant line from further down in the article:
Sheep or goats can be used to help control leafy spurge.