, ~ , ~,N P,I.H. REPR()I)I ( 'TI( ) \

t{l~S EV I E R Animal Reproduction Science 36 ( 1994 ) 197-209

Effect of plane of nutrition and season on body and testicular growth and on semen characteristics in

Boran and Boran X Friesian bulls in Ethiopia

Azage Tegegne *'~, Yifat Dembarga b, Tesfu Kassa b, R. Franceschini a alnternational Livestock Centre fi)r Africa, PO Box 5689. Addis Ababa, Ethiopia

b Faculty of Veterinary Medicine, Addis Ababa University, PO Bo_\ 34, Debre Zeit. Ethiopia

Accepted 18 January. 1994

Abstract

Ten Boran and 12 Boran × Friesian bulls with a mean ( _+ standard error of the mean) age and body weight of 20.2 + 0.5 months and 225.5 _+ 2.5 kg were used over a 1 year period to determine the effect of plane of nutri t ion and season on body and testicular growth and on semen traits. Within genotype, bulls were allocated to either a high (HP) or a low ( LP ) plane of nutrition. Data on body weight, scrotal circumference (SC) and semen character- istics were collected every 2 weeks. Body weight and SC increased ( P < 0.01 ) with age in all t reatment groups. Mean body growth rate was higher ( P < 0.001 ) in bulls on H P than on LP nutri t ion (760 -+ 12.6 vs. 236 _+ 12.6 g day -~ ) and in Boran than in Boran× Friesian bulls (620 _+ 12.8 vs. 375 + 12.3 g d a y - ~ ). Mean SC and changes in SC were higher in bulls on HP than on LP nutri t ion (0.23_+0.01 vs. 0.10_+0.01 mm day -~ ), and in the Boran than in the Boran X Friesian bulls (0.29 _+ 0.01 vs. 0.04 _+ 0.01 mm d a y - ~ ). Season affected changes in SC in both Boran and Boran X Friesian bulls and was highest during the short rainy season and lowest during the dry season. Data from 528 ejaculates showed that the two genotypes reacted differently to season with respect to semen volume, percentage mo- tility and sperm concentrations. Although these semen characteristics were highest during the dry season and lowest during the short rainy season in both genotypes, Boran bulls had better semen traits than Boran × Friesian bulls during the dry season and vice versa during the long rainy season, while genotype differences were minimal during the short rainy sea- son. The percentage of normal sperm cells was higher in bulls on LP than on HP nutrit ion ( P < 0.01 ), in Boran than in Boran X Friesian bulls ( P < 0.05 ), and was highest (P< 0.001 ) during the dry season. It was concluded that genotype differences in semen characteristics in response to season and plane of nutri t ion have to be taken into account in the manage- ment of zebu and crossbred bulls in tropical environments.

*Corresponding author.

0378-4320/94/$07.00 © 1994 Elsevier Science Publishers B.V. All rights reserved SSDI 0378-4320(94)01302-2

198 A. Tegegne et al. / Animal Reproduction Science 36 (1994) 197-209

I. Introduction

The patterns of testicular development, endocrine functions and semen traits are influenced by genetic (Rekwot et al., 1988; Hernandez et al., 1991 ) and en- vironmental factors such as diseases, nutrition, season and location (Entwistle, 1983; Rekwot et al., 1988; Godfrey et al., 1990a, b; Tegegne et al., 1992a). How- ever, there is a disparity in results between studies which compared reproductive development and characteristics ofBos indicus and Bos taurus bulls. Fields et al. (1982) found little difference in postpubertal testicular development between Brahman and Angus bulls in Florida, and Perry et al. ( 1991 ) reported no differ- ences in the patterns of testicular development between Bos indicus and Bos tau- rus derived bulls in Australia. Morris et al. ( 1978 ), however, found slower testic- ular development in Brahman than in Santa Gurtrudis bulls in Texas.

Data on the effect of season on semen traits are also not consistent. Seasonal differences in semen traits and daily sperm production have been reported for different breeds of bulls (Fields et al., 1979; Wildeus et al., 1984; Cardoso and Godinho, 1985; Godfrey et al., 1990b; Goswami et al., 1991 ). In contrast, Her- nandez et al. ( 1991 ) were not able to detect seasonal variations in semen traits in four zebu breeds in Mexico. In Florida, Fields et al. ( 1979 ) showed that testic- ular growth and semen quality were more severely affected during the hotter months in Bos taurus than in Bos indicus bulls. Similarly, Godfrey et al. ( 1990b ) reported that season and location accounted for differences in semen quality and endocrine functions between Brahman and Hereford bulls.

The Boran breed is one of the indigenous cattle breeds in Ethiopia. The breed has been crossed mainly with Friesian cattle for milk production. In Ethiopia, the year is characterized by three distinct seasons: short rainy (March to May), long rainy (June to October) and dry (November to February). Seasonality in con- ception patterns of zebu and crossbred cows under natural mating conditions has been reported (Michalak et al., 1983 ) and seasonal variations in sperm produc- tion potential of zebu bulls has been recorded (Tegegne et al., 1992b), suggesting an important bull influence on herd fertility.

In tropical regions testicular functions, semen output and quality, and mating behaviour of bulls can vary from season to season, depending mainly on availa- bility and quality of feed and climatic conditions (Entwistle, 1983). However, data on the effects of nutrition and season on the patterns of reproductive devel- opment, testicular growth and semen quality of zebu bulls and their crosses with exotic breeds in tropical Africa are limited. This study examined the effect of plane of nutrition and season on body and testicular growth, and on semen traits in postpubertal Boran and Boran × Friesian bulls in the highlands of Ethiopia.

2. Materials and methods

The study was carried at the ILCA Debre Zeit research station, located 50 km southeast of Addis Ababa, 8 °44'N and 38 ° 58'E at an altitude of 1850 m. Ten

A. Tegegne et al. /Animal Reproduction Science 36 (1994) 197-209 199

Boran and 12 Boran × Friesian bulls with a mean ( + standard error of the mean (SEM) ) age and body weight of 20.2 + 0.5 months and 225.5 + 2.5 kg at the start of the study were used. All bulls had satisfactory semen quality at the start of the study. Within genotype, bulls were randomly allocated to either a high (HP) or a low (LP) plane of nutrition designed to yield growth rates of 750 and 250 g day -~, respectively. Bulls were group fed in open shades and had free access to water and mineral blocks. All bulls received good-quality grass hay and chopped alfalfa. Bulls on HP nutrition were provided with additional concentrate mixture composed of noug (Guizotia abyssinica) cake and wheat bran. The amount of feed offered was adjusted based on weekly body weight.

Data on body weight and scrotal circumference (SC) were collected at 14 day intervals. Semen was also collected every 2 weeks from each bull by electroeja- culation and data from a total of 528 ejaculates collected over a 1 year period were used. Semen was evaluated using standard procedures as described by Lun- stra and Echternkamp (1982). Semen volume was recorded. Immediately' after collection, semen samples were pipetted, put on warmed slides and progressive motility score (Table 1 ) and percentage motile sperm cells evaluated. Percentage motile sperm cells was determined on a subjective scale of 0-100% with 10% intervals. A 50 121 aliquot of semen sample was fixed in 2.95 ml of phosphate- buffered saline containing 0.25% glutaraldehyde (Johnson et al., 1976) for sub- sequent determination of sperm concentrations and evaluation of sperm mor- phology. Sperm concentration was determined by haemocytometric count of du- plicate samples. Sperm morphology in terms of percentage normal, percentage sperm cells with abnormal head, body and tail, and cytoplasmic droplets were evaluated from 200 random sperm cells.

Data on rainfall, minimum and maximum temperatures, relative humidity and sunshine hours were collected on a daily basis and averaged across seasons. The short rainy season was the period from March through May, the long rainy season from June to October and the dry season from November to February. Data were analysed by the least-squares analysis of variance technique using the General Linear Models (GLM) procedure (Statistical Analysis Systems Institute Inc.~ 1987). A split-plot model was used with nutrition, genotype and nutri- tion × genotype interaction included in the whole plot. The error term was animal within nutrition ×genotype. The subplot included season as a measure of time,

Table 1 Progressive motility scores used for semen evaluations

Score Description

No sperm moving in a straight line Some sperm showing straight-line movement Most sperm showing slow, straight-line movement Most sperm showing good, straight-line movement Most sperm showing rapid, straight-line movement Most sperm showing very. rapid, straight-line movement

200 A. Tegegne et al. / Animal Reproduction Science 36 (1994) 197-209

along with the appropriate interactions. Age was included in the model as a cov- ariate. Pearson coefficients of correlation were determined between body weight, scrotal circumference, semen characteristics and climatological data.

3. Results

Monthly rainfall, mean minimum and maximum temperatures and sunshine hours during the study period are presented in Fig. 1 and a summary of climatic data for the dry, short rainy and long rainy seasons are presented in Table 2.

4 0 -

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35 0 (.-

3O c c- (/) c 2B

?

0 ~ 0 Mc=xlrnurn temperoture

• ~ • Minimum ternperoture e¢~ Meon aunehlne houru

~ o ~ o _...o..~o~ / o ~ v ~ O ~ O . . ~ . O ~ O 0

* mmJ M A M J O A S 0 N D O F

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80,

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o

r--1 Rolnfall Relative hurnldlty

M A M J d A S O N D d F Month

Fig. I. Climatological data at the ILCA Debre Zeit research station during the study period (March - May, short rainy season; June -October, rainy season; November - February, dry season).

A. Tegegne et al. / Animal Reproduction Soence 36 (1994) 197-209 201

Table 2 Mean ( _+ SEM ) and ranges for climatic data in the short rainy, long rainy and dR' seasons at the l LCA Debre Zeit research station during the study period

Season

Variables Short rainy Long rainy Dr.~ Overall

Maximum temperature (~C) 27.3_+0.2 25.94_0.2 28.3_+0.2 26 .9z l.t~ (26.3-28.9) (24.8-27.5) (25.9-29.7)

Min imum temperature ( ° C ) 14.64_0.1 13.8+0.2 13.8±0.2 13.9+ 1.1 (14.5-14.9) (11.1-14.7) (12.4-14.9)

Relative humidi ty (%) 49.6_+ 1.0 61.5 + _ 1.0 43 .7+0.5 54.1 411.1 (45.7-52.4) (46.0-71.5) (41.8-47.9)

Sunshine hours (h ) 7.8_+0.3 6.9_+0.2 9 .2+0.2 7.8 ~_ 1.5 (7 .3-8 .3) (5 .4-9 .8) (8 .2-9 .8)

Total rainfall ( ram) 115.0 690.7 36.3 842.0

Figures in parentheses indicate ranges.

Table 3 Least-squares means (_+ SEM) for average daily body weight gain ( A D G ) and changes in scrotal circumference ( SC ) during different seasons in Boran and Boran X Friesian bulls raised on two planes of nutrition

Variables Boran Boran × Friesian

HP LP HP LP

A D O ( g d a y ' ) Short rains 1303 _+ 43.0 a 667 _+ 43.0 b 814 _+ 34.0 ~ 179 _+ 33.t; b Long rains 664 _+ 22.4 a 50 +_ 22.4 b 410 -+ 20,5 ~' - 24 ~_ 20.6 h D I ~ . r 634 _+ 24.8 a 402 + 24.8" 734 + 33.9" 139 _+ 33A)"

SC (mm day -I ) Short rains 0.82 +_ 0.05" 0.33 +_ 0.058 0.14 + 0.04" 0.10 - 0.04" Long rains 0.26 _+ 0.03 a 0.16 + 0 . 0 2 b 0.04_+ 0.02 a - 0.04 _- 0.02" D~, 0.06 +_ 0.03" 0.13 _+ 0 . 0 3 b 0.06 -+ 0.04 a - - 0.07 2 0.04 b

abWilhin genotype, figures in a row followed by different superscripts differ significantly ( P< 0.05 I.

The patterns of changes in body weight and scrotal circumference during the three seasons are presented for the two genotypes by nutritional treatment groups in Table 3. The overall mean body growth rate was faster (P< 0.001 ) in bulls on HP than on LP nutrition (760-+ 12.6 vs. 235 + 12.6 g day- ' ) and in Boran than in Boran X Friesian bulls (620+ 12.8 vs. 375_ + 12.3 g day -1 ). These growth rates were, however, influenced by season in a similar pattern in both Boran and Boran X Friesian bulls on HP and LP nutrition. Growth rates were fastest during the short rainy season and slowest during the long rainy season, except in Boran bulls on HP nutrition. Bulls on HP nutrition grew faster than those on LP nutri- tion across all seasons and the Boran bulls grew faster than the Boran X Friesian bulls across all seasons irrespective of nutritional treatment.

The patterns of changes in SC followed a pattern similar to changes in body

202 A. Tegegne et al. / Animal Reproduction Science 36 (1994) 197-209

weight. Bulls on HP nutrition had a faster (P< 0.001 ) overall increase in SC than bulls on LP nutrition (0.23 + 0.01 vs. 0.10 + 0.01 mm day- 1 ). Boran bulls had a faster (P<0.001) increase in SC than BoranxFriesian bulls (0.29_+0.01 vs. 0.04___ 0.01 mm day- ~ ). Moreover, the overall mean changes in SC were slowest (P< 0.001 ) in all treatment groups during the dry season and fastest during the short rainy season (0.35_+0.02 vs. 0.05+0.01 mm day -~ ). Although a similar seasonal trend was observed in changes in SC, a slight decrease in SC was noted in crossbred bulls on LP nutrition during the long rainy season and the dry sea- son. Seasonal influence on changes in SC was more pronounced in the Boran than in the Boran X Friesian bulls. Overall changes in SC were higher in both Boran and Boran X Friesian bulls on HP nutrition than on LP nutrition across all seasons.

Ejaculates which contained sperm cells were obtained at all collection times from all bulls. Mean semen volume (Fig. 2) was higher (P<0.001) in bulls on HP than on LP nutrition (4.9 _+ 0.2 vs. 3.7 _+ 0.2 ml). The effect of plane of nutri- tion on semen volume was more pronounced in Boran × Friesian than in Boran bulls across seasons. There was a significant (P< 0.05 ) season X genotype inter- action effect on semen volume. Boran bulls had higher semen volume during the dry season than crossbred bulls (5.0 + 0.3 vs. 3.8 _+ 0.4 ml), while crossbreds had higher semen volume than Boran bulls during the short rainy season (4.8 _+ 0.3 vs. 3.4 -+ 0.6 ml ) and the long rainy season (4.5 _+ 0.3 vs. 4.2 _+ 0.3 ml). In Boran bulls, semen volume was highest during the dry season ( 5.0 _+ 0.3 ml ), lowest dur- ing the short rainy season (3.4_+ 0.6 ml) and intermediate during the rainy sea- son (4.2+0.3 ml), while relatively little seasonal variation was observed in Boran X Friesian bulls.

Percentage motility was not influenced by either plane of nutrition or genotype.

10-

9-

B-

c 5"

3 "

2 -

1

E~ Boron-Low I Boran-Hlgh

Boron x Friesian-Low Boron x Frienion-High

lini Small Rainy Rainy

SeOSOR

11 1 Or,/

Fig. 2. Seasonal changes in semen volume in Boran and Boran X Friesian bulls raised on two planes of nutrition (bar = SEM).

, t . Tegegne et al. /Animal Reproduction Science 36 (1994) 197-209 203

Percentage motility was, however, lowest (P< 0.05) during the short rainy sea- son (45%) than during the dry (54%) and long rainy (55%) seasons. In Boran bulls, percentage motility was highest during the dry season (65%) and lowest during the short rainy (41%) season, while seasonal changes were smaller in Boran×Friesian bulls (Fig. 3). Motility score was not influenced by plane of nutrition and genotype; however, season tended (P= 0.07 ) to influence motility score, being higher during the dry season (3.0) than during the short rainy ( 2.4 ) and long rainy (2.6) seasons.

Sperm concentration (ml -I ) was not affected by plane of nutrition and geno- type. However, there was a significant (P< 0.001 ) seasonal influence. Mean sperm

1 O0 -

gO-

B0-

7 0 -

N o 80- E

~. 5 0 -

4 0 .

~ 0 .

2O

r - - 1 B o r o n - L o w l i b B o r a n - H l g h

B o r o n x Fr ies ian-Low B o r o n x F'r iesian-High

ljli 711 Small Rainy Raln Dry

Season

Fig. 3. Seasonal changes in percentage sperm motility in Boran and B o r a n × Friesian bulls raised on two planes of nutrition (bar = SEM ).

o

x v

o

ca

800 -

700 -

800 •

5 0 0 •

4 0 0 -

3o0 •

200 •

10o

o

I---1 B o r o n - L o w

l i b B o r a n - H T g h

B o r a n x F d e s i a n - L o w

B o r o n x F d o s l a n - H 1 g h

Small Rainy R a i n y

Season

Dry

Fig. 4. Seasonal changes in sperm concentration in Boran and Boran × Friesian bulls raiscd on two planes of nutrition (bar = SEM).

204 A. Tegegne e ta / . /A nimal Reproduction Science 36 (1994) 197-209

1 O 0 -

go-

B0-

70-

~ 4 0 - E

2O

10

0

I ~1 Boran-Low Boran-H lgh

IS~ Boran x Friesian-Low IR~ Boran x Friesian-Higfi

I11 Small Rainy Rainy

Season

Dry

Fig. 5. Seasonal changes in percentage normal sperm cells in Boran and Boran × Friesian bulls raised on two planes of nutrition (bar = SEM).

Table 4 Significant (P < 0.05 ) coefficients of correlation between body weight (BWT), scrotal circumference (SC) and semen volume (VOL), percentage motility (PMOT), motility score (MOTS), sperm con- centration (CONC) and percentage normal spermatozoa (NORM) in Boran and Boran × Friesian bulls on two planes of nutrition

BWT SC VOL PMOT MOTS CONC NORM

BWT SC 0.78 VOL 0.39 0.35 PMOT 0.25 0.29 MOTS 0.19 0.19 CONC - 0.2O NORM 0.37 0.37

0.32 0.23 0.71 0.14 0.39 0.34 0.35 0.56 0.46 0.39

concentration was highest during the dry season (449 ___ 24 × 106), lowest during the short rainy season (213 + 34 × 106) and intermediate during the rainy season (255-+19X106) . In Boran bulls, sperm concentration decreased from 589 ___ 26 × 106 during the dry season to 207 ___ 32 × 106 during the long rainy sea- son and further to 164_+ 55 × l06 during the short rainy season, while relatively little seasonal variation was observed in Boran X Friesian bulls (Fig. 4). During the dry season, sperm concentration was almost twice as much in Boran bulls as in BoranxFriesian bulls (589_+26 vs. 310_+38X106; P < 0 . 0 0 1 ) , while Boran X Friesian bulls had higher sperm concentrations than Boran bulls during the long rainy season (303_+26 vs. 207___32×106 ) and short rainy season (262_+32 vs. 164_+ 55× 106).

Percentage normal sperm cells was higher ( P < 0.01 ) in bulls on LP than on HP nutrition (64.0 _+ I. 5 vs. 55.7 _+ 1.3% ) and in Boran than in Boran X Friesian bulls (62.1 + 1.7 vs. 57.6 ___ 1.3%). Season also affected ( P < 0.001 ) percentage normal sperm cells, being highest during the dry season (71.2 _+ 1.4%), lowest during the

.1. Teg~\¢,ne et al. / Animal Reproduction .S'cwm'e 36 (I 994) 197-209 21)5

short rainy season (45.2 + 2.4%) and intermediate during the long rainy season (63.1 + 1.3%). The difference in percentage normal sperm cells between the dry and short rainy seasons was, however, similar in the Boran (73.42 1.6 vs. 47.6 _+ 4.4%) and the Boran × Friesian (68.9 + 2.1 vs. 42.9 _+ 2.1% ) bulls ( Fig. 5 ).

Percentage sperm cells with abnormal heads was higher (P< 0.001 ) in bulls on HP than on LP nutrition (8.5+0.8 vs. 4.5+0.9%) and was highest (P<0.01) during the dry season (8.9 +_ 0.8%) and lowest during the short rainy (4.2 +_ 1.4%) season, with no apparent genotype differences. Season had a significant effect on percentages of sperm cells with abnormal body, tail and protoplasmic droplets, while plane of nutrition had no effect. Percentage sperm cells with abnormal tail was highest during the short rainy season (29.42 1.5% ), lowest during the dry season (5.5 +0.8%) and intermediate during the long rainy season ( 11.4_+0.8% ). Similarly, sperm cells with protoplasmic droplets followed a similar pattern. Genotype differences were significant (P< 0.05 ) only on percentage sperm cells with abnormal body, being higher in Boran×Friesian than in Boran bulls (14.5+0.6 vs. 11.8+0.9%).

There were high, positive coefficients of correlation (Table 4) between body weight and SC (0.78), between SC and semen volume (0.35), between percent- age motility and motility score (0.71), sperm concentration and percentage mo- tility (0.39), percentage motility and normal sperm cells (0.56), motility score and percentage normal sperm cells (0.46) and sperm concentration and percent- age normal sperm cells (0.39). Coefficients of correlation between semen char- acteristics and climatic data were generally low. However, moderate values were found between sperm concentration and sunshine hours (0.23) and relative hu- midity ( - 0 . 2 4 ) and between percentage normal sperm cells and sunshine hours (0.21) and relative humidity ( -0 .23 ).

4. Discussion

In our study, plane of nutrition and season influenced body growth rate and changes in SC measurements in both Boran and Boran × Friesian bulls. The pat- terns of change in these traits followed corresponding planes of nutrition in both genotypes. This was in agreement with previous reports which have shown strong associations between plane of nutrition, body weight and scrotal growth (Van- Demark and Mauger, 1964; Reeves and Johnson, 1976; Alkass et al., 1982: Rek- wot et al., 1988; Tegegne et al., 1992c). Earlier studies have also shown genotype differences in body and testicular growth rates (Fields et al., 1979, 1982; Entwis- tle, 1983; Godfrey et al., 1990a; Tegegne et al., 1992a): this is also confirmed in the present study.

Season influenced body and testicular growth in both genotypes. Seasonal ef- fect on body growth and reproductive function is mediated primarily through variations in the quality and quantity of feed (Wildeus and Entwistle, 1984: Par- kinson, 1985). However, such variations occur predominantly under grazing conditions. Our animals were kept and fed in pens throughout the experiment

206 A. Tegegne et al. / Animal Reproduction Science 36 (1994) 197-209

and variations in feed quality and quantity were kept to the minimum possible. The influence of season on body and testicular growth in the two genotypes may result form variations in feed utilization efficiency of animals during different seasons. A number of studies have shown that nutrition and season affect endo- crine and spermatogenic functions and influence testicular growth (N'Dama et al., 1983; Wildeus and Entwistle, 1984; Godfrey et al., 1990b; Tegegne et al., 1992c). Studies have reported high coefficients of correlation between body weight, scrotal circumference and paired testes weight, indicating strong associ- ation between nutrition, testes size and testis function (Wildeus and Entwistle, 1984; Tegegne et al., 1992c). Larger testes size is associated with higher daily sperm production capacity. The influence of season on body and testicular growth agrees with Godfrey et al. (1990b), who found seasonal influences on endocrine and spermatogenic functions, and on testicular growth.

Although the inconsistency of bulls in semen output in response to electroeja- culation is recognized, care has been taken to apply uniform stimulation in these animals. Mean semen volume compared well with Bunaji and Friesianx Bunaji bulls in Nigeria (Rekwot et al., 1988), but was lower than mature zebu bulls in Mexico (Hernandez et al., 1991 ). There was no genotype difference in semen volume and this agrees with previous reports on Bunaji and FriesianxBunaji bulls (Rekwot et al., 1988 ) and four breeds ofzebu bulls (Hernandez et al., 1991 ). High plane of nutrition increased semen volume by 31% and this is in agreement with the findings of Rekwot et al. (1988) who reported that Bunaji and Frie- sian X Bunaji bulls fed a high protein diet had 31% and 46%, respectively, more semen volume than those fed a low protein diet. The absence of seasonal varia- tions in semen volume, although contrary to Rekwot et al. (1987), is in agree- ment with others (Kumi-Diaka et al., 1981; Hernandez et al., 1991 ).

Percentage sperm motility and motility score were comparable with the values reported for Brahman and Hereford bulls (Godfrey et al., 1990b). The absence ofgenotype differences in percentage motility agrees with previous reports (Rek- wot et al., 1988; Hernandez et al., 1991 ). Plane of nutrition did not influence percentage motility and this disagrees with the findings of Rekwot et al. ( 1988 ). Season affected percentage motility and tended to influence motility score. Sim- ilarly, Fields et al. (1979) found seasonal variations in percentage motility in Hereford bulls in Florida. On the other hand, Hernandez et al. ( 199 l ) reported no seasonal differences in percentage motility among four breeds of Bos indicus bulls in Mexico.

Mean sperm concentration in our bulls compares well with the values reported for four zebu breeds in Mexico (Hernandez et al., 1991 ) and Brahman bulls in Texas (Godfrey et al., 1990b), but were higher than the values reported for younger Bunaji and Friesian × Bunaji bulls in Nigeria (Rekwot et al., 1988 ). Our finding that there was no genotype difference in sperm concentrations disagrees with the results of other studies which compared zebu, crossbred and taurus bulls (Fields et al., 1982; Ruttle et al., 1982; Rekwot et al., 1988; Godfrey et al., 1990b). However, the absence of nutrition influence on sperm concentration agrees with the report of Coulter and Kozub ( 1984 ). Seasonal variations in sperm concentra-

A. Tegegne et al. / Animal Reproduction Science 36 (1994) 197-209 207

tions observed in the present study agree with earlier findings (Alva et al., 1982; Mendenez et al., 1983; Rekwot et al., 1987; Godfrey et al., 1990b). Other studies, however, did not find seasonal variation in sperm concentrations in bulls under tropical environments (Kumi-Diaka et al., 1981; Hernandez et al., 1991 ).

The inconsistency in the literature on the effect of nutrition and season on per- centage motility, motility score and sperm concentration could be the result of differences in the age of the animals used, the quality and quantity of feed sup- plied, the duration of feeding, severity in seasonal variations and the genotype of the animals used.

The significant genotype difference in percentage normal sperm cells is in agreement with other findings (Fields et al., 1982; Godfrey et al., 1990b). The slightly higher percentage of normal sperm cells in the Boran than in the crosses could be because of their better adaptation to the tropical environment. Bulls on HP nutrition had about 10% higher abnormal sperm cells than bulls on LP nutri- tion, and this difference was consistently reflected in all segments of the sperm cells. These differences may be associated with higher levels of scrotal fat depo- sition which could affect testicular heat exchange, spermatogenesis and sperma- tozoa morphology, as shown by Wildeus and Entwistle (1986) after scrotal in- sulation. The significant seasonal effect on percentage normal sperm cells is also in agreement with earlier findings (Rekwot et al., 1987; Godfrey et al., 1990b). The higher values in percentage normal sperm cells during the dry season could be associated with the relatively longer sunshine hours and lower relative humidity.

Season influenced percentages of sperm cells with abnormal head, body, tail and protoplasmic droplets. In India, differences in semen traits between Jer- sey × Hariana and Friesian X Hariana crosses in response to seasonal changes have been reported and semen traits were found to be lower during the hot-dry than during the hot-humid autumn and winter seasons (Goswami et al., 1991 ). Simi- lar seasonal influences in spermatozoa morphology have also been reported for other zebu and taurus breeds (Wildeus et al., 1984; Godfrey et al., 1990b).

In summary, this study confirmed earlier findings that differences exist be- tween zebu and crossbred bulls in their response to seasonal influences on testic- ular growth and semen characteristics. Relatively better sperm motility score, sperm concentrations and percentage normal sperm cells were observed in both genotypes during the dry season. In Ethiopia, the dry season is usually character- ized by a relatively higher ambient temperature, longer sunshine hours and lower relative humidity. These may have a positive effect on testicular function and on semen quality in bulls of these genotypes as shown by the moderate coefficients of correlation between these climatological variables and some semen character- istics. Moreover, the effect of the preceding wet season on testicular function and semen quality may be manifested during the subsequent dry season. The superior semen characteristics in Boran than in Boran X Friesian bulls during the dry sea- son could also indicate the relatively better adaptive nature of zebu cattle to trop- ical environments. However, further studies are warranted to determine the pre- cise effects of nutrition, season and other environmental factors and their

208 A. Tegegne et al. / Animal Reproduction Science 36 (1994) 197-209

interactions on testicular function and semen characteristics in zebu and crossbred bulls using relatively larger number of animals. Such data could help in develop- ing strategies to optimize the use of breeding bulls in enhancing genetic progress and in improving reproductive efficiency of cow herds in both natural mating and artificial insemination programmes in the tropics.

5. Acknowledgements

The authors thank Tesfaye Gashaw for the management of animals, Hirut Mu- lugeta and Kassahun Zewdie for technical assistance, and J. Sherington and Amare Atale for assistance with statistical analysis.

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