EFFECT OF HIGH ENERGY DIETS ON THE PRODUCTIVE AND REPRODUCTIVE CHARACTERISTICS OF YOUNG BONSMARA BULLS

Transcription

1 EFFECT OF HIGH ENERGY DIETS ON THE PRODUCTIVE AND REPRODUCTIVE CHARACTERISTICS OF YOUNG BONSMARA BULLS by HERMANUS STEFANUS LABUSCHAGNÉ Submitted in partial fulfillment of the requirements for the degree MAGISTER TECHNOLOGIAE: AGRICULTURE to the Department of Agriculture Management George Campus Port Elizabeth Technikon Supervisor : Dr. L.M.J. Schwalbach Co-supervisor : Mr. G.J. Taylor December 2001

2 ii OPGEDRA AAN MY GESIN Riétte, vir die liefdevolle bystand en ondersteuning deur al die jare. Stefan en Henriétta, julle is my inspirasie vir die lewe. My ouers, vir die bystand en al die jare se opofferings.

3 iii PREFACE AND ACKNOWLEDGEMENTS The poor average annual calving percentage of 60% to 65% in the National Beef Herd suggests that there is a lot of room for improvement in the biological and economical efficiency of beef cattle farming in South Africa. The reproductive performance of a beef enterprise may be limited by several factors, of which management and environment are the most important. Most young stud bulls used in breeding herds today may be sub-fertile for several reasons, of which nutritional management at an early age is very important. In order to attain a better understanding of these factors, research in this area in South Africa should be encouraged. This preliminary study was therefore undertaken to attempt to quantify and qualify the influences of feeding high energy diets to young beef bulls during performance testing in South Africa. This dissertation is presented in the form of two articles augmented by a general introduction, a literature review and general conclusion to create a single unit. Care has been taken to avoid unnecessary repetition. However, with the inclusion of the two articles, some repetition was inevitable. The author wishes to express his sincere gratitude and appreciation to the following persons and institutions: Dr. Luis Schwalbach, University of the Free State, for his competent guidance, dedicated support and friendly optimism.

4 iv Mr. Glen Taylor, Port Elizabeth Technikon, for his initial inspiration to undertake this study and for his continual encouragement, guidance and support. Prof. Beukes, Medical School, University of the Orange Free State for the histological analyses. Dr. Piet van Zyl, veterinarian from Ultimo AI Center for the semen collection and evaluation. Mr. Koot Louw from Agrisperm for all his time and assistance in the semen collection process. Mr. Winston, Port Elizabeth Technikon, for his assistance in the statistical analysis. The Agriculture Research Council, Animal Improvement Institute, for their support and for the time given to me to complete this study. To Sernick Bonsmara, especially Mr. Pieter Booysen for his time and assistance in the data collection. To Umpukane Bonsmara, especially Mr. Hans van Rooyen, for making their animals available for this study. The PE Technikon for the opportunity to complete this study. Above all to God, for abilities and the strength to complete this study.

5 v DECLARATION I, HERMANUS STEFANUS LABUSCHAGNÉ, hereby declare that this dissertation submitted by me to the PE Technikon for the degree MAGISTER TECHNOLOGIAE AGRICULTURE, has not previously been submitted for a degree at any other Technikon. I furthermore cede copyright of this dissertation in favour of the PE Technikon... HERMAN S. LABUSCHAGNÈ George December 2001

6 vi TABLE OF CONTENTS Page PREFACE AND ACKNOWLEDGEMENTS DECLARATION LIST OF TABLES LIST OF FIGURES LIST OF PLATES LIST OF ABBREVIATIONS iii v ix xi xii xiii CHAPTER 1. GENERAL INTRODUCTION 1 2. LITERATURE REVIEW INTRODUCTION BASIC ANATOMY, PHYSIOLOGY AND HISTOLOGY OF THE BULL S REPRODUCTIVE SYSTEM Anatomy of the reproductive system of a bull Physiology of the testis Spermatogenesis Basic histology of the testis AGE AT PUBERTY (AP) TESTICULAR SIZE AND DEVELOPMENT Importance of scrotal circumference Heritability of scrotal circumference 13

7 vii Genotype SEMEN QUALITY AND QUANTITY Testicular thermoregulation Shape of the scrotum NUTRITIONAL INFLUENCES ON BULL FERTILITY Macroscopic and microscopic (histological) effect of high 23 energy diets fed to young bulls on their testicular development. 3. REPRODUCTIVE AND PRODUCTIVE CHARACTE-RISTICS OF YOUNG BONSMARA BULLS FED TWO DIETARY ENERGY LEVELS INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS THE EFFECT OF AGE ON PRODUCTIVE AND REPRODUCTIVE CHARACTERISTICS OF YOUNG BONSMARA BULLS FED A HIGH ENERGY DIET INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS 56

8 viii 5. GENERAL CONCLUSIONS AND RECOMMENDATIONS 58 ABSTRACT 60 OPSOMMING 64 REFERENCES 68

9 ix LIST OF TABLES TABLE 2.1 Mean scrotal circumference and heritability estimates of bulls following performance testing (Phase C) in South Africa. Page Classification of sperm mass movement Mean (±SE) age, final body weight (FBW), body condition score (BCS), average daily gain (ADG) and average daily gain for day of age (ADA) of young Bonsmara bulls fed a high- or a medium energy diet. 3.3 Mean (± SE) scrotal circumference (SC), semen volume (Vol), subjective semen concentration (SconS), objective semen concentration (Scon0), mass movement (MV), percentage linear motility (LM), percentage live sperm (LS), percentage major defects (MD), percentage minor defects (MinD) of young Bonsmara bulls fed a high- or a medium energy diet The contents of the high energy diet Classification of sperm mass movement Mean (± SE) slaughtering age, live body weight at the beginning (BLW) and end body weight (FLW), average daily gain (ADG), average daily gain per day of age (ADA), feed conversion ratio (FCR), warm carcass weight (CWW), cold carcass weight (CWC), carcass dressing percentage (CDP), carcass fat grade (CFG) and body condition score (BCS), for older and younger Bonsmara bulls fed a high energy diet. 49

10 x 4.4 Mean (± SE) scrotal circumference (SC), total scrotal weight (TSW), scrotal fat (SF), scrotal skin weight (SSW), paired testes weight (PTW), scrotal skin thickness (SST), total testes circumference (TTC), total testicle volume (TTV), weight of epididymis / spermatic cord (WESC), volume of epididymis / spermatic cord (VESC) and percentage of bulls with depressed spermatogenesis (PBDS) for older and younger bulls fed a high energy diet. 4.5 Mean (± SE) sperm concentration subjective (SconS) and objective (SconO), sperm mass movement (MV), percentage linear motility (LM), percentage live sperm (LS), percentage major defects (MD) and percentage minor defects (MinD) for older and younger bulls fed a high energy diet

11 xi LIST OF FIGURES FIGURE Page 2.1 Three scrotal shapes observed in beef bulls. 20

12 xii LIST OF PLATES PLATES Page 2.1 Excised scrotum of a bull after slaughter Dissected scrotum of a bull Histological view of the testicular parenchyma with active and inactive seminiferous tubules 27

13 xiii LIST OF ABBREVIATIONS ADA ADG AII AP ARC AV BCS BLW BSE CDP CFG CWC CWW DS ESC FBW FCE FCR FLW FSH HE ICSH LH Average daily gain per day of age Average daily gain Animal Improvement Institute Age at puberty Agricultural Research Council Artificial vagina Body condition score Beginning live weight Breeding soundness evaluation Carcass dressing percentage Carcass fat grade Carcass weight cold Carcass weight warm Percentage dead sperm Epididymis and spermatic cord weight Final body weight Feed conversion efficiency Feed conversion ratio Final live weight Follicle-stimulating hormone High energy group Interstitial cell stimulating hormone Luteinizing hormone

14 xiv LM LS LTC MD ME MinD MV OB PBDS PTW RTC SF SC SconO SconS SST SSW TSW TTC TTV VESC VLT Vol VRT WCC Percentage linear motility Percentage live sperm Left testicle circumference Percentage major defects Medium energy group Percentage minor defects Mass movement Older bull group Percentage of bulls with depressed spermatogenesis Paired testes weight Right testicle circumference Scrotal fat Scrotal circumference Objective semen concentration Subjective semen concentration Scrotal skin thickness Scrotal skin weight Total scrotal weight Total testes circumference Total testicle volume Volume of epididymis / spermatic cord Volume of left testicle Semen volume Volume of right testicle Weight of the carcass cold

15 xv WCW WESC WLT WRT YB Weight of the carcass warm Weight of epididymis/spermatic cord Weight of left testicle Weight of right testicle Young bull group

16 1 CHAPTER 1 GENERAL INTRODUCTION In South Africa the annual average calving percentage in beef herds runs between 60% and 65% (Bosman, 1999). This simply means that nearly two cows are being maintained during one year to produce one calf. Ideally, the annual calving percentage in beef herds should be between 90-95%. The reproductive performance may be limited by several factors, of which management and the environment are often the most important. The most important part of the environment and management affecting the productive performance and reproductive characteristics of any livestock species is nutrition. A reasonable amount of research on this topic has been done on the most important factors limiting reproduction and production of beef females worldwide. Limited research has also been carried out in South Africa over the last decade. As a result of this research, breeders often cull cows that do not conceive during a limited breeding season in an attempt to select for higher fertile offspring. However, it is quite possible that sires with below average reproductive potential are being used to breed female offspring that in time will need to be culled, based on their poor reproductive performance. The culling of poor reproductive females, although repeated every generation, will not, on its own solve the general low fertility rate of the South Africa s National Beef Herd. This can only have a small impact in improving the reproductive potential of a herd, while sires with below average reproductive potential are being used (Brinks, 1994). Very little is known regarding the factors limiting bull fertility under farming conditions in South Africa. Some stud breeders, commercial stock farmers and livestock

17 2 specialists (veterinarians and animal scientists) are complaining about the poor reproductive efficiency of the bulls available for sale in the industry. The people involved in semen collection and preservation (freezing), are also facing difficulties regarding the quality and the freeze ability of semen collected from bulls conditioned for sales and/or shows. They all blame the intensive feeding and the fat body condition of the bulls for these problems. There is some scientific evidence that indicates that high energy fed bulls have a low reproductive efficiency (Coulter, 1994). In order to evaluate these perceptions, adequate scientific studies must be done on bulls under local conditions and practices. If there are negative effects on bull fertility emanating from intensive feeding periods, it is important to evaluate and quantify the extension and the duration of these negative effects. If these perceptions are correct, the beef industry will have to review the current practices on bull conditioning, so that these practices do not impair the bull s fertility. Stud farmers in South Africa have long ago realized that fat bulls fetch better prices at sales than leaner bulls. The market place, whether it is on the farm or at a show-sale, is regulated by supply and demand. If the commercial cattle farmers and stud breeders pay a premium price for these obese breeding bulls, the stud breeder will continue to provide, these fat bulls that need to be intensively fed. As part of the National Animal Improvement Scheme, young pedigree beef bulls (weaners) are submitted to an intensive feeding test period of 84 days at a centralized testing center. During this period the feed conversion ratio, as well as the growth rate of these young bulls are evaluated on a standard high dietary energy diet. Animals that pass this procedure receive a merit award, according to their performance and are then used for breeding purposes. In general, the better the

18 3 performance (based on average daily gain and feed conversion ratio), the higher the value of the animal in the market place. This test is performed at a pre-pubertal age and the young bulls have the tendency to deposit fat. Are there any negative consequences for its future reproductive life? The aim of this study is to evaluate the effect of intensive feeding of young bulls on their subsequent potential fertility.

19 4 CHAPTER 2 LITERATURE REVIEW 2.1 INTRODUCTION This literature review intends to cover basic reproductive aspects on bull fertility. Aspects such as the occurrence of puberty in the bull, basic anatomy, physiology and histology of the bull s reproductive system as well as the most important reproductive parameters, characteristics used to evaluate potential fertility and the most important factors affecting bull fertility will be discussed. Particular attention will be given to the effect of nutrition (mainly energy) on bull fertility and sexual activity. 2.2 BASIC ANATOMY, PHYSIOLOGY AND HISTOLOGY OF THE BULL S REPRODUCTIVE SYSTEM Anatomy of the reproductive system of a bull The reproductive system of the bull like all other mammalian farm animals consist of two testes contained in the scrotum outside of the body cavity, accessory organs including ducts, glands and the penis. The testes produce spermatozoa (the male sex cells, also named sperm) and testosterone (the most important male sex hormone). The scrotum provides protection and a favorable environment for optimum spermatogenesis (± 2-4 C lower than body temperature). The remaining structures assist the spermatozoa to reach the ovum in a condition conducive to its fertilization. These structures include the epididymis and the ductus deferens in each

20 5 testis, the ampulae and the accessory sex glands (the vesicular glands, or Cowper s gland, the prostate, and the bulbourethral gland), the urethra and the penis. The accessory sex glands produce the greater part (seminal plasma) of the ejaculate, or semen, which serves as a transport media for the sperm, as well as providing nutrition and acting as a buffer, neutralizing excess acidity in the female genital tract (Frandson, 1986a). The testis derives its blood supply from the testicular artery, which branches directly from the aorta, a short distance behind the renal artery on the same side. This testicular artery lengthens as the testis descends into the scrotum. The testicular vein parallels the course of the artery, except that in the vicinity of the testis it is more tortuous and convoluted. This mass of veins just above the testis is called the pampiniform plexus and plays an important role in the thermoregulation of the testis (Frandson, 1986a) Physiology of the testis The testis can be considered as both an endocrine (glands which empty their secretory products directly into the blood stream) and an exocrine gland (glands that empty their secretory products on an epithelial surface, usually by means of ducts). The endocrine function of the testes consist mainly of the production of testosterone, the most important male sex hormone, from the interstitial cells (also called the cells of Leydig). Hormones, such as testosterone, with its masculinizing effect are known as androgens. The testes are the main source of androgens, but small amounts are also produced by the adrenal cortex, the female ovaries, and the placenta (of

21 6 pregnant females). Lack of libido (sex drive) and inability to produce offspring are two of the most obvious results of castration (removal of the testis) due to a lack of testosterone and sperm cells. Testosterone promotes the development and function of the accessory glands, resulting in the development of secondary sex characteristics (body conformation and shape, as well as muscling characteristics), and controls the secretion of LH (luteinizing hormone) and ICSH (interstitial cell stimulating hormone) in the male. Testosterone promotes protein anabolism, resulting in increased body size in the male as compared to the female. The bone structure also responds to testosterone and therefore the bones of bulls become larger and thicker than in the female or ox (Frandson, 1986b). Spermatogenesis is initiated by FSH (follicle stimulating hormone) from the adenohypophysis of the pituitary gland, but testosterone is necessary for completing the process. The hypophyseal gonadotrophins directly control germ-cell mitosis and meiosis and indirectly control maturation of spermatids (spermatogenesis). LH stimulates the interstitial cells (Leydig cells) of the testes to produce testosterone. The testosterone production is controlled by a feedback mechanism to inhibit further production of LH. FSH is necessary for the final maturation of spermatids. LH controls the secretion of testosterone; and prolactin enhances LH in maintaining testosterone production. The testes produce an appreciable amount of oestrogen (female sex hormone), presumably by the Sertoli cells in the seminiferous tubules. The function of oestrogen in the male is obscure, but it may act to inhibit FSH secretion from the adenohypophysis (Frandson, 1986b).

22 7 The accessory sex glands function only in the production of seminal fluid, which is not absolutely essential for fertilization as long as the ph of the female genital tract is not too acidic. Functional activity of the accessory sex glands depends on testicular androgens. Smooth muscle fibers in the accessory glands empty the secretion at the time of ejaculation, in response to the autonomic nerves, including the parasympathetic pelvic nerve and the sympathetic hypogastric nerve (Frandson, 1986b). The seminal fluid provides nutrition, buffer and extending (dilution) properties to the semen, which are important for sperm cell survival and motility in the female s genital tract Spermatogenesis The germinal epithelium, primary cells of the male sex cells, makes up the periphery of the seminiferous tubules. These primary sex cells are constantly dividing, and as new cells form, potential sperm cells migrate towards the lumen of the tubules, develop tails, and become spermatozoa. Spermatogenesis is the process by which primary sex cells in the testis produce sperm. During meiosis in spermatogenesis, the number of chromosomes is reduced to half the number normally found in the somatic cells of a species. Chromosomes are the structures in the nucleus of each cell that are basically composed of genes, the hereditary determiners, which are transmitted from one generation to the next. Spermatogenesis as such thus involves a series of steps in the formation of sperm. Spermatogonia, generalized cells at the periphery of seminiferous tubules, increase in number by mitosis, a type of cell division in which the daughter cells are nearly identical with the parent cell (same number of chromosomes). Primary

23 8 spermatocytes, produced by Spermatogonia, migrate towards the center of the tubule and undergo meiotic division, in which the chromosomes from each pair goes to each of the two secondary spermatocytes. Thus, the chromosome number is halved in the secondary spermatocytes. The two secondary spermatocytes formed from each primary spermatocyte divide by mitosis to form four spermatids. Each spermatid undergoes a series of nuclear and cytoplasmic changes (spermiogenesis) and differentiates from a non-motile cell to a potentially highly motile cell (cell capable of movement) by developing a flagellum (tail) to form a spermatozoon. Sperm are the germ cells, which, after maturing while passing through the epididymis, are capable of fertilizing an ovum following capacitation in the female tract. Sperm become actively motile when exposed to the fluids secreted by the accessory glands. However, many of the sperm degenerate and are absorbed by the epithelial cells of the epididymis and ductus deferens, and many sperm are excreted in the urine. Of the four spermatozoa developed from each primary spermatocyte, two contain the Y chromosome to produce male offspring (XY), and two contain the X chromosome to produce female offspring (XX) when united with the ovum (which contains only the X chromosome). The acrosome system (acrosome and head cap) of the sperm is derived from an acrosomic vesicle formed within the Golgi complex of the spermatid. The vesicle becomes flattened and forms the head cap. The Sertoli cells also called the sustentacular cells, or nurse cells, are found scattered among the germ cells within the seminiferous tubules. These Sertoli cells apparently supply nutrition to the maturing spermatids and possibly transport adrenogens to the germinal cells. The movement of spermatozoa through the male reproductive tract is mostly passive, because motility of spermatozoa usually becomes apparent only after

24 9 being exposed to the secretions of the accessory sex glands at the time of ejaculation. A sperm can move through the epididymis in about 8 to 11 days in the bull. Most spermatozoa are stored in the tail of the epididymis where they may survive for an unknown period of time, up to weeks or months in the immobile state (Frandson, 1986b). The sperm production process is a continuous process that requires 45 to 60 days from spermatogenesis to ejaculation (Elmore, 1985) Basic histology of the testis Each testis consists of a mass of seminiferous tubules surrounded by a heavy fibrous capsule called the tunica albuginea. A number of fibrous septa, or trabecula, pass inward from the tunica albuginea to form a framework, or stroma, for support of the seminiferous tubules. These trabeculae unite to form a fibrous cord, the mediastinum testis. The rete testis consists of anastomosed canals within the mediastinum testis. These canals are interposed between the seminiferous tubules and the ductuli efferentes that join the ductus epididymides in the head of the epididymis. The Leydig cells, which secrete the male hormone, testosterone, are located in the connective tissue in between the seminiferous tubules (Frandson, 1986b). 2.3 AGE AT PUBERTY (AP) Puberty in bulls has been defined as the age at which the first ejaculate containing a minimum of 50 x 10 6 /ml total spermatozoa with at least 10% progressive motility is produced (Lunstra et al., 1978). According to Wolf et al. (1965), bulls also need to manifest libido and have adequate penile development to permit normal ejaculation. Functionally, puberty is the earliest age at which males can impregnate females or

25 10 serve the artificial vagina. Lindner- (1960) and Mann et al., (1967) reported that the time at which it first becomes possible to collect a bull ejaculate coincides approximately with the age at which androgens begin to be secreted in greater quantities by the testes and when the fructose content in the seminal vesicles rise rapidly. In a study, Skinner (1981) found the time of the first appearance of spermatozoa in the ejaculate of bulls to be at 38 to 42 weeks of age. It was found that both the total number and the concentration of sperm in the ejaculate increased steadily as age advanced. In some bulls the first spermatozoa are immotile, but both motility and concentration increased with age. Lunstra and Echternkamp (1982) stated scrotal circumference (SC) to be a more accurate predictor of when a bull reaches puberty, regardless of breed, compared to either age or body weight. Bulls reached puberty (50 x 10 6 /ml sperm with a minimum of 10% motility) at an average SC of 27.9 cm. The testicular development in young beef bulls and the onset of puberty is also largely determined by genetics (Coulter, 1994). Pruitt and Corah (1986) reported that higher levels of dietary energy did not hasten neither the age at puberty nor the age at first mating. This is in contrast to Bratton et al. (1959), who reported that high levels of dietary energy hastened, while low levels of energy delayed, the age at the appearance of the first motile sperm production in Holstein bulls. Because higher levels of energy increase weight gains without reducing age at first mating and puberty, weight at first mating and puberty increased as the level of energy in the diet increased. Even though increasing body weight by feeding higher levels of energy, did not hasten puberty, when corrected for breed and energy level. Bulls that were heavier at 1 year of age were younger at puberty (r= -0.37, P<0.01) and first mating (r= -0.26, P=0.07). In general Bos taurus bulls reach puberty earlier than Bos indicus ones (Pruitt and Corah, 1986). Nolan et al. (1990) demonstrated

26 11 that the same endocrine changes occurring in Bos taurus bulls also occur in Bos indicus bulls. These changes, however, were delayed due to the older age at puberty in the Bos indicus bull, but the changes were similar. Additional dietary energy enhanced the onset of puberty primarily via enhanced testicular function, as measured by serum testosterone, testicular testosterone, Leydig cell size and sperm production. In South Africa bull calves enter the post weaning intensive performance test (Phase C) between the age of 21 and 38 weeks of age. The test period is 16 weeks (4 weeks for adaptation and 12 weeks of testing) and puberty occurs in approximately this period. Wolf et al. (1965) observed that the separation between the penis and the sheath was complete at 9 to 10 months of age. The very high correlation between SC and AP indicate that age at puberty may be estimated using SC, a much easier and practical measurement. Toelle and Robinson (1985) reported that SC was also genetically favorable related to several reproductive traits. Lunstra and Echternkamp (1982), reported a correlation of r=0.98 (negative correlation) between breed means (8 breeds) for SC of bulls, with age at puberty in related heifers (daughters and grand-daughters). 2.4 TESTICULAR SIZE AND DEVELOPMENT Importance of scrotal circumference Scrotal circumference is an important indicator when examining beef bulls for breeding soundness. It is favorably correlated to testes mass, sperm production, semen quality, age at puberty, body weight and age in young bulls (Swanepoel and

27 12 Heyns, 1990). Bulls with small testicles have reduced sperm production and poor semen quality, these bulls have also a decreased proportion of functional seminiferous tubules, reduced sperm output, and an elevated percentage of morphologically abnormal sperm (Cates 1975; Coulter and Foote, 1979). Studies conducted by Cates (1975) on 1944 beef bulls of various ages and breeds demonstrated that the probability of a beef bull having satisfactory semen quality, based on the standards of the Society for Theriogenology, increased dramatically as the scrotal circumference increased from 30 to 38cm. Only 23% of 155 bulls with a scrotal circumference of 32 cm were classified as having satisfactory fertility. In contrast, 88% of 136 bulls with a scrotal circumference of 38 cm were classified as satisfactory fertile. Semen quality improved very little in bulls with a scrotal circumference measurement greater than 38 cm when compared to bulls with a 38cm SC (Coulter, 1994). Thompson et al. (1992) reported that SC was not an accurate predictor of sperm morphology or motility when a SC of 32 cm was used to predict the recommended minimal standards for semen quality. In addition, there was no significant linear relationship between SC and either the degree of germinal epithelial loss or the percentage of Grade 4+ seminiferous tubules in the bulls completing the performance test. Knights et al. (1984) reported a favorable genetic relationship of SC with measures of semen quality and quantity. In general, as SC increases in yearling bulls, mass motility, percentage normal sperm, semen volume, sperm concentration and total sperm output increased while the percentage of sperm abnormalities decreased. The genetic and phenotypic correlations between SC and body growth reported parameters in the literature are generally favorable. The genetic correlation between

28 13 SC and birth weight appears to be relatively low (r= 0.08), whereas the correlation between SC and yearling weight is relatively high, r= 0.63 (Smith et al., 1989). This suggests that selection for larger SC (early puberty) and faster growth rate is compatible in young bulls. Selection for increased SC should simultaneously result in increased growth from birth to yearling ages, while holding birth weights relatively constant (Brinks, 1994). Hoogenboezem (1995) reported that bulls with inferior testicular development at a young age showed an increase in scrotal circumference with both age and body weight, but those bulls with superior development at a relatively young age maintained that advantage throughout life. Therefore, the probability of finding bulls with smaller than average testes among bulls selected for weaning weight would be smaller than in bulls selected for growth rate in a feedlot (Hoogenboezem, 1995). This conclusion is also supported by the results of Swanepoel and Heyns (1986) in Simmentaler bulls. Scrotal circumference is an easy parameter to measure, it is highly repeatable and the heritability estimates range from 0.26 (Meyer et. al., 1990) to 0.78 (Heyns, 1987). It is favorably related to semen characteristics and is a good predictor of age at puberty in bulls and its related females. Selection for scrotal circumference will be useful not only because it is compatible with selection for growth, but also because improved fertility will result in increase productivity in both male and female offspring (Brinks, 1994) Heritability of scrotal circumference Since nearly 90% of the genetic change in a beef herd, over time results from the sire s genetic contribution, heritable reproductive characteristics of bulls need to be

29 14 identified and used to genetically improve the reproductive potential of both male and female offspring (Brinks, 1994). There are several reports in the literature regarding the heritability of SC in yearling beef bulls on both an age and weight adjusted basis. These studies indicate that SC in yearling bulls is a moderate to highly heritable trait (h²>0.40) and that selection should be very effective in improving SC (Brinks, 1994). The heritability of SC in bulls that were performance tested in South Africa under the centralized intensive feeding phase (Phase C of performance testing-arc; 2001, Unpublished data) show similar values as found in the literature (Table 2.1). Table 2.1 Mean scrotal circumference and heritability estimates of bulls following performance testing (Phase C) in South Africa Breed Average h² Afrikaner Beefmaster Bonsmara Limousin Drakensberger Pinzgauer S.A. Angus Simbra South Devon Source; ARC AII, 2001, Unpublished Genotype The vast majority of reports in scientific literature on bull fertility quote studies involving Bos taurus breeds. However, it is important to be careful when extrapolating from Bos taurus to Bos indicus genotypes, as breed differences may be

30 15 significant. Genotypic and phenotypic differences exist for scrotal circumference between Bos taurus and Bos indicus cattle types. Entwistle (1980) showed that differences exist in the physical structure of the Bos indicus testes regarding length and width, when compared to that of Bos taurus. The length: width ratio of 2.0 for Bos indicus is significantly higher than the 1.8 of Bos taurus. In the phase C test of the South African Beef Cattle Improvement Scheme, the minimum recommended SC for all breeds is 28.0 cm at the end of the test (33-50 weeks old). However, for Brahman and Tuli breeds the recommended minimum is 24.0 cm. The average SC for the different breeds tested under phase C of the South African Beef Cattle Improvement Scheme are presented in Table 2.1. The age at measurement of SC should taken into consideration due to differences in the sexual maturity rates of various breeds of cattle. Swanepoel and Heyns (1986) reported breed differences in the rate of increase in the SC. Bos taurus breeds (Hereford and Simmentaler) have a relatively low rate of increase in SC, when compared to the Bos indicus and/or Zebu breeds (Afrikaner and Nelore). The Bonsmara follows the Bos indicus pattern with a relatively high rate of increase in SC, while the Santa Gertrudis follows the Bos taurus trend (Hoogenboezem, 1995). These differences may be due to the genetic composition of the two breeds (Bonsmara: Bos indicus x Bos taurus and Santa Gertrudis: Bos indicus x Bos taurus). Breed differences in SC tend to be small, except when breeds differ widely in weight or age at puberty (Lunstra et al., 1978). These breed differences may also be maintained at older ages (Entwistle, 1980). It must be recognized that Continental breed bulls have a different body composition compared to the British breeds. It is commonly accepted that Continental breeds deposit backfat later and to a lesser degree than the British breeds. However, many Continental bull breeds have the tendency to accumulate excess fat deposits in the

31 16 neck of the scrotum, which are likely to impair the thermoregulation of the testes. Therefore, although backfat is not excessive, optimum reproductive capacity still appears to be compromised by scrotal fat deposition (Coulter, 1994). Furthermore, it seems as if British breeds are more susceptible to the deleterious effects of high energy diets than Continental x British crossed bulls and should be fed only moderate energy diets from weaning to minimize the negative effects on their reproductive capacity (Coulter and Foote, 1997). No reports are available in the literature on tropical breeds under tropical and subtropical conditions. Very little is known about the effect of nutrition on the fertility of South African bull breeds under the local conditions. 2.5 SEMEN QUALITY AND QUANTITY In broad terms, semen quality refers to the viability and structure of spermatozoa as determined by progressive sperm motility and morphology. Semen quality in the bull is an accurate reflection of the functional status of the sperm producing epithelium of the seminiferous tubules in the testes, the maturation, transportation and storage functions of the epididymis. The quality of semen produced by an individual bull is the product of the bull s inherent genetic make up, less any detrimental influences induced by the environment in which the bull was reared and maintained (Coulter, 1994). Semen quality in young beef bulls has been shown to improve for up to 16 weeks following puberty (Lunstra and Echternkamp, 1982). Therefore, considerable caution must be exercised when assessing the semen quality of young post puberal beef bulls (Coulter, 1994). The same author reported that some sperm abnormalities e.g., knobbed acrosome defect and the dag defect are known to be inherited as

32 17 recessive traits. Other abnormalities are speculated to be the result of genetic deficiencies, but specific modes of inheritance have not yet been identified (Barth and Oko, 1989). Heritability estimates for semen quality traits such as progressive sperm motility and the proportion of spermatozoa with primary and secondary abnormalities in young beef bulls have generally been inconsistent and low to moderate in value (range to 0.44) (Coulter, 1994). In a study conducted by Kastelic et al. (1996), insulation of the scrotal neck (simulating fat deposited within the scrotal neck) resulted in a significant decrease in morphologically normal sperm in bulls. This seems to be the result of impaired thermoregulation. It would seem as if the thermoregulatory mechanism maintaining the testis at an ideal temperature may have been tampered with, or affected by the increased scrotal insulation (either artificially applied or naturally, due to increased fat deposits), resulting in decreased semen quality. Skinner (1981) also found that both the total number and the concentration of spermatozoa in the ejaculate increased steadily with age. In some bulls the first spermatozoa at puberty were immotile, but both motility and concentration increased with age. It was also found that in bulls fed a high energy diet, the sperm motility actually declined and the number of abnormal spermatozoa increased, as the animal grew older, although these differences were not significant until 76 weeks of age. It was also reported that the abnormal spermatozoa produced by the high energy fed bulls were similar to those observed during heat stress of short duration. In this study the nutritional planes were reversed at 104 weeks of age. This trend towards increased sperm abnormalities nevertheless continued, indicating probable irreversible damage to the testis being induced.

33 Testicular thermoregulation Thermoregulation of the testes may be the most important single factor influencing the spermatogenetic process and the resultant quality of semen produced by the bull (Coulter, 1994). The scrotum as well as the cremaster muscles plays an important function in the thermoregulation of the testis. The testis must be 2-4 C below body temperature for optimal spermatogenesis (Marcus, et al., 1996). Infra-red thermography is a non-invasive technique that provides a pictorial image of an object s infra-red emissions (radiated heat energy), allowing a good estimation of an object s surface temperature to an accuracy of 0.18ºC (Coulter, 1994). According to the same author thermography has been used to diagnose scrotal/testicular pathology, particularly varicocele in man and inflammation in bull testes. Using the ram as a model, Coulter (1988) confirmed that scrotal surface temperature as measured by thermography was positively and significantly correlated (P< 0.001) with both subcutaneous (r= 0.95) and surrogate testicular temperature (r= 0.91). The temperature differential between scrotal surface (30.11±0.2ºC) and deep testicular temperature (34.88±0.2ºC) was 4.77ºC in the ram at an ambient temperature of 25.61ºC. In studies examining the relationship between scrotal surface temperature (as measured by infra-red thermography) and semen quality traits, bulls with scrotal surface temperature gradients (Coulter, 1994) of between 4.0 and 6.0ºC from the base to the apex of the scrotum had a lower incidence of primary sperm defects compared to bulls with gradients of less than 4ºC. Bulls with gradients greater than 6ºC did not differ significantly. The exact mechanism by which the feeding of high energy diets affect semen production and quality is not clearly known. However, circumstantial evidence indicates the probable involvement of impaired thermoregulation and possibly a stress induced hormonal imbalance. It appears that

34 19 decreased fertility is due to fat deposition both within the scrotal tissue overlying the testes / epididymis and fat deposition in the neck of the scrotum over the pampiniform plexus (Coulter, 1994). This is most likely the reason why a cryptorchid animal with both testes retained in the abdominal cavity is likely to be sterile. Spermatogenesis does not occur normally unless the testes are 3 to 4ºC cooler than body temperature, a condition provided by the scrotum (Marcus et al., 1996). The same author also found, in a case report on a cryptorchid bull, that the higher temperature in the retained testicle is the main factor responsible for impaired semen quality, since the elevated testicular metabolic rate changes its normal physiology. However, the relatively high temperature of the abdomen does not interfere with the production of testosterone, so the cryptorchid animal has the behavior and appearance of a normal male, except that no testes are evident and no normal spermatozoa are produced (Frandson, 1986b). Skinner (1981) noted that body-testicular temperature differences increase with age, probably as the scrotum becomes more pendulous.

35 20 A B C Figure 2.1 Three scrotal shapes observed in beef bulls are the straight-sided scrotum (A), the normal scrotum (B), and the wedge-shaped scrotum (C). Adapted from Cates (1975) Shape of the scrotum Scrotal shape in the beef bull is known to influence testicular development and function. As scrotal shape is a conformational trait, it would be expected to have a high heritability. Basic scrotal shapes have been recognized in the beef bull as: normal or bottle-shaped, straight-sided, and wedge-shaped scrotum Coulter (1994). Bulls having a normal scrotum with a distinct neck (Figure 2.1, Bull B) generally have the best testicular development and function. This scrotal configuration permits optimal thermoregulation of the testes under a wide range of ambient temperatures. Often bulls with straight-sided scrotum (Figure 2.1, Bull A) have only moderate testicular size. The straight-sided neck of the scrotum is generally the result of fat deposits in this area that may impair testicular thermoregulation. Occasionally, young bulls having this shape scrotum may lose body condition and approach a more normal scrotal configuration as they mature and

36 21 the scrotum becomes more pendulous. Wedge-shaped scrotums (Figure 2.1, Bull C) are pointed toward the apex of the scrotum and tend to hold the testes close to the body wall. Bulls with this scrotal configuration have small testes and seldom produce semen of satisfactory quality (Coulter, 1994). 2.6 NUTRITIONAL INFLUENCES ON BULL FERTILITY Energy is probably the most important nutritional consideration in beef cattle production, provided that adequate levels of protein are ingested. Animals require energy to grow and to keep the body functioning. Carbohydrates and fats are the primary sources of energy in the diet. One factor that may have significant and long term effects on bull fertility is nutrition. Diets adequate in protein, vitamins, minerals and energy appear to hasten the onset of puberty in beef bulls (Abdel- Raouf, 1960). However the feeding of high energy diets to post-puberal beef bulls is of no benefit to their reproductive capacity including semen quality and may substantially impede the reproductive potential. Skinner (1981) fed two groups of Hereford bulls from 12 weeks of age the same diet containing 14.5% crude protein and 74.9% digestible energy. In one group the feed was given ad libitum while in the other group feed was restricted. From 104 to 130 weeks of age the nutritional levels were reversed to see if the effect of fattening could be altered. It was evident that fertility of the bulls on the high plane of nutrition was detrimentally affected. Indications were that it might be extremely difficult for a bull to get rid of residual fat once it has been laid down in the scrotum. A medium-level energy diet fed to Hereford and Angus bulls from weaning at 6 to 7 months of age, until 15 months of age resulted in 52% greater total epididymal sperm reserves than in bulls fed a high energy diet (Coulter, 1988).

37 22 Similarly Hereford and Angus bulls fed a medium energy diet from weaning to 15 months of age, when bulls might be used for breeding as yearlings, had a 12% greater daily sperm production per gram of testicular parenchyma, than bulls fed on a high energy diet. Bulls in the medium energy diet groups at 15 months of age had 76% greater caput-corpus epididymal sperm reserves in the first year, and 89% greater caput-corpus epididymal sperm reserves in the second year, and 52% greater cauda epididymal sperm reserves compared to high energy diet bulls (Coulter et al., 1987). Coulter (1994) suggested that although the feeding of high versus medium levels of dietary energy to young beef bulls had no significant effect on the amount of lipid present in the testicular parenchyma. Bulls fed a high energy diet, as described by Coulter and Kozub (1989), had 34% more total scrotal lipids than bulls fed the medium energy diet (13.7±0.1 vs. 10.2±0.7 mg lipid/g scrotal tissue respectively). The correlation coefficient between total scrotal lipid and epididymal sperm reserves was 0.26 (n=55). The corresponding coefficient (r) between backfat thickness and epididymal sperm reserves was (n= 55). Highly insulative lipids are deposited within the scrotal tissue and may reduce the radiation of heat from the scrotal surface, thereby increasing testicular temperature. This may interfere with the thermoregulation of the testis and reduce sperm production and in some cases semen quality. Coulter (1994) noted that yearling beef bulls fed on a high versus moderate energy diets have a higher average scrotal surface temperature (28.6 vs. 28.0ºC respectively) as measured by infra-red thermography. In most cases, regardless of age, bulls fed high energy diets had substantially reduced reproductive potential, compared to bulls fed medium energy diets (Coulter, 1994).

38 Macroscopic and microscopic (histological) effect of high energy diets fed to young bulls on their testicular development. Skinner (1981), compared different parameters like body weight, the weight of the testes and the epididymis, the extra-gonadal sperm reserves, inguinal and testicular fat covering and content, testicular weight as a percentage of body weight for different growth planes in bulls but little differences were found. It was reported that no apparent differences could be seen in the histology and histochemistry of the testes in each nutritional group. The amount of fat covering the testes and spermatic cord were, however, significantly greater in the bulls on a high plain of nutrition from 52 weeks of age. When the nutritional planes were reversed at 104 weeks of age, no reduction of inguinal fat content to any great extent was observed. In addition, the testes weight when expressed as a percentage of body weight, were lower in the high plane fed bulls. Histological observations of the spermatic cord at 76 weeks of age showed a large increase in fat deposition in the high energy fed group, between the vessels of the pampiniform plexus. Marcus et al. (1996) reported following a histological examination that no sperm were found in the retained testicle of a cryptorchid bull. The Sertoli cells showed fat degeneration and fibrotic tissue surrounded the tubulli seminiferi. There was severe atrophy which laced most of the layers of the reproductive cells involved in spermatogenesis. The remaining cells observed were most probably the lining cells of the seminiferous tubules. Groups of interstitial cells responsible for testosterone synthesis were present and some of these were atrophied. The cauda epididymis was formed by ducts lined by normal epithelial cells but devoid of spermatozoa. The ducts were surrounded and separated by a large amount of fibrous tissue. In a study conducted at the Meat Animal Research Center (MARC), Clay Center, Nebraska (Coulter, 1998) it was

39 24 found that pregnancy rate was lower in cows mated by bulls with abnormal scrotal temperatures (Coulter, 1998). It was suggested by Coulter that excess fat deposition in the neck of the scrotum, leads to reduced bull fertility. Skinner (1981) suggested that it might be extremely difficult for a bull to get rid of residual fat once it has been accumulated in the neck of the scrotum. This suggests that once the fat is deposited in the neck of the scrotum, during a short period of high energy intake in bulls, its effects on fertility can be long term.

40 25 Plate 2.1 Excised scrotum of a bull after slaughter Fat deposition Testicles Epididymis

41 26 Plate 2.2 Dissected scrotum of a bull Scrotum Fat Testicles Pampiniform Plexus Head of the epididymis Body of the epididymis Tail of the epididymis

42 27 Plate 2.3 Histological view of the testicular parenchyma with active (a) and inactive (b) seminiferous tubules.

43 28 CHAPTER 3 REPRODUCTIVE AND PRODUCTIVE CHARACTERISTICS OF YOUNG BONSMARA BULLS FED TWO DIETARY ENERGY LEVELS 3.1 INTRODUCTION The average annual calving percentage of 60% to 65% in the national beef herd in South Africa (Bosman, 1999) suggests that there is a lot of room for improvement and an ideal figure could be between 90% and 95%. The reproductive performance of a cow calf operation may be limited by several factors, of which management and the environment are often the most important. Reproductive performance has a greater impact on economic returns than either growth rate or product quality in beef farming (Trenkle and Wilham, 1977). In most cow calf operations, females that do not conceive during a limited breeding season are often culled in an attempt to select for highly fertile offspring. It is however possible that sires with below average reproductive potential are being used to breed female offspring that in time will need to be culled due to their poor reproductive performance. The culling of females with poor reproductive performance, although repeated every generation, will not, on its own solve the general low fertility rate of the South African s National Beef Herd, as long as sires with below average reproductive potential are not being used for breeding (Brinks, 1994). According to Chenoweth (1981), approximately 30% of bulls used for breeding in the beef industry have reproductive problems and it appears that the variation in the reproductive potential of beef bulls is vast (Coulter, 1994). Most beef cattle breeders in South Africa have little or no information on the reproductive status of their bulls, particularly their yearlings. In many cases, bulls are not

44 29 assessed for fertility prior to sale or use (Godfrey and Lunstra, 1989). Reproductive efficiency of both bulls and females contribute to the expressed reproductive performance of the beef herd and the use of sub-fertile bulls can decrease the fertility of the herd (Brinks, 1994). Scrotal circumference (SC) is a trait frequently used as a predictor of bull fertility. SC also provides a good indication of puberty in young bulls and moderate, but favourable correlations have been found between SC and semen quality. Studies reviewed by Coulter and Foote (1979) and Brinks (1989) suggest that SC measurements in bulls are of value for the prediction of potential sperm production and breeding soundness. In addition, SC has a moderate to high heritability. This is important since heritability estimates of semen traits are generally low (Pearson et al., 1984; Smith et al., 1989). High genetic correlations were recorded between SC in the bull and age at puberty in half-sibling heifers (Brinks et al., 1978; King et al., 1983; Toelle and Robinson, 1985). Scrotal circumference is also an important measurement, which makes up 40% of the total breeding soundness evaluation endorsed by the Society for Theriogenology (Ball et al., 1983). However, SC is a growth trait that may be affected by genetic, environmental or individual bull differences (Randel, 1994). Nutrition is an environmental effect that may adversely affect semen quality and this effect is difficult to quantify. Abdel-Raouf (1960) reported that diets adequate in protein, vitamins, minerals and energy appear to hasten the onset of puberty in beef bulls. However, Coulter (1994) cautioned that the feeding of high energy diets to post-pubertal beef bulls may be of no benefit to their reproductive capability, including semen quality and may reduce their reproductive potential.