Differences between timed-AI (TAI) pregnancy rates (PR) of beef bulls and collections within bull are difficult to determine due to limited number of females per breeding group. PR to TAI between 6 Angus bulls and between collections (combined 2 ejaculates) within bull were examined in large Brazilian beef herds. In 4 bulls, 3 collections obtained in a 2-week period were packaged in different colored straws. In 2 bulls, a single collection was packaged in 3 straw colors as controls. Straws contained 20 million total spermatozoa (0.5 ml). Nelore females (n = 6003) in 75 groups on 4 farms were synchronized for TAI. Every bull was represented in each group, and collection within bull was randomized across groups. Parity and body condition score (BCS) were recorded at TAI. Pregnancy was diagnosed approximately 35 d post-TAI (5614 complete records). Data were analyzed using GLIMMIX procedure of SAS with main effects of sire, collection(sire), farm, parity(farm), and BCS (≤2.75 or ≥3 on a scale of 5). Estrotect patches were used on a subset of cows (n = 3334; 2 farms) to assess estrus activity. PR was greater (P < 0.05) when BCS was ≥3 and if females displayed estrus. PR differed (P < 0.05) between sires and tended to differ between collections within sire (P = 0.09; Table 1). The numeric range in PR was similar among straw colors for both individual collections and a control (Sire 3), illustrating that the impact of binomial variation, even within large sample sizes such as these, cannot be ignored. In conclusion, large and economically significant differences in fertility exist between bulls used for TAI, and knowledge of bull fertility is crucial for the consistency and success of beef TAI programs. Table 1. Pregnancy rates (PR; %) to timed-AI (TAI) for bulls and individual collections within bull

Bull	TAI PR per bull		SEM (%)	TAI PR by straw color
n	TAI PR	Purple	Red	Yellow
1	828	46.1a	2.7	47.7	44.8	45.8
2	1,109	44.7a	2.4	47.3	40.4	46.6
31	735	41.3ab	2.9	42.3	44.7	36.8
4	1,026	39.2ab	2.6	31.8	42.3	43.7
51	841	35.7bc	2.6	35.5	35.8	35.7
6	1,075	30.6c	2.1	26.5	34.5	30.7

^a–cMeans within the same column not sharing a common superscript differ significantly, P < 0.05. ¹All straws from sire are from the same collection.