INTRODUCTION

During the period of the mid 1990's the pork industry experienced a revolution in breeding management practices. Although accurate statistics are not available, there is an indication that AI may soon account for 50 percent or more of the total matings in the US. These changes in breeding management practices already have had a significant influence on the structure of the industry that supplies genetics. As new reproductive technologies and techniques are developed, even more rapid changes can be anticipated.

Following is a brief update on gender preselection, the effects of frequency of semen collection and effects of nutrition on sperm output of boars.

GENDER PRESELECTION

Manipulation of spermatozoa as a means of controlling the sex of offspring has been a goal for many years. A practical, highly efficient means of preselecting boar ejaculates for the production of either male or female offsping would have a significant influence on both human and animal reproduction.

Garner (1984) reviewed various differences between X and Y bearing chromosomes and the approaches which had been used to separate them. These included physical differences in the chromosomes, motility, surface charges, surface molecules, internal cellular enzymes and DNA content. He concluded at that time that only DNA content could be used to consistently identify X and Y bearing spermatozoa.

More recently, Johnson, 1997, reviewed his studies with gender preselection utilizing the USDA- Beltsville Sperm Sexing Technology. This technique separates Hoechst 33324 stained X and Y spermatozoa by using a flow cytometry/cell sorter system. DNA content of X bearing spermatozoa is about 3.6% greater than that of Y bearing spermatozoa. When stained with Hoechst 33324, a vital fluorochrome, the differential fluorescence is detected with an UV laser. Each cell is encased in a droplet, which is given a positive or negative charge based upon DNA content. Charged droplets are passed through an electrostatic field where individual X and Y spermatozoa are deflected into collection tubes.

Rath et al (1997) reported the birth of piglets resulting from the use of in vitro fertilization (IVF) and surgical insemination techniques with spermatozoa preselected for sex based upon the USDA technology. Of the two litters born as a result of IVF, one resulted in 100% females and one with 100% males. One litter produced from the surgical insemination technique resulted in 88% females and the other 86% males.

Commercial application of this technology at the present time is limited by the relatively slow speed at which cells can be sorted and by the cost and expertise requirements. The present USDA technology will produce about 4 to 5 million cells per hour. A typical boar ejaculate contains from 30 to 80 billion spermatozoa and most commercially produced semen contains 3 to 4 billion cells per dose. However, when combined with IVF or surgical insemination techniques where success with only 200,000 to 400,000 cells has been demonstrated, the technology may soon be applied. Presently, the high-speed cell sorter costs about $200,000 and obviously a team of highly trained technicians would be required to perform cell sorting and IVF/surgical insemination (Johnson, 1997).

REFERENCES

Garner, D.L. 1984. Semen sexing - An overview of separation of X- and Y- spermatozoa. Proc. 10th An. Conf. on AI and Reprod.

Johnson, L.A. 1997. Personal communication.

Johnson, L.A. 1997. Advances in gender preselection of swine. J. Reprod. And Fert. Supp. (In press)

Rath, D., L.A. Johnson, J.R. Dobrinsky, G.R.Welsh and H. Nieman. 1997. Therio. 47:795-800.

COLLECTION FREQUENCY EFFECTS ON SEMEN PRODUCTION OF BOARS

Collection/ejaculation frequency is one of the major factors affecting both quantity and quality of sperm produced by AI boars. A collection frequency that optimizes both the total number of cells produced per unit time and their "quality" would be ideal. However, there may be somewhat of an inverse relationship between these two parameters. High collection frequencies (every 24 or 48 hours) may yield the maximum number of sperm per unit time, but subsequent fertility may be compromised. The ability of the boar to maintain an adequate level of libido may also become a factor at high collection frequencies. In practice, semen demand and the labor efficiency of collecting and processing ejaculates must also be considered when determining the optimum collection frequency.

Spermatogenesis a continuous process and a mature boar can accumulate 120-160 billion sperm in the cauda (tail) epididymides where they await ejaculation in a quiescent state. When this reservoir is at capacity, a single collection can yield about 50-60% of the epididymal reserves (Bonet et al., 1991). If a boar is not collected and does not masturbate, the majority of the excess cells will be excreted in the urine. When a boar is collected at a high frequency (i.e. every 24 hours, for 10 days) total sperm in the ejaculate will drop dramatically for 3-4 days until epididymal reserves stabilize and then the total sperm per collection will become relatively constant. This state of stabilization of the boar's daily sperm output (DSO) is representative of his daily sperm production (DSP). DSO is calculated by dividing the total number of cells in the ejaculate by the number of days since last collection. Once stabilized, DSO will represent 80-90% of the boar's DSP (Swierstra, 1973, 1974). DSP is the actual number of sperm produced daily and can be estimated directly through quantitative testicular histology or indirectly by DSO.

In order to get an estimate of DSO, the epididymal reserves must be depleted. Several methods have been suggested to accomplish this. A boar may be collected 3 times per week for 5-6 weeks and the fifth or sixth week used to estimate DSO. A more rapid alternative is to deplete the boar's reserves with 4-5 collections on a single day and then use the following 3-4 days for DSO estimation (Cameron, 1987, 1985). Once epididymal reserves are depleted it takes about 6-7 days of sexual rest to restore them because a mature boar can produce about 15-20 billion sperm per day.

It has been suggested that once the cauda epididymis is depleted it forces sperm in the caput (head) and corpus (body) of the epididymis to pass through at an increased rate, possibly interfering with their maturation. However, the rate of passage through the caput and corpus epididymis, where sperm maturation occurs, is due to constant smooth muscle contractions in bulls and is probably not influenced by ejaculation frequency. The cauda (tail) epididymis, which is normally not contracting, is the only segment under the control of smooth muscle contractions during ejaculation in the bull (Amann, 1986). The same is probably true for the boar and it seems unlikely that frequent ejaculations could increase the rate of passage of sperm through the caput and corpus epididymis.

Increased semen collection frequency can result in increased DSO (Kemp et al., 1991, 1990, 1988), but this effect may be temporary. Kemp found no effect of ejaculation frequency on any microscopically evaluated parameters of sperm quality, but subsequent fertility was not accessed. High collection frequency decreases semen volume and sometimes sperm motility, but the most dramatic drop is in total sperm per ejaculate. Levis (1986) demonstrated that boars rested for 5 days and then collected on 12 or 24 hour intervals had 33-41% and 59-66% less total motile sperm in their second and third ejaculates, respectively, as compared to their first collection following the sexual rest period.

Collection 3 times per week, on Monday, Wednesday and Friday, resulted in greater volume, concentration and total sperm per ejaculate than continued 24 or 48 hour collection intervals (Cameron, 1985). Cameron hypothesized that this 3 times a week scheme may have resulted in a greater output because the boars appeared to maintain a higher level of libido. Furthermore, Swierstra (1973) found sperm output per unit time was greater on a 72 hour than on a 24 hour collection interval. Thus, a boar's level of libido may be a conflicting factor when estimating DSO at high collection frequencies.

Studies on the effect of collection frequency on semen quality are somewhat contradictory. Strzezek et al. (1995) found that high collection frequencies resulted in decreased sperm motility, concentration, total sperm and increased percent abnormal sperm and sperm with damaged membranes. Osmotic resistance-test values (ORT) were also decreased, which could have major implications when freezing semen. Furthermore, collections at 2 day intervals as compared to 3.5 day intervals resulted in reduced motility, volume, total sperm and ORT values (Schilling and Vengust, 1987). However, no significant differences were found in the phospholipid composition of the sperm of boars on a 24 hour versus a 72 hour collection frequency (Johnson et al., 1969). Other parameters, such as respiratory activity of sperm (Fülöp, 1996) and levels of important biochemical compounds (Strzezek et al., 1995, 1996) are also altered by high collection frequency.

Even though high collection frequencies have been found to negatively impact many sperm quality parameters in vitro, little research on in vivo fertility had been conducted. Sweirstra (1976) found sows inseminated with equal amounts of motile sperm (2.5 billion) from boars on 24 hour and 72 hour collection intervals had significantly different pregnancy rates (83% vs. 70%, receptively). However, a boar by ejaculation frequency interaction suggested that some boars were more fertile when collection interval was increased while others were more fertile when it was decreased.

Since collection frequency can impact many ejaculate parameters, a boar should not be evaluated based on a single ejaculate. Variation between boars and between ejaculates from the same boar can be reduced through stabilization of epididymal reserves. Thus, comparisons of sperm production and semen quality between boars cannot be made until this is accomplished. Variation between studies in the age of boars, collection frequency and methods of sperm quantity and quality evaluation implemented has produced a wide variety of experimental results.

Generally, high semen collection frequencies do not appear to be beneficial in terms of the number of sperm harvested per unit time and may have a negative impact on sperm function. This is important when sperm are to be stored at 17-18º C for use in AI, but becomes even more critical when the sperm collected are to be frozen. The interval between collections should be long enough for epididymal reserves to be restored, but not so long that the epididymal storage capacity is exceeded and sperm are lost through excretion. Swierstra (1971) found boars on a 3 day collection interval had negligible sperm loss due to absorption in the epididymides or excretion in the urine.

Thus, for maximum sperm harvest, it appears that a collection frequency of 3 times per week for mature boars (>12 months) and 2 times per week for young boars (7-10 months) may be optimum. However, in practice, the efficiency of collecting and processing ejaculates and semen demand must be considered when assigning boars to a collection schedule. The current practice of collecting boars 1 to 2 times every 7-10 days may provide a schedule which optimizes both sperm harvest and labor efficiency. Studies are lacking which provide data on the subsequent fertility of the occassional "high demand" sires, which are collected 2 times per day, once or more per week.

REFERENCES

Amann, R.P. (1986) How a bull works. 11th Tech. Conf. on A.I. and Reprod. 6-18.

Bonet, S., Briz, M., and Fradera, A. (1991) The sperm quality and fertility of boars after two different ejaculation frequencies. Scientia gerundenesis 17: 77-84.

Cameron, R.D.A. (1987) Sexual development and semen production in boars. Pig News and Information, 8: 389-396.

Cameron, R.D.A. (1985) Measurement of semen production rates of boars. Aust. Vet. J. 62: 301-304.

Fülöp, L. (1996) Influence of collection frequency on respiratory activity of boar semen. Reprod. Dom. Anim. 31: 241-242.

Johnson, L.A., Gerrits, R.J., and Young, E.P. (1969) Quantitative analysis of porcine spermatozoa and seminal plasma phospholipids as affected by frequency of ejaculation. J. Reprod. Fert. 19: 95-102.

Kemp, B., Bakker, G.C.M., Den Hartog, L.A., and Verstegen, M.W.A. (1991) The effect of semen collection frequency and food intake on semen production in breeding boars. Anim. Prod. 52: 355-360.

Kemp, B., Vervoort, F.P., Bikker, P., Janmaat, J., Verstegen, M.W.A., and Grooten, H.J.G. (1990) Semen collection frequency and the energy metabolism of A.I. Boars. Anim. Reprod. Sci. 22: 87-98.

Kemp, B., Grooten, H.J.G., Den Hartog, L.A., Luiting, P., and Verstegen, M.W.A. (1988) The effect of high protein intake on sperm production in boars at two semen collection frequencies. Anim. Reprod. Sci. 17: 103-113.

Levis, D.G. (1986) Reproductive management of the boar. George A. Young Swine Conference Proceedings. Lincoln, Nebraska, U.S.A.

Schilling, E., and Vengust, M. (1987) Frequency of semen collection in boars and quality of ejaculates as evaluated by the osmotic resistance of acrosomal membranes. Anim. Reprod. Sci. 12: 283-290.

Strzezek, J., Demianowicz, W., Kordan, W., Torska, J., Wysocki, P., and Holody, D. (1996) Biochemical status of boar spermatozoa and seminal plasma before and after a 10-day depletion test. Reprod. Dom. Anim. 31: 245-246.

Strzezek, J., Kordan, W., Glogowski, J., Wysocki, P., and Borkowski, K. (1995) Influence of semen-collection frequency on sperm quality in boars, with special reference to biochemical markers. Reprod. Dom. Anim., 30: 85-94.

Swierstra, E.E., and Dyck, G.W. (1976) Influence of the boar and ejaculation frequency on pregnancy rate and embryonic survival in swine. J. Anim. Sci. 42: 455-460.

Swierstra, E. E. (1974) A comparison of regular ejaculation with sexual rest on semen characteristics and reproductive organ weights in young boars. J. Anim. Sci. 39: 575-581.

Swierstra, E.E. (1973) Influence of breed, age, and ejaculation frequency on boar semen composition. Can. J. Anim. Sci. 53: 43-53.

Swierstra, E.E. (1971) Sperm production of boars as measured from epididymal sperm reserves and quantitative testicular histology. J. Reprod. Fert. 27: 91-99.

NUTRITIONAL EFFECTS ON SEMEN PRODUCTION OF AI BOARS

Compared to natural service boars, AI boars are more costly, replaced more rapidly and impact the litter size and farrowing rate of several hundred more sows. The goal should be to provide a nutrition program that will maximize the production of high quality semen. The idea that the conventional sow gestation ration provides adequate nutrients for AI boars has been confirmed by some studies and challenged by others. Kemp and Den Hartog (1989a) reviewed the literature related to the influence of energy and protein intake on boar reproductive performance. The focus of this review will be on those nutrients which have been associated with libido, sperm production and fertility.

ENERGY, CRUDE PROTEIN AND SPECIFIC AMINO ACIDS

The total energy requirements of the boar can be divided into energy for maintenance, growth, mating activity and semen production. If the boar is housed below his lower critical temperature (LCT) energy for heat production may need to be considered as well. This temperature is approximately 20°C (68°F) and is dependent on the boar's body weight and environmental factors (Kemp et al., 1989b). Energy for maintenance represents about 60-90% of the total energy requirements, while mating activity and sperm production combined only represent about 5% (Close, 1994). Kemp et al. (1990) estimated the energy required for mating activity and semen production and found it to be negligible compared to the energy requirement for maintenance and gain.

A boar's energy requirement for maintenance is mainly a function of his body weight. Close and Roberts (1991) developed a formula based on data from several studies to estimate the maintenance requirement. The optimum growth rate (weight gain) for a boar is not well defined,

but is often targeted to be between 0.5-1.1 lb. per day. It appears that the rate of gain should be near 1 lb. per day in a young boar and should gradually decrease to 0.5 lb. or less as the boar matures.

Kemp (1991) outlined calculations of a boar's total energy requirement based on the available data. Kemp et al. (1989b) calculated total energy requirement of a boar to be 34.2 to 36.9 MJ ME/day as body weight increases from 330 to 770 lb. Similarly, Close and Roberts (1991) calculated total energy requirement to be 27.5 to 37.4 MJ ME/day as body weight increases from 220 to 770 lb.

The level of protein intake has been linked to sperm production in numerous studies. Low protein intake (7% CP vs. 16% CP diet) resulted in decreased semen volume, total sperm output and increased sperm concentration (Louis et al., 1994a). The difference in sperm output between treatments did not seem to be due to differences in sperm production because estimates of sperm production from testicular homogenization-resistant sperm nuclei counts were not different. This experiment also demonstrated a reduction in libido. Boars on the low protein diet took more time to mount the dummy and start ejaculating and stayed on the dummy for a shorter period than boars on the control diet. The author suggests that this difference in libido may be the cause of the difference in semen output between the dietary treatments. Reduction in both protein (7% vs. 16% CP) and energy (6.1 vs. 7.7 Mcal/day ME) intake was found to be a larger factor than reduction in energy intake alone in decreased libido in a companion experiment (Louis et al., 1994b). Thus, restricting energy intake to reduce boar weight gain must be done cautiously and must not compromise protein requirements.

A high level of crude protein intake yielded no increases in sperm quantity or quality when Kemp et al. (1988) compared isocaloric diets of 14.5 % CP versus 22.2% CP. However, the addition of specific amino acids to diets at 1988 NRC requirements has produced some improvements in sperm output. Addition of 6 and 9 g of lysine to an NRC basal diet (12.36% CP) produced an increase in total sperm and a decrease in percent abnormal sperm (Moon and Kim, 1990). Addition of 7 and 14 g of methionine to the same NRC basal diet produced an increase in semen volume and total sperm (Kim and Moon, 1990). However, in a similar type of study, Ju et al. (1985) found no effect on semen production with addition of 7.2 g methionine or 9.2 g lysine to an 11.78% CP corn-soy diet.

Restriction of the weight gain of a growing boar through limiting feed intake, not only limits energy intake but also crude protein (CP) intake. Reduction in crude protein intake has been shown to reduce libido and sperm production (Louis et al., 1994a, 1994b). Even though this practice has been used for natural service boars, it is generally not necessary in the case of the AI boar for a number of reasons. Since AI boars only mount the "dummy sow" and not real sows or gilts their size is not as much of a concern. Granted, there must be large enough crates or pens to accommodate mature boars. Another reason is that AI boars are often turned over at a high rate and are replaced by a genetically superior boar in approximately 1-2 years time. Furthermore, there is evidence that restrictions in energy and or crude protein intake can limit the sperm production of boars. This is obviously not ideal since producers want to maximize the quantity and quality of sperm an AI boar produces.

Feeding boars ad libitum is not the answer as they can become fat, lethargic and often lose libido. However, 13 month old boars fed a 16.6% CP, 5.9 MJ/lb. ME diet at three different feed intakes, high (H=12.6 lb., ad lib.), medium (M=8.0 lb.) and low (L=4.2 lb.) had significantly different sperm outputs. The M and H boars produced 46 and 69% more sperm, respectively, than the boars on the L feed intake (Kemp et al., 1989c). Thus, it should be noted that sperm output is not being maximized because feed intakes presently common for AI boars are near the L feed intake treatment.

Based on the available data a 14.5% CP diet with 5.7 MJ ME/lb. fed at the level of 6-7 lb. per day was recommended by Kemp (1991). Sow gestation rations usually do not contain the level of CP, but may contain this level of ME. Recommendations that range from 250-390 g of CP intake per day have been made for boars. The body weight, targeted growth rate, level of mating activity (collection frequency) and environmental conditions the boar is housed in should be used to determine the feeding level.

VITAMINS AND MINERALS

Vitamin levels recommended for sow gestation diets are generally sufficient for boar diets. However, some studies have found improvements in sperm quantity and quality with supplemental vitamin C and E. This has encouraged some nutritionists and AI boar stud managers to increase the levels of vitamin C and E in boar rations.

The role of vitamin E as an antioxidant does not explain why it is required for maintenance of spermatogenesis in many animals. The deleterious effect of vitamin E deficiency on germ cell proliferation and differentiation does not seem to be related to testosterone levels or modification of gonadotropin feedback loops (Cooper et al., 1987). Nonetheless, supplemental vitamin E has been shown to increase total sperm output (Westendorf and Richter, 1977) and sperm concentration (Brzezinska-Slebodzinska et al., 1995) in boars. A recent study by Marin-Guzman et al. (1997) found boars fed a diet deficient in vitamin E (initiated at weaning) had lower sperm motility compared to boars given 220 IU/kg supplemental vitamin E. The antioxidant function of vitamin E in protecting the sperm plasma membrane seems to come from within the sperm since it is concentrated there, but not in seminal plasma. The reason that vitamin E is required for maintenance of spermatogenesis remains unknown.

Pigs can synthesize vitamin C (unlike guinea pigs and humans), but during periods of stress they seem to have an increased requirement. Vitamin C, like vitamin E, is an important antioxidant, but vitamin C also seems to be functioning in a variety of biochemical processes that are not well understood. Supplemental vitamin C has shown some signs of improving semen quality during periods of high ambient temperature. Ivos et al. (1971) found an increase in the conception rate of sows bred by boars supplemented with vitamin C during the summer and Lin (1985) demonstrated an increase in total sperm per ejaculate during the summer in vitamin C supplemented boars. However, a study in which half the sows and boars on five different farms were supplemented with vitamin C (4 g/head/day) during the summer produced no significant improvement in sow reproductive performance (Greer et al., 1987). Thus, it is still unclear whether or not supplemental vitamin C will provide substantial improvements in the fertility of boars under stressful conditions.

Selenium (Se) and zinc (Zn) are two minerals that have been demonstrated to have crucial roles in testicular function and spermatogenesis. Concentration of Se in the testes is regulated by gonadotrophic hormone levels and increases greatly during puberty. Se and Zn seem to have vital roles in spermatogenesis because a substantial deficiency in either results in structural abnormalities in the sperm produced. Decreased motility is also often evident as a result of this structural damage.

Initial studies on the effect of Se deficient diets on boars found no apparent reduction in fertility (Henson et al., 1983; Segerson et al., 1981). However, Marin-Guzman et al. (1997) recently found boars fed a Se deficient diet (0 ppm Se), initiated at weaning, had increased sperm abnormalities, reduced sperm motility and decreased fertility when gilts were inseminated as compared to boars fed a diet with 0.5 ppm Se. Se and Zn are definitely necessary in trace amounts for normal spermatogenesis. However, sow gestation rations usually have about 0.3-0.4 ppm Se and about 100 ppm Zn. These levels seem to be sufficient for boars and there is no evidence to suggest that higher levels are necessary.

In summary, the optimum feeding strategy to maximize the reproductive efficiency of AI boars has not been determined. However, one may want to consider one of the commercially available diets formulated specifically for stud boars. Both the quantity fed and the nutrient content of the diet may influence semen output. Additionally, various genotypes may require different feeding strategies. Compromising a boar's libido and sperm production through inadequate nutrient supply could be costly. AI boars are expensive animals and their fertility will influence the reproductive efficiency of several hundred females.

REFERENCES

Brzezinska-Slebodzinska, E., Slebodzinski, A.B., Pietras, B., and Wieczorek, G. (1995) Antioxidant effect of vitamin E and glutathione on lipid peroxidation in boar semen plasma. Biological Trace Element Research 47: 69-74.

Close, W. (1994) Nutrition for optimum breeding: boar feeds. Feed Management 45: 21-22.

Close, W.H., and Roberts, F.G. (1991) Nutrition of the working boar. Recent Advances in Animal Nutrition, Oxford: Butterworth-Heinemann pp 21-44.

Colenbrander, B., and Kemp, B. (1990) Factors influencing semen quality in pigs. J. Reprod. Fert., Suppl. 40: 105-115.

Cooper, D.R., Kling, O.R., and Carpenter, M.P. (1987) Effect of vitamin E deficiency on serum concentrations of follicle-stimulating hormone and testosterone during testicular maturation and degeneration. Endocrinology 120: 83-90.

Greer, E.B., Gardner, I.A., and Wright, G.L. (1987) Failure of dietary vitamin C supplementation to prevent seasonal infertility in pigs. Aust. J. Exp. Agric. 27: 343-347.

Henson, M.C., Kattesh, H.G., Hitchcock, J.P., and Kincaid, S.A. (1983) The effects of dietary selenium on growth and selected reproductive parameters in young boars. Anim. Prod. 37: 401-407.

Ivos, J., Doplihar, C., and Muhaxhiri, G. (1971) Thermic stress as a factor of disturbances in the reproduction of pigs and possibility of prevention of these disturbances by the addition of ascorbic acid. Veterinarski Archiv 41: 202-216.

Ju, J.C., Cheng, S.P., and Yen, H.T. (1985) Effects of amino acid additions in diets on semen characteristics of boars. Journal of the Chinese Society of Animal Science 14: 27-35.

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

Kemp, B., Vervoort, F.P., Bikker, P., Janmaat, J., Verstegen, M.W.A., and Grooten, H.J.G. (1990) Semen collection frequency and the energy metabolism of A.I. Boars. Anim. Reprod. Sci. 22: 87-98.

Kemp, B., and Den Hartog, L.A. (1989a) The influence of energy and protein intake on the reproductive performance of the breeding boar: a review. Anim. Reprod. Sci. 20: 103-115.

Kemp, B., Verstegen, M.W.A., Den Hartog, L.A., and Grooten, H.J.G. (1989b) The effect of environmental temperature on the metabolic rate and partitioning of energy intake in breeding boars. Livestock Prod. Sci. 23: 329-340.

Kemp, B., Den Hartog, L.A., and Grooten, H.J.G. (1989c) The effect of feeding level on semen quantity and quality of breeding boars. Anim. Reprod. Sci. 20: 245-254.

Kemp, B., Grooten, H.J.G., Den Hartog, L.A., Luiting, P., and Verstegen, M.W.A. (1988) The effect of high protein intake on sperm production in boars at two semen collection frequencies. Anim. Reprod. Sci. 17: 103-113.

Kim, K.H., and Moon, S.J. (1990) Effect of methionine levels on semen quality of boars. Korean J. Anim. Sci. 32: 800-804.

Lin, H.K. (1985) Studies on improving semen quality of working boars fed diets with addition of vitamin C in Summer. Ann. Res. Rep. Anim. Ind. Res. Inst. TSC. p 59.

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

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

Marin-Guzman, J., Mahan, D.C., Chung, Y.K., Pate, J.L., and Pope, W.F. (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: 2994-3003.

Moon, S.J., and Kim, K.H. (1990) Effect of lysine levels on semen quality of boar. Korean J. Anim. Sci. 32: 767-771.

Segerson, E.C., Getz, W.R., and Johnson, B.H. (1981) Selenium and reproductive function in boars fed a low selenium diet. J. Anim. Sci. 53: 1360-1367.

Westerndorf, P., and Richter, L. (1977) Ubersicht für Tierernährung 5: 161-184.