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MEETING A YOUNG BULL'S NUTRITIONAL NEEDS IS A CHALLENGE

Steve Blezinger
Ph.D.

Last issue I started a series discussing trends in the cattle industry in the coming years. In light of this being the A.I./ Herd Bull issue, I felt it would be useful to deviate from that series to “Shoot the Bull” in a manner of speaking. I'll get back to the trend series in the upcoming issue. On several occasions in the past, we have discussed management and feeding of bulls. Considering the importance of the bull within your cow herd, the influence of a relatively small group of genetics on the total outcome of your calf production system, appropriate development of young bulls is essential and bears reiteration. Although many cow-calf producers purchase all the bulls they use and never develop any themselves it is important that all cattlemen understand these concepts and can identify an appropriate program. We'll take a look at some important components of a sound bull feeding and development program as well as some of the more important nutritional concerns currently under scrutiny.

Understanding Bull Nutrition and Feeding

Bulls are a challenging group of cattle to provide proper nutrition. They are a relatively small group but can take up a lot of space. The tendency is to run all bulls together and hope that they won't do much damage to the facilities or each other. But, nutritional needs vary due to age and condition, so if young and old bulls are run together some bulls may not get the nutrition they need and others may get too much. Obviously this creates either over or under feeding within a group, both of which can be very detrimental. Yearling bulls on performance tests are commonly placed on high energy diets. These bull need to be "let down" from the time they are purchased until they are turned out with cows. A mistake made occasionally is to turn the bulls that have been on a high grain ration out on very lush pasture or place them on straight grass or possibly alfalfa hay. This can lead to digestive upsets or imbalances, thus leading to potential reproductive problems. Research shows that it takes 60 days for sperm development (spermatogenesis).

The gain for yearling bulls prior to the breeding season should be about two pounds per day. This would require a diet containing 10-11 percent protein and 60-70 percent TDN (dry matter basis) or .75 megacalories per lb. net energy maintenance and .42 megacalories per lb. net energy gain. This can be supplied by 6-10 lbs. of grain per day and full access to medium quality hay. Any hay fed should be free from molds and green in color, if possible.

A mineral and vitamin mix should be offered that contains adequate calcium, phosphorus, and vitamin A. An exact mineral is difficult to recommend since it depends so greatly on what your forage base is and what grain mix you may be feeding. Remember that in many situations, forages are relatively high in calcium and potassium while low in phosphorus and magnesium. In many situations, various grain components are the opposite. Minerals can be provided free-choice or in your grain mix (this is preferable). As I've said before, it is vital that you have your forages tested for nutrient content, especially mineral levels. If forages are weathered and/or of low quality, an intramuscular injection of 3 million IU of vitamin A is often recommended although daily feeding, in my opinion is more effective. The following is a listing of target nutrient levels I recommend for a bull of the type shown:

CHART 1 (see below)

Growing Out Young Bulls

Young bulls should attain 1/2 their mature body weight by 14-15 months of age. Extremely low levels of energy intake early in life delays the onset of puberty. Feeding excess energy may reduce both semen qual-ity and serving capacity. This is thought to be due to excess fat deposition in the scrotum, insulating the testes and increasing testicular temperature.

How Much Does a Young Bull Need to Gain?

Debates continue with regards to grain-based tests versus pasture based tests. It is felt by some producers that bulls that do well on forage will relay this performance to their off-spring. The alternative argument for grain-based test programs is that we determine their maximum genetic potential for gain. For example, suppose a breeder has one bull that gained three pounds per day and another gained only 1.8 pounds a day on the same diet. Rate of gain in the feedlot is about 50 percent heritable (degree to which a parent passes a specific trait on to it's offspring, in this case, rate of gain).

The difference in rate of gain between the bulls is 1.2 lb. Multiply the 1.2 by the 50 percent heritability and the result is .6 lb. per day. Since 1/2 the inheritance comes from the dam and 1/2 from the bull, divide 0.6 by 2, which gives 0.3 lb. Thus calves sired by the bull that gained three pounds a day should gain .3 lb. more daily than calves sired by the bull that gained only 1.8 lbs. a day (if bulls bred to same herd of cows). This doesn't take into account, however how much the cow contributes (or removes) from the calves' gain performance.

What to Feed Growing Bulls

There is no one single diet that will meet the requirement of all bulls. In other words you can't be all things to all people -- or in this case, bulls. Requirements are based on bull age, frame, body condition, desired rate of gain, and environment. The following are a couple of examples of diets for growing bulls. The first column represents the requirements of a young bull, average weight of 700 lbs in average condition. The second column are the requirements for a 900 lb. average weight bull in good condition. Both are gaining 2 to 2.5 lbs per head per day. Note the differences.

CHART 2 (see below)

Many combinations of forages and grains can be used to provide these levels. Your best bet is to start with the forage base you have available on your operation and go from there.

Recent Topics of Concern in Bull Nutrition

ZINC LEVELS: It has been known for some time that a deficiency of zinc can reduce male fertility. Zinc plays a role in the production, storage, and secretion of individual hormones as well as in the effectiveness of receptor sites. Zinc affects the production and secretion of testosterone, insulin and adrenal corticosteroids. Spermatogenesis and the development of the primary and secondary sex organs in the male are also impacted by dietary zinc levels. Of additional interest has been the source of the zinc fed. Some studies suggest that organic sources (commonly called chelated) of minerals are more biologically available than inorganic (metallic) sources of minerals. In a study by Arthington et al. (1995) it was determined that bulls fed 60 ppm of zinc had higher liver zinc concentration than bulls fed 40 ppm of zinc supplied by zinc sulfate but had similar zinc liver concentrations to bulls fed 40 ppm zinc supplied by 1/3 zinc proteinate and 2/3 zinc sulfate. No differences in average daily gain or scrotal circumference were detected. Percent normal sperm cells was highest for the 40 ppm of zinc supplied by 1/3 zinc proteinate and 2/3 zinc sulfate (68.9 percent). Percent normal sperm cell levels for the 60 ppm zinc levels and 40 ppm zinc level supplied by zinc sulfate were 62.5 percent and 55.8 percent, respectively. This suggests that the use of organic zinc in growing bull diets may improve subsequent fertility measures. However, inorganic zinc at an increased level (60 ppm) also improved fertility. But more importantly this indicates that for growing bulls, the NRC recommended level of 30 ppm in the diet may be too low. Zinc must be present in the diet at all times because animals have only small amounts stored in the body. A combination of zinc concentrations in plasma (<0.6 to 0.8 ppm) and feed (<40 ppm) would be good indicators of status.

COTTONSEED PRODUCTS AND BULL FERTILITY: Recently we discussed the use of cotton products in cattle feeds. Gossypol is a naturally occurring substance found in the pigment glands of cotton-seed that can be toxic if fed for a prolonged period at high concentrations. Whole cottonseed has a significantly higher level of free gossypol than meal or hulls. Concern about feeding gossypol containing products to bulls arose when Chinese researches discovered that gossypol is a potent male contraceptive in humans. Gossypol appears to be more damaging to reproductive function in young males near puberty than to older, mature males. Studies where bull fertility has been hindered have involved feeding cottonseed products at high levels and/or for long periods of time. In routine feeding practices, the use of 3-5 lbs. of cottonseed meal is most unlikely to expose the breeding animals to the levels of gossypol needed to cause reproductive problems.

Conclusions

This brief overview once again only touches on a relatively complex component of a complete cattle program. Extensive study and experience will help you understand and establish a sound bull development program.

Dr. Steve Blezinger is a nutritional and management consultant with an office in Sulphur Springs, Texas. He can be reached at P. O. Box 653 Sulphur Springs, TX 75483, by phone at (903) 885-7992 or by e-mail at sblez@unicomp.net


Chart 1: Approximate Nutrient Requirements 
for 1,250 lb Bull Gaining 2 lbs.

Dry Matter Intake,      26.0         Cobalt, ppm                .50
lbs.                              .68         Copper, ppm             12.0
Net Energy Maint,       .40          Iodine, ppm                .75
Mcal/lb                      11.0          Iron, ppm                100.0 
Net Energy Gain          .60         Manganese, ppm      40.0
Mcal/lb                        .30         Selenium, ppm           .25
Crude Protein, %         .65         Zinc, ppm                   .50
Calcium, %                  .15         Vitamin A, IU/lb        2000
Phosphorus, %            .08          Vitamin D, IU/lb         125
Potassium, %              .10          Vitamin E, IU/lb          7.5 
Magnesium, % 
Sodium, % 
Sulfur, %

 

Chart 2: Animal Requirements:

Dry Matter Intake, lbs.                         17.5                19.7
Net Energy Maintenance, Mcal/lb         .75                  .80
Net Energy Gain, Mcal/lb                      .47                  .51
Crude Protein, %                                 12.0                11.5
Calcium, %                                            .55                 .48
Phosphorus, %                                       .27                 .26
Potassium, %                                         .65                 .65
Magnesium, %                                       .20                 .16
Sodium, %                                             .08                 .08
Cobalt, ppm                                          .50                 .50
Copper, ppm                                       12.0               12.0
Iodine, ppm                                           .75                .75
Iron, ppm                                           100.0             100.0
Manganese, ppm                                 40.0               40.0
Selenium, ppm                                      .25                .25
Zinc, ppm                                             50.0              50.0
Vitamin A, IU/lb                                   2000              2000
Vitamin D, IU/lb                                     125               125
Vitamin E, IU/lb                                      7.5                7.5

 

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