Feeding Young Beef Bulls Influences their Reproductive Capacity

British Breeds

Research conducted at the Agriculture Canada Research Station, Lethbridge, over an eight-year interval indicates that the feeding of high versus medium energy diets to young British breed beef bulls is detrimental to their reproductive capacity. Experiments have been carried out where the feeding of high versus medium energy diets to strains of Hereford and Angus bulls was compared. High energy diets consisted of 80 percent concentrate (barley, 60%; oats, 10%; beet pulp, 10%) and 20 percent forage (alfalfa or alfalfa-straw (70:30) cubes), while the medium energy diet was forage alone. Bulls were fed either high or medium energy diets from weaning until slaughter at 12, 15 or 24 months of age.

At slaughter, sperm production by the bulls was estimated by epididymal sperm reserves. In most cases, regardless of age, bulls fed high energy diets had substantially reduced reproductive potential compared to bulls fed medium energy diets. For example, feeding medium versus high energy diets to Angus and Hereford bulls from weaning to 15 months of age increased paired testes weight by 6 percent, efficiency of sperm production by 13 percent, daily sperm production 19 percent and epididymal sperm reserves by 52 percent. Table 1 summarizes the results for epididymal sperm reserves in four experiments. It should be noted that, with the exception of 24-month-old Hereford bulls in 1983, at the end of these experiments the average bull fed the high energy diet was carrying less backfat than bulls of comparable age and breed being marketed by the beef industry today. Along with a reduction in sperm reserves, quality of semen and the libido of bulls fed high energy diets were reduced. For example, in 24-month-old Hereford bulls slaughtered in 1983, bulls fed the high energy diet had one half the motility and one third as many normal sperm as bulls fed the medium energy diet. Also, fat bulls had eleven times fewer services during libido testing than did lean bulls. To be fair, this is an extreme case as the bulls in question were obese, but it shows what can happen.

Results of the Lethbridge research supports the findings of a South African study. Feeding high versus normal levels of energy did not advance the age at which spermatozoa started to appear in the ejaculates of young Hereford bulls. Further, although sperm motility was seen to increase with age in the normal energy group as expected, sperm motility rose only marginally in the high energy group. This author states that the significant difference in the number of abnormal spermatozoa in the high energy group from 76 weeks of age onward can probably be related to the significant smaller body-testicular temperature difference that seems to result from the excess fat laid down in the neck of the scrotum.

In addition to the detrimental influence on reproductive traits, it would be expected that bulls fed high energy diets would have a much greater probability of acquiring foot and leg problems and consequently reduced longevity.

Why Hereford and Angus bulls carrying excess condition have the inferior reproductive capacity described is not clear. It is speculated that part or all of the problem results from deposition of fat in the neck of the scrotum and/or in the scrotal tissue itself. This excess fat may insulate the testes and increase testicular temperature, thereby reducing the number and quality of sperm cells produced. The sperm production process, spermatogenesis, is extremely sensitive to temperature change.

Continental Breeds

Where do Continental breed bulls fit into the picture? There has been very little research of this type conducted on bulls of Continental breeds. Results of a recent study conducted at Kansas State University showed no effect on either seminal characteristics or serving capacity, of feeding three different levels of energy to Hereford and Simmental bulls from weaning for a period of 200 days followed by grazing for 38 days. However, it should be noted that Hereford bulls in this study fed the lowest of three levels of energy had backfat thicknesses similar to bulls of the same breed and comparable age fed the high energy diet referred to earlier in the Lethbridge Research Station research. Further, it should be noted that the mean progressive motility of all Hereford bulls in the Kansas State study was only 29.2 percent following the entire feeding period, a level unacceptable for breeding bulls. Therefore, the Simmental bulls may have already been influenced even by the lowest energy diet.

Preliminary results are now available from a three year field trial conducted by the Lethbridge Research Station. This trial was designed to assess the effectiveness of different criteria used to evaluate the reproductive capacity of young beef bulls used for multiple-sire natural service under range conditions. Numerous measurements, including backfat thickness, were taken immediately before the breeding season. A total of 277 bulls representing five composite "breeds" of crossbred bulls were included in the analysis. Bulls were composed of greater than half of one or more of the Brown Swiss, Charolais, Chianina, Geibvieh, Limousin, Romanola, and Simmental breeds. Bull fertility was determined by blood typing calves to determine their paternal parent. The mean backfat thickness of all bulls was 1.5 + .07 mm (range 0 to 7 mm). There was a highly significant negative contribution of backfat thickness to bull fertility, or expressed another way, as backfat thickness increased, bull fertility decreased. This is the first time backfat thickness has been demonstrated to have a direct negative affect on beef bull fertility. To optimize reproductive capacity, it is recommended that cattle ranchers select bulls with minimum backfat thickness.

It must be recognized that Continental breed bulls have a different body composition than the British breeds. It is commonly recognized that Continental breeds lay down backfat later and to a lesser extent than do the British breeds. However, many Continental breed bulls have excess fat in the neck of the scrotum that likely impairs thermoregulation of the testes. Therefore, although backfat is not excessive, optimum reproductive capacity still appears to be compromised. Experiments are currently being planned that will add substantial information about the effects of dietary energy of the reproductive status of young Continental breed bulls.

Many bulls available within the industry today are deficient because of reproductive and/or feet and leg problems. Many of these problems may be prevented through improved management of young bulls, particularly the level of energy fed during growth performance testing and preparation for show and/or sale. The market place, whether it be on the ranch or at a show-sale, is regulated by supply and demand. If commercial cattle ranchers pay a premium for fat bulls that have a higher probability of being reproductively deficient, that is exactly what the seed stock producer will continue to provide. Commercial cattle ranchers must "start the ball rolling by discriminating against fat bulls. Many astute cattle ranchers have already recognized the "fallacy of fat". They now purchase their bulls as yearlings or long yearlings and grow, rather than fatten, their bulls for use at two years of age.

Regardless of the backfat thickness a bull has when purchased, before you turn it out to breed, subject it to a thorough evaluation of reproductive capacity by a professional experienced in this area.

Table 1. Depression of sperm reserves in bulls fed high vs. medium energy diets

Bull			Dietary energy
Age (mo)	Breed	Year	Medium	High	Percent reduction in sperm reservesa
24	Hereford	1978	37.7b	9.3b	75
		1979	35.7	23.1	35
		1983	40.6	19.8	51
	Angus	1978	33.4	29.2	13
		1979	33.1	28.6	14
15	Hereford	1980	33.3	22.1	34
		1981	30.9	17.9	48
	Angus	1980	28.2	25.5	10
		1981	34.3	17.9	48
12	Hereford	1982	9.3	5.5	41
	Angus	1982	10.4	7.5	27

This information was prepared by Dr. Glenn H. Coulter.

Adapted from Beef Herd Management Reference Binder and Study Guide 422-1