Distribution of the breeding females 
Terrain 
Water availability 
Carrying capacity--feed intensity 
Pasture adaptation 
Pasture size 
 Bull variation is caused by: 
Age 
Condition 
Mating ability 
Libido 
Fertility 
Sperm reserve 
Social behavior 
Injury 

 Management decisions include: 

Length of breeding season 
Reproductive diseases 
Breeding intensity 
Amount of observation 

        Most of these factors must be considered to define the optimum bull to female ratio.  The following graph illustrates how difficult it is for producers to know what the optimum cow:bull ratio should be.  This study, conducted in Colorado, shows the percentage of synchronized females that bulls impregnated when given the opportunity to breed 3 to 51 synchronized females.  Notice that some bulls had a poor percentage pregnant even when exposed to small numbers of females and some bulls had a high percentage pregnant even though they were running with 30 or more synchronized females.  The vertical axis depicts the percentage pregnant after one opportunity to mate with the group of synchronized females. 
  
 
 Figure 1. Effect of number of females exhibiting estrus on the percentage pregnant by each bull in single-sire mating of estrus synchronized females. (Pexton, et al. 1990) 

         Proper management during the breeding season should result in each female being bred by a single fertile bull each time she is in estrus.  Bull overlap (more than one bull breeding a cow in heat) is not desirable, primarily because it does not enhance pregnancy rates.  Disadvantages of bull overlap are increased risk of bull injury (through competition for estrous females), additional pressure from social dominance and the extra costs incurred by purchasing and maintaining more bulls. 
 Recent research at an Eastern Colorado research station, where the average carrying capacity is 25 acres per-animal-unit-year, showed similar conception rates for bull to female ratios of 1:25 and 1:50.  This research was conducted with multi-sire breeding pastures.  All of the bulls were experienced bulls that had previously passed a breeding soundness examination. 
          Bull overlap can be decreased by eliminating bull congregation within breeding pastures.  This  can be achieved by dividing the breeding herd into separate pastures or by using pastures that have natural barriers that reduce mixing of breeding groups.  In addition, riders can be used to keep bulls well distributed among breeding groups. 
 These large cow to bull ratios can reduce bull costs on very large ranches with minimal risk.  On a small 50 to 100 cow operation, using just one bull that happens to undergo an injury or disease could spell disaster for an entire calf crop. 
 Recommendations for smaller herds that will utilize only one bull per pasture may need to be more conservative.  A time honored rule-of-thumb is to place about the same number of cows or heifers with a young bull as his age is in months.  For instance a bull that is 14 months old going into his first breeding season should be expected to breed 14 or 15 cows; whereas as a two-year old bull may be placed with 20 - 25 cows.  Mature bulls that have been examined by a veterinarian and have passed a breeding soundness exam can be placed with 25 - 35 cows and normally give good results.