I'm in the fight against Global (de)Worming.
Quit deworming healthy adult cows. Period.
Other than an occasional first-calf heifer that looked thin while nursing her first calf, I didn't deworm an adult cow in our herd in the last 15 years.
After about 2-2.5 yrs of age, most cattle will have developed resistance/tolerance/resilience to worms, and nematode parasites will have no significant impact on productivity for >90% of cows. You'll save a boatload of $$$, too.
When you deworm the entire herd (if you're using an efficacious drug at the correct dosage and route), you're killing off ALL the worms that are still susceptible to that class of dewormer that you're using... so all that's left to lay eggs that hatch out as the next generation of worms on pasture are the resistant ones.
So... if you deworm everything on the place with, say, ivermectin, this spring... all that's left to lay eggs are the ivermectin-resistant worms. Then, when you 'rotate' to a different class of dewormer... say, Panacur/Safeguard, this fall - and deworm everything... well, all that is left to lay eggs are the ones that are resistant to both Ivermectin AND Panacur/Safeguard.
But... if you'd quit deworming all adult cows - or even just 10% of them (targeted non-selection)... you'll preserve some 'refugia'... that portion of the population of worms that are not having selection pressure placed on them for resistance. The worms that make up the refugia still have some 'susceptible' genes that can help to 'dilute' the resistance genes in that breeding population of worms.
Pasture management is important, too. 75% of infectious L3-stage nematode larvae are in the bottom 4 inches of forage, 15% in the next two inches above that, 10% in the next two inches above that... so... the shorter you graze your pastures, the more infectious worm larvae your cattle are exposed to.