We are looking for a cattle-safe, pre-emergent fertilizer to apply in December. Thistles and johnsongrass are taking over one of our pastures.
You are asking two questions: a fertilizer question and a herbicide question.
First, an answer for the fertilizer question. All dry formulations of pasture fertilizers containing nitrogen, such as ammonium nitrate or urea, are safe for grazing livestock. Timing of the application is very important to get the desired response. If growth is anticipated of cool-season grasses this fall and into the winter, then application now would be appropriate. If your climate is such that the growing season is basically over, applying the fertilizer next spring would be the better option. It is suggested that a soil test be conducted to fully determine the types and amounts of nutrients that the pasture may need.
For the herbicide question, one would have to know more about the pasture type, climate and location, and target weeds. It appears to be somewhat south because of the johnsongrass issue. As a warm-season perennial grass, this is difficult without complete pasture renovation. The thistles could be sprayed now with something like Grazon herbicide. There wouldn't be any pre-emergent herbicides that would be effective.