TECHNICAL ARTICLES Swine

Nutritional factors affecting the reproductive efficiency of the working boar

INTRODUCTION

In recent years natural mating systems dominated the pig industry and it was generally assumed that a normal boar ejaculate contains more than enough sperm to impregnate a sow. This has led to the misconception that boars do not have any special dietary requirements for optimum reproductive performance.

Most boars were fed limited amounts of a gestation diet and very little consideration was given to the effect of nutrition on male reproductive efficiency. These incorrect feeding programs reduced the reproductive efficiency of the breeding herd and resulted in higher than normal culling rates for boars.

With the change in the industry towards artificial insemination (AI) more emphasis was placed on improving the reproductive performance of the boar. AI stations soon realized that they could improve their profitability by increasing the number of additional dose of semen collected from a single ejaculate.

Before the nutritional factors that may influence male reproductive performance can be reviewed, it is important to first look at the formation of sperm (spermatogenesis) and the composition of sperm and it’s associated fluid (seminal plasma).

BOAR SEMEN

SPERMATOGENESIS

Boars usually reach puberty between 4 and 8 months of age, and it can be defined as the first time that fertile spermatozoa (sperm) are detected in a single ejaculate.

Testicular development mainly occurs during adolescence, when the growing boar requires a well balanced diet with optimum levels of amino acids, vitamins and minerals. Well developed testes usually produce sperm of a better quality and quantity. The process of spermatogenesis in the boar is estimated to take about 36 to 45 days. Sperm (spermatozoa) is produced by the seminiferous tubules in the testis from where the still, unfertile, spermatozoa progress to the epididymis. This is the transport duct from the testis to the vas deferens (sperm cord). In the epididymis the spermatozoa are concentrated, stored and maturate (become fertile). This process may take between 5 and 25 days. From the epididymis the sperm move along the vas deferens to the accessory glands and urethra, from were it is expelled as semen during ejaculation.

Boars produce about 15 to 20 million spermatozoa per day, with approximately 200 million per milliliter in a single ejaculate (Bearden & Fuquary 1992).

SPERM

Sperm are highly specialized cells that do not grow or multiply and have no physiological function other than fertilizing the ovum and transferring the genetic potential of the male animal to his offspring.

Sperm consists of two structures; the head and tail. The most important structures in the head are the nucleus, containing the parental hereditary material (DNA) and the acrosome, a cap like structure, covering the anterior part of the cell.

The tail consists of three structures, the midpiece, mainpiece and endpiece. The purpose of these structures is to propel the spermatozoa like a whip lash towards the site of fertilization in the female reproductive tract (Hafez 1922).

During the fertilization process the acrosome enable the spermatozoa to penetrate the cell membrane of the ovum. Upon entering the cytoplasma of the ovum the tail is detached from the head at the midpiece and the nuclei of the spermatozoa and ovum merge to form the new embryo. Heredity deformation of the acrosome and tail will increase the incidence of male infertility.

Norma l spermatozoa of all mammalian species are about 50 to 60 ų long and are very similar in shape. They do not contain the same cytoplasmic characteristics as most other living cells, except for a lipoprotein cell membrane, which in the case of mammalian sperm mainly consists of omega 3 fatty acids.

SEMEN

Semen comprises of spermatozoa and seminal plasma (fluid portion), with the latter being produced by the accessory glands. Seminal plasma contributes 95 to 98% of semen volume, while spermatozoa only contribute about 2 to 5% of the total volume (Bearden & Fuquary 1992).

Seminal plasma is important to convey sperm from the male to the female reproductive tract. It contains the energy sources (fructose and sorbitol) for sperm metabolism and protects the sperm against oxidation. Most of the constituents of the seminal plasma are not present at such high concentrations anywhere else in the body. The synthesis of these organic compounds is directly stimulated by testosterone (male hormone)(Hafez 1922).

The average chemical composition of boar semen is outlined in table 1.

Table 1: Average chemical composition of semen from different species (mg/100 ml)

The latest research indicated that seminal plasma may also be involved in the successful implantation of the embryo in the uterus during in vitro fertilization and embryo transfer. It is speculated that the seminal plasma may contain some substance that suppresses the female immune system, but still allows her the ability to prevent uterine infections (Orsi 2007).

NUTRITIONAL EFFECTS

The nutrient requirements of the working boar are influenced by the following factors: age of the boar, genetic line, health status, frequency of ejaculation and environmental conditions such as housing and ambient temperature (Estienne & Harper; 2004).

From a reproductive prospective is the precise dietary requirement of the boar probably not as important as the potential positive and negative effects of specific nutrients on sperm production (Wilson et al. 2004).

The effect of nutrition on the reproductive performance of the boar can be measured in the following categories; libido, semen characteristics and fertilization ability of sperm (Close and Cole. 2000; Wilson et al. 2004).

Literature on the nutrient requirement for optimal sperm production is already limited and with the significant genetic improvement over the last 20 years, most of this data became inadequate for modern genotypes (Estienne & Haper 2004).

Furthermore, when determining the influence of nutrition on the reproductive performance of the boar it is important to take into account that spermatogenesis, as mentioned earlier, may take up to 35 to 48 days. Any feeding study reporting a pre-experimental period less than six weeks, will therefore not give a true response of the feed effect, but rather a good reflection of pre-trial conditions (Estienne & Harper; 2004).

To complicate maters even more, researchers need to take in consideration when designing experiments, that there is a substantial variation in semen characteristics between boars. As a result, large numbers of experimental boars are required to conduct meaningful experiments with statistically significant detectable differences between treatments.

Nutrient requirements for optimum reproductive performance

The nutrient requirement of the boar can be calculated from the requirement for maintenance, bodyweight gain, semen production, mating activity and heat production (Close and Cole 2000).

Energy

• Maintenance

Although, it is generally recognized that the growing boar has a higher energy requirement for maintenance than females or castrated males, information is limited and usually based on the requirements of mature sows.

Close and Roberts (1991) described the relationship between bodyweight (W, kg) and energy requirement for maintenance (M, MJ DE/day) as;

M = 0.795W 0.665

Therefore, the maintenance energy requirement for a 100 kg and 300 kg boar will be 17 MJ DE/day and 35.3 MJDE/day respectively as indicated in Table 2 (Wilson et al. 2004).

Table 2. Thermoneutral feeding level for boars based on a factorial approach

• Growth

The optimum growth rate for breeding boars is not well defined. According to Marchesi (2004), young boars need to gain 500 to 600 grams per day, while mature boars (250 to 400 kg) only need to gain 200g/day. Wilson et al. (2004) suggested that the bodyweight gain for young boars (150 – 250 kg) should be about 400g/day. From the calculations of Close and Cole (2000) the energy requirement for bodyweight gain decreases from 10.7 to 2.1 MJ DE/day with a reduction in weight gain from 0.5 to 0.1 kg/day.

In the mature boar the nutritional objective should change from feeding for full expression of testis development in the pre-adolescent boar to feeding for full expression of semen production, although nutrition is equally important for both stages (Althouse 1991).

A rule of thumb for mature boars is to maintain a condition score of 3, even though sperm production is only maximized if the boar continues to gain weight throughout its productive life. To the contrary, diets that are formulated to achieve moderate to high bodyweight gain may improve sperm production, but will compromise boar longevity.

On average boars used for both natural mating and semen collection are kept in the breeding herd for much shorter production periods than in the past. With continuous selection for genetic improvement, boars are frequently replaced by animals with a better performance index.

Stud breeders on the other hand may still keep boars for longer periods than commercial producers. If semen production is optimal, these boars may have the same life time sperm production as boars fed for maximum sperm production over a shorter period.

It is therefore important to determine the objective with a specific feeding program; either feed for maximum sperm production or for longevity.

• Mating activity

The energy requirement of the boar for mating activity will depend on bodyweight and frequency of use. Close and Roberts (1991) indicated a value of 18KJ/kg W 0.75 per day for boars mounting a dummy sow. It is, however, very difficult to determine the precise energy requirement for mating activity.

• Semen production

Close and Cole (2000) calculated the energy requirement for semen production based on the assumptions that the boar ejaculates once a day with an approximate semen volume of 250 ml. The energy content of this ejaculate was estimated at 1.04 MJ, and the protein content at 37g/kg. Since the seminal plasma consists mainly of sugars such as fructose, the coefficient of energy utilization for carbohydrates (0.6) was used. This resulted in a daily energy requirement for semen production of 0.45 MJ DE.

The largest proportion of the energy requirement is for maintenance, which contributes between 60 and 90 percent of the total requirement while the requirement for mating behavior and semen production do not contribute more than 0.5 percent.

Tables 3 outline the energy requirements for working boars as cited by Close and Cole (2000) & Marchesi (2004).

Table 3 : Dietary energy requirements for adult working boars (diet based on a DE of 3250 kcal/kg)

These recommendations are based on boars kept at a thermoneutral temperature of 20 0C. Boars, however, are usually kept in pens with poor environmental conditions. In these conditions the feed allowance must be adjusted to prevent the animal from losing body condition. Wilson et al. (2004) recommends an increase in feed allowance of 0.08 kg/boar/day for each 1 0C drop below 20 0C.

Feed allowance

The amount of feed that is offered to the boar will be determined by the age of the boar, genetics, frequency of use, environmental conditions and nutrient density of the diet. In general, overfeeding can cause obesity, leg problems and a reduction in libido, while underfeeding may result in a decline in libido and sperm production (Marcheli 2004, Wilson et al. 2004).

According to the recommendation of Marchesi (2004), young boars starting to work, with a bodyweight of 130 to 160 kg require 2.5 to 2.6 kg feed per boar per day, while the mature boar (290 to 360 kg) only needs 3.0 to 3.4 kg/boar/day. .

Boars used for natural mating are usually culled because they become too heavy. The most obvious method to lower bodyweight is to reduce feed intake. But by reducing feed intake, nutrient intake is also reduced. This decreased intake of some nutrients may have a detrimental effect on libido and semen parameters (Louis et al. 1994). Wilson et al. (2002) reported that boars fed a restricted low nutrient dense diet had a decrease in both semen volume and sperm concentration.

In a study by Stevermer et al. (1961) the level of feed intake had a direct influence on seminal plasma composition. Underfeeding resulted in a drop of 30% and 60% in fructose and citric acid respectively. The amount of ergothioneine in the semen fluid was also lower in underfed boars vs. control boars. Another plasma constituent, inositol, was also directly correlated with feed intake. Inositol is associated with fructose and glucose metabolism, while citric acid may be an indicator of androgenic activity (male behavior). Ergothioneine prevents a decrease in sperm motility due to oxidation.

Feeding intervals and time

Most boars are fed twice a day, although some producers and AI stations still tend to feed only once a day. This practice can cause digestive disturbances in the working boar. According to Marchesi (2004) the stomach may twist due to the huge amount of air that the boar swallow while eating greedily. This air will interact with the gastric acids and result in dilatation of the stomach. In extreme cases this large gassy, twisted stomach can lead to respiratory problems and/or heart failure.

Boars are usually sensitive to any change in feeding pattern, such as changes to the feeding time, feed interval or feed form. If boars are used to consume mash or pellets, it is not advisable to make any sudden change in feed form. Rather make any dietary changes gradually to accustom the boars to the new diet.

Protein and Amino Acids

Published information on the amino acid requirements of the adult boar for optimum reproductive performance is limited and nearly all data is derived from gestating sows.

A protein deficiency in rats and probably all mammals reduces the production of GnRH (gonadotropin releasing hormone) and serum concentrations of LH (luteinizing hormone), FSH (follicle stimulating hormone) and testosterone, consequently affecting semen production adversely (Louis et al; 1994).

Louis et al. (1994) reported that boars fed a low protein diet (7% crude protein) compared to boars on a high protein diet (16% crude protein), took longer to start ejaculating and also remained mounted on the dummy sow for much shorter periods The energy density of these two diets was however, constant. The low protein diet also resulted in a reduction in semen volume per ejaculate, but increases sperm concentration. Total sperm production was consequently lower due to the reduced semen volume.

Louis et al. (1994) also found that boars with a lower protein intake had lower estradiol-17 ß (estrogen) levels. Since boars have much higher levels of circulating estrogen (female hormone than most males of other species, it was demonstrated that the hormone is involved in the maintenance of the boar’s libido and semen volume.

In a second study Louis et al. (1994) confirmed the loss of libido with a decrease in protein intake, but also found that if both protein and energy intake was reduced, more than half of the boars refused to mount the dummy sow and ejaculate. If only energy density was reduced only a quarter of the boars refused to mount, which was considered to be normal.

Boars with a lower protein intake (7.7 g lysine per day) also spent less time on the dummy sow than the boars on the higher protein intake (18.1 g lysine per day). While boars fed protein dense diets, regardless of energy density, had better semen volumes and sperm outputs than boars fed diets lowered in both energy and protein density.

Boars with a higher protein intake had significantly larger epididymides than boars on the lower crude protein diets. This could have contributed to the larger semen volumes and sperm output of the boars on the protein dense diets. Libido was the first limiting factor for reproductive performance for boars on a low plane of nutrition, followed by sperm production. Therefore, loss of libido or sexual aggressiveness can be considered as the first indication that the boar is under nutritional stress.

Both Louis et al. (1994) and Wilson et al. (2002) indicated that protein and amino acid intake are important for libido, semen volume and sperm output. Louis et al. (1994) concluded that energy levels in the diet must be optimized for bodyweight gain and protein and amino acids for optimum reproductive performance.

Close and Roberts (1991) indicated that a protein intake of approximately 260 g/day and an ideal amino acid profile (protein that supply amino acids to the pig in the amounts required for optimum production) similar to that of the pregnant sow was satisfactory to meet the daily protein requirement of the working boar.

Vitamins and minerals

The effect of micronutrients on reproductive performance of boars is not well documented. The feed intake of boars is usually restricted and with the limited knowledge available on micronutrient requirements for optimum sperm production, it may well be that the micro nutrients is unknowingly in short supply.

Audet et al. (2004) reported that folic acid, is the only vitamin that is readily transferred to seminal plasma with a strong positive correlation between blood and seminal plasma concentrations of this vitamin. Folic acid is essential for DNA synthesis and a dietary deficiency will have a negative impact on spermatogenesis.

Marin-Guzman et al. (2000) indicated that selenium is involved in several functions within the testes. Firstly, it has a functional role in sperm development. Secondly, in the developing testis, it is associated, with the amount as well as proper development of sertoli cells. These cells are the nursing cells for the spermatogonium in the seminiferous tubules and influence the number of spermatozoa produced. Lastly, selenium has an important role in glutathione peroxide activity which helps to prevent the sperm against oxidation. A selenium deficiency can therefore, cause a reduction in the concentration of normal sperm as well as decrease in sperm motility. This effect can probably be aggravated by more frequent ejaculations.

Jacyno et al. (2005) also reported a positive effect of organic selenium and vitamin E on sperm morphology. Singleton & Belstra (1997) indicated that Vitamin E has an important role in protecting the sperm cell membrane against oxidation, sperm motility and spermatogenesis. Although the results from the literature are inconsistent, supplemental vitamin C, in a period of severe stress may be beneficial for sperm production. With the supplementation of vitamin C, producers must take into consideration the duration of the process of spermatogenesis and supplementation must already start in spring to get an effect in the hot summer months. This may explain some of the observed inconsistencies in the reviewed data.

Audet et al. (2004) reported that the supplementation of both fat soluble and water soluble vitamins improve the number of sperm per ejaculate and sperm motility during a period of intensive semen collection. However, no improvement in any other semen characteristics or libido was reported.

Marin-Guzman et al. (2000) indicated that selenium is involved in several functions within the testes. Firstly, it has a functional role in sperm development. Secondly, in the developing testis, it is associated, with the amount as well as proper development of sertoli cells. These cells are the nursing cells for the spermatogonium in the seminiferous tubules and influence the number of spermatozoa produced. Lastly, selenium has an important role in glutathione peroxide activity which helps to prevent the sperm against oxidation. A selenium deficiency can therefore, cause a reduction in the concentration of normal sperm as well as decrease in sperm motility. This effect can probably be aggravated by more frequent ejaculations.

Jacyno et al. (2005) also reported a positive effect of organic selenium and vitamin E on sperm morphology. Singleton & Belstra (1997) indicated that Vitamin E has an important role in protecting the sperm cell membrane against oxidation, sperm motility and spermatogenesis. Although the results from the literature are inconsistent, supplemental vitamin C, in a period of severe stress may be beneficial for sperm production. With the supplementation of vitamin C, producers must take into consideration the duration of the process of spermatogenesis and supplementation must already start in spring to get an effect in the hot summer months. This may explain some of the observed inconsistencies in the reviewed data.

Audet et al. (2004) reported that the supplementation of both fat soluble and water soluble vitamins improve the number of sperm per ejaculate and sperm motility during a period of intensive semen collection. However, no improvement in any other semen characteristics or libido was reported.

Some vitamins and minerals may have an indirect effect on sperm production and sperm characteristics but a direct effect on reproductive soundness. Calcium and phosphorus are such important minerals in the boar diet, not from a direct effect on sperm production, but for structural soundness of the boar (Close & Roberts 1991). Optimum bone development will not only enable the boar to effectively mount the sow or dummy, but to remain in this position to deposit an adequate amount of semen for fertilization or AI semen sachet production (Wilson et al. 2004).

Zinc on the other hand, has a dual function, indirectly for foot health (Close & Roberts 1991) and directly for testicular function (Wilson et al. 2004). A zinc deficiency may result in underdevelopment of the Leydig cells. These cells are stimulated by LH to produce testosterone. Underdeveloped cells are less sensitive to LH stimulation, and may cause lower circulating testosterone levels.

Biotin may aid in the prevention of foot problems (Close & Cole 2000).

Fatty Acids

The lipid content of sperm cell membranes is uniquely different across species, with mammalian sperm considered as the richest source of omega 3 in nature. As with humans, pigs also depend on dietary supplementation of omega 3 and a deficiency in the working boar may result in a decrease in reproductive performance (Hazzladine 2006; Wilson et al. 2002).

Supplementation of omega 3, especially DHA (docosahexaenoic acid) may improve sperm production and motility, and reduce the percentage of abnormal sperm (Rooke et al 2001). Wilson et al. (2002) indicated that it is beneficial to the boar under stress conditions and Estienne & Haper (2007) reported that omega 3 supplementation improves the number of AI doses per ejaculate.

It is evident from this data that omega 3 (DHA) supplementation may aid in improving the semen characteristics of the sub fertile boar.

ANTI-NUTRITIONAL PROPERTIES

Certain anti-nutritional factors within feed my adversely influence semen production and characteristics. Mycotoxins such as zearalenone may have detrimental effects on fertility.

High intakes of zearalenone, an estrogenic mycotoxin, may decrease libido in adult boars, while an increased intake of zearalenone in pre-pubertal boars may reduce testis size with a corresponding reduction in semen production in adulthood (Close & Cole 2000), (Young & King 1986).

Pigs are some of the most sensitive animals to vomitoxin or deoxynivalenol (DON) toxicity. This toxin is produced by Fusarium graminearum and form part of the type B trichothecene (Wilson et al. 2004). As indicated by the name this toxin cause the animal to start vomiting and thereby reducing feed intake and performance.

Aflatoxin B 1 can affect semen characteristics and fertility in breeding boars. Boars with very high semen residues of Aflatoxin B 1, had lower sperm numbers, more morphological abnormal sperm and decreased sperm survival rates, with a subsequent decrease in fertility (Close & Cole 2000).

Several studies have shown an improvement in semen quality when the source of mycotoxins were reduced in the diet by either adding mold inhibitors or binding agents to the diet. (Wilson et al. 2004). It is, however, important to determine the mycotoxin status of suspicious raw materials before it is used in any boar diet.

CONCLUSION

Research on the nutrient requirements for optimum reproductive performance of the boar is very limited. Enough evidence is, however, available to support the fact that nutrition has a direct effect on the reproductive efficiency of the working boar.

Nutrient requirements may vary for the different genotypes, environmental conditions, management systems and health status, but with the information available nutritionists can already optimize boar diets for an improvement in libido and semen characteristics.

The feeding cost of a boar diet contributes a very small proportion of the total production cost of a natural mating breeding herd. Since the reproduction performance of several sows in the herd may depend on the performance of a single boar, nutritional deficiencies which may affect the ability of the boar may be costly. AI stations on the other hand can improve their profitability by using proper boar diets .

Future research is essential to improve our understanding of the influence of nutrition on male reproduction. Certain nutrients may be identified or the nutrient requirements predicted more accurately for optimum male reproductive performance.

Over the past three decades there has been a remarkable improvement in reproductive technology. Sophisticated breeding models were first developed, followed by DNA mapping and ultimately, cloning. Despite these scientific advances in animal breeding, sperm is still needed to transfer the genetic superiority of male animal to his progeny . Why do we not then feed the male to optimize his reductive performance?

Date published: 2008-03-12

Author:
G. Pretorious

Publication: N/A