Yearling bulls should have a body condition score of 5.5 to 6.5 (on a 9-point scale) at the start of the breeding season. Once body condition has been assessed, management is needed for over-conditioned bulls and thin bulls. If yearling bulls are over-conditioned they need to be “let down” to prepare them for the breeding season. Gradual changes are needed when changing diets to be more or less concentrated to reduce the possibilities of metabolic disorders and impaired breeding performance. Because mature sperm is produced over a 60-day period before ejaculation, nutritional effects of over- or underfeeding on sperm quantity and quality will have some carryover effect.
The general method of stepping down bulls is to gradually replace a portion of the concentrate in the ration with forage over several weeks until the bulls are consuming forage or forage plus a supplement. Ideally, this should start at least 30 to 60 days prior to turn-out, and yearling bulls should continue to gain 1.5 to 2.0 pounds per day. Thin bulls should be put on a ration with a higher level of energy to increase rate of gain. If bulls purchased or previously in your ownership are in good condition, you will need to ensure that they are adapted to high-forage rations prior to turn out.
Source: South Dakota State Ag Extension