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Virginia Cooperative Extension -
 Knowledge for the CommonWealth

Boar Feeding and Nutrition

Livestock Update, October 2004

Mark J. Estienne and Allen F. Harper, Tidewater AREC, VA Tech

When compared to other classes of swine, nutritional research focusing on the breeding boar has historically been rather limited. Reasons for this relative lack of attention include the fact that mature boars did, and still do, comprise a relatively small part of the entire swine population. Additionally, long ago it was determined that a typical boar ejaculate contains many more sperm cells than are necessary to impregnate a single sow. Because natural mating systems dominated the industry, there was little incentive for investigating nutritional approaches for increasing the average number of sperm cells produced in an ejaculate from say 50 billion to 75 billion. It was common for swine producers to feed boars a gestating sow diet and assume that male reproductive efficiency would not be seriously impacted. Today, however, artificial insemination is the most common mating system in the swine industry and each additional dose of semen processed from an ejaculate has monetary value.

Another factor that may have limited research in this area is the large variation displayed among boars with regard to reproductive characteristics such as semen volume, sperm concentration, sperm motility or measures of sexual behavior. To conduct meaningful research, detect statistically significant treatment differences, and draw sound conclusions, large numbers of experimental boars are generally needed which sometimes presents logistical problems for researchers. That spermatogenesis in boars requires 6 to 7 weeks is another consideration. Experiments investigating the effects of various nutritional regimens on sperm production need a preliminary period of at least this long before actual effects of treatment can be critically evaluated.

Finally, when examining the effects of graded levels of nutrients on reproduction in the boar, semen and libido characteristics may not be particularly sensitive measures. This may be in contrast to other nutritional research where a relatively small change in a specific nutrient results in easily demonstrated changes in performance. For example, Figure 1 shows the response in gain/day and lean gain/day to increasing dietary protein in gilts. Such a well-defined response curve seems much more difficult to produce if response criteria involve semen characteristics or libido.

The objective of this paper is to review some of the practical research that has been conducted to examine the effects of feeding and nutrition on reproduction in the adult boar, paying particularly close attention to more recent findings. When older research is cited, the reader is reminded of the genetic changes that have occurred in the swine industry in the past 20 years. Thus, past nutritional recommendations may or may not be completely applicable to all modern genotypes (e.g., extreme lean, Meishan cross, etc.). Moreover, nutritional requirements of boars may be impacted by factors such as health status, ambient temperature and the frequency of semen collection.

Figure 1. Effect of dietary crude protein intake on overall and lean gain/day in growing-finishing gilts (from Cromwell et al., 1993)

Nutrient Requirements of Sexually Active Boars
In 1998, the National Research Council (NRC) published the most recent Nutrient Requirements of Swine. Contained within this document are nutrient requirements for sexually active boars. As noted above, research focusing on the nutrition of the boar has been limited, necessitating the use of many older studies, and in some cases, conjecture, in determining NRC recommendations. Thus, the requirements put forth may or may not be applicable to all modern genotypes. Indeed, it is common for boars housed at many commercial studs to be fed rations that contain specific nutrient levels exceeding NRC recommendations. Nevertheless, contained in Table 1 are the NRC requirements for selected nutrients. The table can be used to put into context, the results of experiments described below.

Effects of Protein and Energy Intake on Reproductive Characteristics
According to the NRC, sexually active boars require 6.530 Mcal of metabolizable energy, 260 g of total protein, and 12 g of lysine per day. Daily energy requirements can be partitioned for three energy-demanding processes: maintenance, growth and reproductive functions.

Table 1. Selected energy and essential nutrient requirements of sexually active boars (90% dry matter) (from NRC, 1998).
Feed intake4.4 lbs/day
ME intake6.530 Mcal/day
Crude protein 260 g (13.0 % of diet)
Required amount/day
Lysine 12.0 g (0.60 % of diet)
Selenium 0.3 mg
Vitamin A 8,000 IU
Vitamin D3 400 IU
Vitamin E 88 IU
Vitamin K (menadione) 1.0 mg
Biotin 0.4 mg
Choline 2.5 mg
Folacin 2.6 mg
Niacin 20.0 mg
Pantothenic Acid 24.0 mg
Riboflavin 7.5 mg
Thiamin 2.0 mg
Vitamin B6 2.0 mg
Vitamin B12 0.03 mg
Linolenic Acid 2.0 g (0.1 % of diet)

Maintenance requirements are greater for larger boars and increase in colder environments. Boars may enter studs 9 months of age or less and in an actively growing state. The ideal rate of growth for boars housed in studs, however, is the subject of considerable debate and research is needed to characterize semen production and libido in boars fed to grow at different rates. Finally, the energy costs for mounting an artificial sow and ejaculating once or twice weekly are negligible compared with the energy costs of maintenance and growth.

Based on a review of the scientific literature, Kemp (1991) and Kemp and Verstegen (1991) concluded that a prolonged period of protein and energy restriction decreases the production of sperm cells. It appears, however, that the deleterious effects of under-nutrition are more pronounced when protein, rather than energy, is limited.

Research conducted by Louis et al. (1994a, b) focused on the effects of protein and energy intake on semen characteristics and libido in boars. In the first experiment (Louis et al., 1994b), 20 sexually mature, Landrace x Large White boars were fed either high protein (324 g crude protein and 16.6 g lysine per day) or low protein (146 g crude protein and 6.2 g of lysine per day) diets for 23 weeks. Boars on both diets received 6.82 Mcal of metabolizable energy per day.

During the first seven weeks of the study, there was a trend for boars fed the low protein diet to take longer to mount an artificial sow and begin ejaculating. There were no effects of treatment on semen characteristics (volume, sperm concentration, sperm motility, etc.).

From Week 8 to the conclusion of the study, however, boars fed the low protein diet required more time to mount the artificial sow and start ejaculating, had shorter duration of ejaculations, and ejaculated less semen than did boars fed the high protein diet. Total sperm output and sperm motility were similar between treatment groups.

Alterations in blood concentrations of reproductive hormones may at least partially explain these results. Several hormones, including luteinizing hormone (LH) and follicle-stimulating hormone (FSH), are released into the blood stream from the pituitary gland, a garden pea-sized organ that is located just below the brain.

Secretion of LH and FSH stimulates spermatogenesis and the testicular secretion of testosterone and estradiol, two steroid hormones that together are responsible for maintenance of libido. Blood levels of estradiol increase with age in boars (Estienne et al., 2000) and are inversely related to the time required to mount and begin ejaculation once boars are in the presence of an artificial sow (Louis et al., 1994b). Estradiol concentrations were higher in boars that readily mounted an artificial sow than in boars that refused to do so.

In the experiment of Louis et al. (1994b), from Week 8 to the conclusion of the study, blood concentrations of LH and testosterone were similar between treatment groups. The concentration of estradiol, however, was greater for boars fed the high protein diet than for boars fed the low protein diet.

In the second experiment (Louis et al., 1994a), 24 sexually mature, crossbred boars (Landrace x Large White) received one of three diets (8 boars per treatment): 1) low-energy and low-protein, 2) low-energy and high-protein, or 3) high-energy and high-protein. The low-energy and high-energy feeds provided 6.1 and 7.7 Mcal of metabolizable energy per day, respectively. The low-protein diet provided 7.7 g of lysine per day while the high-protein diet provided 18.1 g of lysine per day. Each boar was allowed a total of 4 to 5 pounds of feed daily.

Semen was collected twice weekly for 27 weeks. During the course of the experiment, boars consuming the high-energy and high-protein diet gained more weight than did boars in the other treatment groups. Average daily gain during the experiment for boars eating the high-energy and high-protein feed was 0.83 pounds. Boars consuming the low-energy and high-protein diet gained more weight than did boars eating the low-energy and low-protein feed (0.37 pounds and 0.20 pounds per day, respectively).

Boars consuming high-protein and either high- or low-energy diets had similar semen and ejaculation characteristics during Weeks 8 through 27 of the study. However, animals in these treatment groups produced 60% more semen and 33% longer durations of ejaculations than did boars consuming the low-energy and low-protein feed.

Libido was significantly affected by treatment. Five of eight boars consuming the low-energy and low-protein diet consistently refused to mount the artificial sow. In contrast, only two of eight boars consuming the high-protein and low-energy diet, and zero of eight boars consuming the high-protein and high-energy ration failed to mount the artificial sow.

Several important conclusions can be drawn from these experiments. First, if protein, or both energy and protein intake are reduced, libido and semen quality in boars are adversely affected. However, energy intake of adult breeding boars can perhaps be reduced to control their weight gains without seriously compromising reproductive performance. Secondly, reduced libido (e.g., time necessary to mount an artificial sow and begin ejaculation) preceded altered semen characteristics in boars that were chronically protein restricted. Thus, a reduction in sexual aggressiveness may be considered an early "caution flag" that boars are perhaps being nutritionally challenged. Finally, decreased libido may be a consequence of suppressed testicular secretion of estradiol.

Effects of Vitamin Intake on Reproductive Characteristics
Few experiments have been conducted in boars to assess the effects of vitamins on reproductive function and most of the existing studies have focused on young developing boars. The following is a summary of pertinent work.

Recently, Audet et al. (2004) conducted an experiment during which the effects of dietary supplements of vitamins on semen characteristics and libido were determined. Duroc, Landrace or Yorkshire boars that ranged in age from 6 to 10 months received one of four daily diets: Basal diet supplemented with "industry levels" of vitamins (n = 9), Basal diet supplemented with 1000 mg Vitamin C (n = 11), Basal diet supplemented with fat soluble vitamins (100,000 IU Vitamin A; 10,000 IU Vitamin D3; 600 IU Vitamin E; and 10 mg Menadione [Vitamin K]) (n = 9), and basal diet supplemented with water soluble vitamins (4000 mg choline; 400 mg Pantothenic Acid; 100 mg Riboflavin; 40 mg folic acid; 500 mg Niacin; 20 mg Thiamin; 60 mg Pyridoxine; 0.4 mg Vitamin B12; and 5 mg Biotin) (n = 11). All boars received 6.6 pounds of feed daily containing 8.3 Mcal of metabolizable energy, 15.3% crude protein and 1.06% lysine. The vitamin premixes were given as a top dressing of 50 g.

Diets were fed during a one month acclimation period, a three month period during which boars were trained to mount an artificial sow and allow semen collection, a five-week, regular collection period during which boars were collected three times every two weeks (8 ejaculates), a two-week intensive collection period during which boars were collected daily, and a 10-week recovery period during which boars were collected three times every two weeks (15 ejaculates).

Throughout the experiment measures of libido (interval between entering collection area and start of ejaculation and duration of ejaculation) and blood concentrations of estradiol were similar among treatments. Moreover, there were no treatment effects on the number of sperm cells per ejaculate or the percentage of motile sperm cells during the regular collection period.

During the intensive collection period, there was a tendency for the number of sperm cells per ejaculate to be greater in fat soluble vitamin- (25.92 billion) and water soluble vitamin- (26.67 billion) supplemented boars compared with controls (24.12 billion). Although these differences were relatively small, they approached statistical significance.

During the recovery period, the number of sperm cells per ejaculate was similar between groups. However, the percentage of motile sperm cells for boars given supplemental fat soluble vitamins (88.8%) and for boars given supplemental water soluble vitamins (89.1%) tended to be greater than the percentage of motile sperm cells for controls (87.0%). The biological significance of these small differences in the percentage of motile sperm cells is questionable. Finally the percentage of sperm cells with abnormal morphology was similar among groups during the recovery period. Audet et al. (2004) concluded that supplementation of boar diets with high levels of Vitamin C, fat soluble vitamins or water soluble vitamins had no appreciable effects on semen or libido characteristics in boars.

Marin-Guzman et al. (1997) studied the effects of vitamin E and selenium supplementation to boar diets. From weaning to 9 months of age, and through a 16-week experimental period, boars were fed a basal diet, the basal diet supplemented with selenium (0.23 mg/pound of diet), the basal diet supplemented with Vitamin E (100 IU/pound of diet), or the basal diet supplemented with both selenium (0.23 mg/pound of diet) and Vitamin E (100 IU/pound of diet). Diets were consumed on an ad libitum basis from weaning to approximately 319 pounds of body weight and thereafter were limit fed to individual boars at a rate of 4.4 pounds per day. During the experimental period semen was collected three times weekly. Boars fed the basal diet displayed decreased sperm motility and an increase in the percentage of sperm cells with abnormal morphology compared with the supplemented groups. The effects of added selenium on semen characteristics were more pronounced than the effects of added Vitamin E, and selenium supplementation resulted in greater fertilization rates when gilts were bred with semen from the experimental boars. It should be noted that current U.S. Food and Drug Administration (FDA) regulations allow up to 0.136 mg of added selenium/pound of diet for all pigs (NRC, 1998).

Effects of Fatty Acids on Reproductive Characteristics
Linoleic acid (an omega-6 fatty acid) is the only fatty acid for which NRC has established requirements for sexually active boars (0.1% of diet). The effect of dietary supplementation of various fatty acids, particularly the omega-3 fatty acids, on semen and libido characteristics in boars, however, has received increasing interest by swine researchers. The omega-3 fatty acids are linolenic, eicosapentaenoic (EPA) and docosahexaenoic (DHA).

Rooke et al. (2001) conducted an experiment during which boars (age range from 395 to 761 days) were fed daily 5.5 pounds of either a control diet (n = 5) or the control diet supplemented with tuna oil (13.6 g/pound of diet) (n = 5). Tuna oil is a rich source of omega-3 fatty acids. Boars in both groups were fed Vitamin E (134 mg/pound of diet) to serve as an antioxidant. An antioxidant is necessary to prevent oxygen from altering the biological activity of omega-3 fatty acids.

Semen was collected twice weekly at 3, 5, and 6 weeks of feeding the experimental diets. Supplementing the diet of the boars with tuna oil increased the proportion of viable sperm cells and the percentages of sperm cells with progressive motility, normal acrosome morphology, and normal morphology.

PROSPERM (Minitube America, Inc., Minneapolis, MN) is a commercially available product that contains DHA, Vitamin E and selenium. In a commercial field trial (Spermnotes, Volume V, Issue 1- Spring 2001, pages 4-5) thirty-five boars were reportedly fed diets with or without PROSPERM for 16 weeks. Sperm concentration (502 million for control, 584 million for supplemented), number of sperm/ejaculate (74.1 billion for control, 83.4 billion for supplemented), and sperm motility score (3.9 for control, 4.5 for supplemented) were increased by PROSPERM. Four hundred, seventy-eight gilts were mated via artificial insemination to boars that received the supplement or those that did not. Significant improvements were demonstrated for conception rate (83% for control, 90% for supplemented) and number of pigs born alive (10.2 for control, 10.6 for supplemented). Remaining to be determined is the relative contribution of each of the components of PROSPERM (DHA, Vitamin E and selenium) toward the overall positive effect on reproduction.

Effects of L-Carnitine on Reproductive Characteristics

L-carnitine is a vitamin-like compound synthesized in the liver, kidney, and brain that is involved in energy metabolism by sperm cells. There have been reports that supplementation of diets with L-carnitine increase sperm production and enhances sperm motility in several species. For example, feeding L-carnitine at a rate of 227 mg/pound of diet increased sperm concentrations in roosters (Neuman et al., 2002).

At Virginia Techıs Tidewater Agricultural Research and Extension Center in Suffolk, VA we recently conducted two experiments to assess the effects of dietary L-carnitine (Carniking; Lonza, Inc., Fairlawn, NJ) supplementation on semen characteristics in boars (Kozink et al., 2004). In Experiment 1, young, postpubertal boars that were 258 days of age were used. Boars were fed daily 4.4 pounds of a control diet (n = 9) or the control diet plus 500 mg L-Carnitine (n = 9). Semen was collected weekly from Week 0 to 15 and on 4 consecutive days during Week 16. Experiment 2 was similar to Experiment 1 except boars (n = 10 per treatment) were 504 days of age. For the weekly and intensive collections in both experiments there were no positive effects of L-carnitine on sperm cells/ejaculate or on sperm motility.

In contrast to our results, results of a commercial field trial (Akey Swine Newsletter, August 2000, page 1) suggested that dietary supplementation with L-carnitine enhanced boar performance. One hundred, eighty boars (high lean growth genotype) received a control diet or a diet supplemented with a "low" or "high" level of Carniking for 16 weeks. Feeding high levels of L-carnitine reportedly increased semen volume and the number of viable sperm cells produced.

Although research focusing on the nutrition of the sexually active boar is limited, some general conclusions can be drawn. A prolonged period of restricted protein, or both energy and protein, adversely affects libido and semen quality in boars. Reduced libido, perhaps due to decreased estradiol concentrations, precedes altered semen characteristics in boars that are chronically protein restricted. Recent data suggest that there are no exceptional positive effects of supplementing large levels of Vitamin C, fat soluble vitamins, or water soluble vitamins on boar semen or libido characteristics. With regard to reproductive performance, however, there are data to support the addition of selenium and vitamin E to boar diets. Recent evidence also supports the notion that dietary supplementation with omega-3 fatty acids improves semen characteristics. Finally, results from studies investigating the effects of supplemental L-carnitine on reproduction in boars are equivocal and will require additional study.

Audet, I., J.P. Laforest, G.P. Martineau, and J.J. Matte. 2004. Effect of vitamin supplements on some aspects of performance, vitamin status, and semen quality in boars. J. Anim. Sci. 82:626-633.

Cromwell, G.L., T.R. Cline, J.D. Crenshaw, T.D. Crenshaw, R.C. Ewan, C.R. Hamilton, A.J. Lewis, D.C. Mahan, E.R. Miller, J.E. Pettigrew, L.F. Tribble, and T.L. Veum. 1993. The dietary protein and (or) lysine requirements of barrows and gilts. J. Anim. Sci. 71:1510-1519.

Estienne, M.J., D.S. Broughton, and C.R. Barb. 2000. Serum concentrations of luteinizing hormone, growth hormone, testosterone, estradiol, and leptin in boars treated with n-methyl-d, l-aspartate. J. Anim. Sci. 78:365-370.

Kemp, B. 1991. Nutritional strategy for optimal semen production in boars. Pig News and Information. 12:555-558.

Kemp, B. and M.W.A. Verstegen. 1991. Nutrition and sperm production. Reprod. Domestic Anim. Supplement 1:287-296.

Kozink, D.M., M.J. Estienne, A.F. Harper, and J.W. Knight. 2003. Effects of dietary L-carnitine supplementation on semen characteristics in boars. Theriogenology 61:1247-1258.

Louis, G.F., A.J. Lewis, W.C. Weldon, P.M. Ermer, P.S. Miller, R.J. Kittok, and W.W. Stroup. 1994a. The effect of energy and protein intakes on boar libido, semen characteristics, and plasma hormone concentrations. J. Anim. Sci. 72:2051-2060.

Louis, G.F., A.J. Lewis, W.C. Weldon, P.S. Miller, R.J. Kittok, and W.W. Stroup. 1994b. The effect of protein intake on boar libido, semen characteristics, and plasma hormone concentrations. J. Anim. Sci. 72:2038-2050.

Marin-Guzman, J., D.C. Mahan, Y.K. Chung, J.L. Pate, and W.F. Pope. 1997. Effects of dietary selenium and vitamin E on boar performance and tissue responses, semen quality, and subsequent fertilization rates in mature gilts. J. Anim. Sci. 75:2294-3003.

National Research Council. Nutrient requirements of swine, 10th ed. Washington, DC: National Academy Press; 1998.

Neuman, S.L., T.L. Lin, P.Y. Hester. 2002. The effect of dietary carnitine on semen traits of White Leghorn roosters. Poul. Sci. 81:495-503.

Rooke, J.A., C.C. Shao, and B.K. Speake. 2001. Effects of feeding tuna oil on the lipid composition of pig spermatozoa and in vitro characteristics of semen. Reproduction 121:315-322.

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