Date of this Version
An increasing number of Bos indicus-blood bulls are being used in crossbreeding programs for commercial beef production in the U.S., but there is little information available on pubertal development, sperm production and semen quality for young bulls of this type. Puberty in young Bos taurus bulls has been defined in a variety of ways (e.g., first ability to serve, first sperm production, first ejaculation of motile sperm, etc.), but all of these criteria are costly and labor-intensive to determine. We have shown previously (Lunstra et al., 1978) that, among young Bos taurus beef bulls of various breeds reared in the same environmental and management conditions, puberty occurs when a scrotal circumference of 28 cm is achieved, regardless of large differences in body wt and age at puberty among and within different breeds of bulls. In that study, puberty was defined as the age at which a bull first produced an ejaculate containing ≥ 50 million sperm with ≥ 10% progressive motility. In addition, we (Lunstra and Echternkamp, 1982) and other researchers have shown that the major characteristics of semen quality improve linearly during the first 12 to 16 wk after reaching this criterion of puberty, and that these improvements in semen quality are highly correlated (r=.44 to .75) with the steady linear increase in scrotal circumference that occurs during this post-pubertal timeframe in Bos taurus beef bulls. Thus, scrotal circumference appears to be an accurate and easily-obtained measurement that provides a relatively reliable predictor of age at puberty in young Bos taurus beef bulls. However, similar studies on Bos indicus-blood bulls are needed before this relationship can be assessed or confirmed in zebu-blood bulls.
It is not reasonable to attempt to use young bulls for natural mating or semen processing immediately after reaching this initial criterion of puberty (i.e., first ejaculate containing ≥ 50 million sperm with ≥ 10% progressive motility). It does not become economically feasible to use young beef bulls for collection and processing of semen for artificial insemination until ejaculates containing ≥ 500 million sperm with ≥ 50% progressive motility can be produced, and this is also a reasonable criterion for young bulls to reach before being used for natural mating. However, no studies using this more stringent semen criterion of puberty have been conducted in either Bos taurus or Bos indicus bulls. Because selection and use of superior sires at the youngest possible age is desired for natural mating and artificial insemination, further investigations of the relationships between postpubertal changes in testis size and aspects of semen quality are needed in young Bos taurus and Bos indicus beef bulls.
The following study was conducted to determine the age at which young Bos taurus and Bos indicus crossbred beef bulls reach this revised criterion of puberty (i.e., first ejaculate containing ≥ 500 million sperm with ≥ 50% progressive motility), and to evaluate interrelationships between pubertal age, testis size, and body wt in these specietypes of beef bulls.