Read the label. The label has the directions, restrictions, cautions, application rates and methods. Herbicides require registration in each state. There may be some variation in restrictions and use from state to state. There may be a supplemental label as well.
Higher application rates result in longer residual life. Ideal of course is to use the lowest rate that will kill your weeds. Application rates are based on ideal and uniform application. That will be affected by many factors. Overlap being the biggest. Nobody wants unsprayed streaks with weeds. So overlap occurs. That means sections with double the rate or more. Nozzles have a fairly uniform pattern when new. When they wear, not as much. How old and worn are your nozzles?
The popular pasture herbicides with residual (Grazon P+D, GrazonNext, Duracor) have label restrictions on hay. From the Duracor label:
"Pasture and Rangeland Restrictions • Do not use grasses treated with DuraCor in the preceding 18 months for hay intended for export outside the United States. • Hay from areas treated with DuraCor in the preceding 18 months can NOT be distributed or made available for sale off the farm or ranch where harvested unless allowed by supplemental labeling. • Hay from areas treated with this product in the preceding 18 months can NOT be used for silage, haylage, baylage, and green chop unless allowed by supplemental labeling. • Do not move hay and silage made from grass treated with DuraCor within the preceding 18 months off farm unless allowed by supplemental labeling. • Do not use hay from areas treated with DuraCor within the preceding 18 months or manure from animals feeding on hay treated with DuraCor in compost. • Do not use grasses treated with DuraCor in the preceding 18 months for seed production."
"Restrictions in Hay or Manure Use - Do not use aminopyralid-treated or florpyrauxifen-benzyl-treated plant residues, including hay or straw from areas treated within the preceding 18 months, in compost, mulch, or mushroom spawn. - Do not use manure from animals that have grazed forage or eaten hay harvested from treated areas within the previous 3 days, in compost, mulch, or mushroom spawn. - Do not spread manure from animals that have grazed or consumed forage or hay from treated areas within the previous 3 days on land used for growing broadleaf crops. 4 Specimen Label Revised 11-29-19 - Manure from animals that have grazed forage or eaten hay harvested from treated areas within the previous 3 days may only be used on pasture grasses, grass grown for seed, wheat, and corn. - Do not plant a broadleaf crop (including soybeans, sunflower, tobacco, vegetables, field beans, peanuts, and potatoes) in fields treated in the previous year with manure from animals that have grazed forage or eaten hay harvested from aminopyralid-treated or florpyrauxifen-benzyl-treated areas until an adequately sensitive field bioassay is conducted to determine that the aminopyralid and florpyrauxifen-benzyl residue in the soil is at level that is not injurious to the crop to be planted. - To promote herbicide decomposition, plant residues must be evenly incorporated in the surface soil or burned. Breakdown of aminopyralid and florpyrauxifen-benzyl in plant residues or manure is more rapid under warm, moist soil conditions and may be accelerated by supplemental irrigation."