By Kaitlynn Abell, DVM
It has been well documented and practiced to place multiple bulls in a breeding pasture during a given breeding season. Reproductive performance has been documented on bulls in multiple-sire pastures at the US Meat Animal Research Center with progeny data. Previous data has shown that the progeny data amongst these bulls is highly variable. It has been speculated that this variability, or lack of equal distributions between calves sired per bull, is potentially effected by libido, social dominance, and lack of conception following a successful mount. With the use of progeny data, conception risk and patterns of conception were analyzed from 5 multiple-sire pastures from the US MARC database from the spring 2010 calf crop.
Parentage was tested for all calves via genotyping from individual cows and bulls. A total of 681 calves were born to 729 cows that were exposed to 34 bulls (3 years old) in a 62 day breeding season. The mean bull to cow ratio was 1:22 (range of 1:15- 1:29). The mean cumulative pregnancy rate was 93% (range 89-97%). Bulls were ranked based on the number of calves sired per bull; a bull with rank 1 sired the greatest number of calves within a pasture, rank 2 is the bull with the least number of calves sired within a pasture, and rank 2 bulls were all other bulls. Observed percentage of calves sired within a location was determined based on total calves sired by rank 1 bulls per pasture. Expected percentage of calves sired per bull was determined based on an equal distribution of calves sired per total number of bulls within a location. The results for the expected and observed percentage are shown in table 1.
Calving intervals were analyzed in 21 day intervals and were based on a 278 day gestation length. Interval 1 consisted of days 1-21, interval 2 consisted of days 22-43, and interval 3 consisted of anything greater than 43 days of the predicted calving season. Analyzing all 5 pastures within 21d intervals, interval 1 contained the greatest percentage of calves per cows exposed, ranging from 48 – 59%, as shown in Figure 1.
The number of calves born per cows that were exposed to bulls in the multiple-sire pastures based on reproductive rank 1, 2, or 3 within calving intervals is shown in Figure 2. In interval 1; rank 1, 2, and 3 bulls were statistically different. Overall, bulls with rank 1 sired the greatest percentage of calves in interval 1 and 2, compared to rank 2 and 3 within the same intervals.
Sires within the 5 pastures differed significantly in the cumulative number of calves sired and the number of calves sired in the first and second 21-day intervals of a 63 day breeding season. Rank 1 bulls sired 29-68% greater calves than expected in each breeding pasture. Further research is needed to determine potential causes of difference of calving distributions among bulls in multi-sire pastures.