Mineral metabolism and boar fertility

ALLTECH CD 2002: Mineral metabolism and boar fertility: observations from Latin America to Europe

1Animal Science Department, The Ohio State University, Columbus, OH, USA,
2Alltech France, Goussainville, France,
3Alltech de México, México City

Introduction

With increasing emphasis on fewer boars and the use of artificial insemination in many herds, the awareness of having structurally sound boars in good body condition that are producing semen of high quality is paramount to achieving high reproductive performance. The production of ejaculates of high quality subsequently allows a greater dilution of the semen to attain large number of service dosages per animal.

Although half of the genetic pool of the herd resides in the boars, there is relatively little research that has evaluated the effect of nutrition on boar semen quality. Most of the studies that have been conducted have centered on protein and energy needs. These trials have generally concluded that adequate protein is provided when boars are fed diets containing 14 to 15% protein or a lysine level of 0.70%. Feed intake when restricted will reduce sperm production and thus adequate energy should be provided to the boar to maintain sperm output (Colenbrader and Kemp, 1990). Mineral nutrition for the boar is almost completely lacking, particularly with the microminerals.

Mineral metabolism in the boar

Mineral needs can be subdivided into two major areas: (1) those needed for growth and development, and (2) those needed for sperm production. Figure 1 presents the relative amount needed for both phases of the boar. Both macro- and microminerals are needed for both hard and soft tissue development. This is clearly evident from the many studies that have demonstrated the need for both macro- and microminerals with pigs of other ages and production phases. One would not expect the boar to have different requirements than the gilt or barrow when the requirements are expressed on a quantity of tissue growth basis. Larger quantities would be needed for boars because of the larger quantity of these tissues deposited in the growing boar.

Obviously the skeletal structure and the musculature of the male of most species, including the boar, are bigger than the female or castrate, and thus larger quantities of minerals are needed to maximize their formation. Macromineral needs would thus be expected to differ from the other genders more so during the growth and development phase than during maturity where maintenance needs largely prevail. Microminerals, used largely as co-factors of enzymes or hormones, are also needed in higher levels during the growing phase because of the larger demands of the greater tissue growth. These larger quantities, except possibly for calcium (Ca) and phosphorus (P), can probably be calculated on a lean gain basis depending on the rate of growth and the total amount of the various tissues developed. The other tissue in the boar that, however, differs from the other genders is the testis, as it would be expected to require different nutrients than the other soft tissues of the body. This is perhaps the area where the greater contribution to boar nutrition can be made.

The testis is made of a long network of tubules lined with germ cells and Sertoli cells. The germ cells are where spermatogenesis occurs after sexual maturity. Sperm are produced in these tubules in a continual manner, but quantities can be influenced by environmental, seasonal, and nutritional factors. The Sertoli cells provide nurture and support for the development and maturity of the newly formed spermatogonia. When these immature spermatids are not properly developed, sperm output will be lowered. The Sertoli cells and the Leydig cells (which lie between the seminiferous tubules) both produce hormones; the latter cells principally produce testosterone and inhibitin, which control the release of the hormones (GnRH, FSH, LH) and direct the development of other sexual characteristics. These hormones can provide either positive or negative feedback to the hypothalamus that controls various aspects of male reproduction and semen characteristics.

Although we commonly think that reproduction starts when we collect semen or at sexual maturity, we need to consider that spermatogenesis starts well before that point. Spermatogenesis or sperm production has been shown to occur in approximately 90% of the seminiferous tubules by 5.4 months of age in growing boars (Marin-Guzman et al., 2000a), suggesting that factors influencing their development may have later consequences.

Leg development and structural soundness The minerals associated with the formation of the skeletal frame are largely Ca and P, although other microminerals are incorporated into the mineral crystal of bone. Because the ratio of Ca and P is important for the absorption of each nutrient, both are normally discussed together albeit they have different functions. Phosphorus has its largest concentration in bone, but it is also used for muscle formation and body metabolism, whereas most of the Ca is found in bone tissue. Consequently, during the period of rapid growth of both muscle and bone the requirement for P is high. If an inadequate dietary P level is provided, then muscle growth will generally be curtailed prior to the growth of bone tissue because of the higher priority of bone formation over muscle. Bone mineralization, however, continues beyond that for maximum bone growth, thus giving rise to thicker bones of greater shaft width and strength.

Hickman et al. (1983) demonstrated that body growth and bone ash responded to dietary Ca and P levels during early growth periods, whereas when muscle growth was declining or upon maturation of body size, high dietary levels of Ca and P did not yield additional gain (Table 1). It is noted that even when growth rate of the boars was similar at the heavier body weight, bone mineralization continued to increase as the dietary Ca and P level increased.

During the early growth phase of the boar, the Ca and P requirements of boars are substantially higher than those of gilts or market pigs. Kornegay et al. (1981) and Kornegay and Thomas (1981) suggested that the Ca and P requirements of the boar are approximately 125% higher that the NRC requirement. Their results also revealed that neither mineral had any affect on foot soundness (Kornegay and Thomas, 1981).

Selenium

Selenium (Se) has been well recognized for its role in preventing oxidative damage in the cytoplasm of the cell because of its incorporation into the enzyme glutathione peroxidase (GSH-Px), but its role in male reproduction is less well defined. A study reported by Marin-Guzman et al. (1997) demonstrated that the feeding of 0.50 ppm Se (sodium selenite) resulted in the expected higher liver Se content and liver GSH-Px activity in this tissue, but the Se content and GSH-Px activity was also higher in the testis of the boar (Table 2).

It is of interest to note that even when pigs were fed the diet without Se, testis GSH-Px activity increased with pig age (weight), suggesting that Se was being incorporated into the functional proteins of the testis even when the diet was inadequate in Se. There was no apparent effect of dietary Se on the volume of semen, sperm concentration in the strained semen, or in the amount of total sperm produced. Later studies with these boars demonstrated that sperm production was higher when Se was added to the diet (Table 6). The Se content and GSH-Px activity of the semen was higher (P<0.01) when the 0.50 ppm Se diet was fed (Table 3). Most of the Se and GSH-Px activity was located in the sperm cell, not in the plasma of the semen (Table 3).

Sperm production occurred in the young boars at 5.4 months of age (Table 4). The number of spermatids (immature sperm cells) tended to be higher (P<0.10) at 6.2 months of age when dietary Se was fed, but clearly was higher at later ages. Although there was a higher production of sperm at each measurement period when dietary Se was fed, it was not until 9 months of age that the Se effect was clearly evident (Table 4). The Sertoli cells, the cells responsible for nurturing the developing spermatids, were higher in boars at 6.2 months of age (P<0.10) and at maturity (P<0.01) when dietary Se was provided. This suggests that not only the developing testis responded to dietary Se, but that Se affected the cellular components of the testis throughout growth and maturity. Consequently, sperm development was affected by dietary Se during the early stages of testis development.

Sperm motility was initially lower, but continued to decline in a linear manner when the diet was not fortified with Se (Figure 2). In contrast, sperm motility was higher and remained higher showing no decline over the 16 week collection period when the diet was fortified with Se. Although it is typical for the percentage of normal sperm to decline as the boar ages, the decline was greater when the diet was inadequate in Se (Figure 3). It is of interest to note that at the 16 week period the percentage of normal sperm was approximately 3-fold higher when the Se-fortified diet was fed.

The major morphological differences between the sperm of Se-adequate or Se-deficient boars were the percentages of bent (tail bent) and shoehook (coiled tail end) sperm tails (Figure 4). These morphological changes increased for both groups of boars with age, but the greater proportion occurred in Se-deficient boars. When the morphological characteristics of the tail midpiece were examined under electron microscopy, it was evident that the damaged membrane surrounding the tail and the abnormal and oval shaped mitochondria occurred when boars were Sedeficient (Figure 5). Electron microscopy of the tail midpiece when boars were fed Se showed that the membrane had a close apposition to the tail and the mitochondria were more ovoid in shape (Figure 6).

Examining the ATP (energy yielding component of mitochondria) concentration of the sperm demonstrated that the ATP present in the mitochondrial fraction had less activity than the ATP in the sperm of Se-fed boars (Table 5). This indicates that Se-deficient boars produced sperm of poor morphology, that the tail was the principle area of abnormality, and that the sperm utilized energy less effectively.

Semen from the above boars was used to inseminate a group of mature gilts. The results demonstrated that only 73% of the oocytes were fertilized when the diet was deficient in Se, whereas 99% of the oocytes were fertilized in Se-fed boars (Table 6). Fewer sperm reached the oocyte, as evidenced by the lower number of accessory sperm in the zona membrane, when boars had been fed the low Se diet. This confirms that the sperm from the Se-deficient boars had a lower motility, and less energy to reach the egg than boars fed the 0.50 ppm Se diet.

Zinc

Animals deficient in zinc (Zn) have lower growth rates and smaller testicles that are less developed than animals fed adequate Zn. The Leydig cells of the testis are primarily responsible for testosterone production. In Zn deficiency it has been shown that the Leydig cells are abnormal with a concurrent loss of epithelial tissue in the seminiferous tubules in the testis (Hesketh, 1982). High levels of dietary Ca are known to chelate Zn in the intestinal tract and thus reduce its availability for absorption.

Because boars are commonly fed diets higher in Ca than during the other production phases, the diet must be fortified with adequate Zn in order to allow an adequate amount to reach the testis. Subsequent research has demonstrated that dietary Zn at levels of 80 to 150 ppm Zn is adequate to maintain sperm production in breeding boars, but lower levels resulted in lowered semen quality and sperm production (Liao et al., 1985).

Organic minerals

A study conducted in central Mexico in a commercial boar stud farm evaluated several organic mineral BioplexesTM (Alltech Inc.) on semen quality in adult breeding boars. The organic minerals added to the diets were copper (Cu), Zn, managanese (Mn), chromium (Cr), and Se. These mineral sources had fortified the diet for a 1-year period. There were a total of 10 boars used in this study, of varying ages. The results demonstrated that after feeding the added mineral complex for the 1-year period, the average number of doses per ejaculate increased from 10.9 to 23.4 (Figure 7).

Although the effects of organic vs. inorganic minerals on boar reproductive responses are unknown, these results suggest that these supplemental mineral forms dramatically improved semen production and quality.

Summary

Research on minerals for breeding boars is extremely limited. The macromineral requirements, except for Ca and P, would be expected to be similar to those of gilts or barrows when expressed on a body lean basis. The need for microminerals for the development and function of the cells of the testis are unknown. Several microminerals are needed for hormone production, whereas others play a role in the structure of the sperm and cells of the testis. The only microminerals that have been investigated in the boar are Se and Zn, and both have demonstrated functions that are unique for the male. The use of organic minerals in boars may be desirable because of their potentially higher availability and lower interference with other minerals prior to absorption. One study with a combination of organic minerals has implied a benefit in improving semen quality and sperm production.

References

Colenbrader, B. and B. Kemp. 1990. Factors influencing semen quality in pigs. J. Reprod. Fert. 40:105.

Hesketh, J.E. 1982. Effects of dietary zinc deficiency on leydig cell ultrastructure in the boar. J. Comp. Path. 92:239.

Hickman, D.S., D.C. Mahan and J.H. Cline. 1983. Dietary calcium and phosphorus for developing boars. J. Anim. Sci. 56:431.

Kornegay, E.T. and H.R. Thomas. 1981. Phosphorus in swine. II. Influence of dietary calcium and phosphorus levels and growth rate on serum minerals, soundness scores and bone development in barrows, gilts and boars. J. Anim. Sci. 52:1049.

Kornegay, E.T., H.R. Thomas and J.L. Baker. 1981. Phosphorus in swine. IV. Influence of dietary calcium and phosphorus and protein levels on feedlot performance, serum minerals, bone development and soundness in boars. J. Anim. Sci. 57:1182.

Liao, C.W., S.C. Chyr and T.F. Shen. 1985. The effect of dietary zinc content on reproductive performance of the boar. In: Proc. of 3rd AAAP Anim. Sci. Congress, Seoul, Korea Republic 2:613.

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. 75:2994.

Marin-Guzman, J., D.C. Mahan and J.L. Pate. 2000a. Effect of dietary selenium and vitamin E on spermatogenic development in boars. J. Anim. Sci. 78:1537.

Marin-Guzman, J., D.C. Mahan and R. Whitmoyer. 2000b. Effect of dietary selenium and vitamin E on the ultrastructure and ATP concentration of boar spermatozoa, and the efficacy of added sodium selenite in extended semen on sperm motility. J. Anim. Sci. 78:1544.