Study site The experiment was carried out on 15 breeding bull of five types of genotypes at the Central Cattle Breeding Station and Dairy Farm, Savar, Dhaka. The farm was surrounded by plain land with plenty of green vegetation. The highest and lowest ambient temperature of the experimental area was recorded as 37-380C and 15-210C, respectively. The average humidity was 76.6%.
Experimental bulls: Experimental bulls were BCB-1 pure breed, BCB-1 × Limousin (Limousin), BCB-1 × Simental (Simmental), BCB-1 × Charolais (Charlaise), BCB-1 × Brahman (Brahman). Among those bulls that are being utilized for the semen production at BLRI, 15 bulls of five types of genotypes were selected for this particular study. All the bulls were kept intensively under the same management conditions being given 2 kg concentrate and 9 to 10 kg hay per day, mineral lick every 1.5 to 2 months during dry period (1.25 kg/bull) and green feeds at the time of availability. The bulls were weighed every month and the scrotal circumference was taken once during the study period. The age of the bulls was determined by the date of birth from the record book maintained by AI center and ranged from 36 to 90 months. Detailed information on each bull such as age, body weight, health status, vaccination was obtained from the record books of the bull station.
Examination of bulls Before the introduction to the collection schedule, the bulls were examined physically with a special emphasis to the palpation of the scrotum and testicles and rectal palpation of the pelvic genitalia was carried out once every two weeks. The sexual behavior of the bulls was observed, including libido, erection, mounting and grasping of the dummy, protrusion of penis and thrust and ejaculation. The scrotum was inspected with respect to size, shape and freedom from skin disease.
Physical examination The examination was based on internal and external components. A trans-rectal examination was used to evaluate the health of secondary sex organs - urethra, prostate, seminal vesicles, ampullae and vas deferens. Palpation of the testicles and epididymis and examination of the sheath and penis were done to detect abnormalities that could affect breeding performance.
Clinical Examination of Bulls The bulls were thoroughly examined especially with regard to the locomotion body condition score, rectal temperature, breathing rate and pulse rate. The nutrition condition of the bulls was scored into 1-5 scale17. The bulls used to be weighed routinely once in a month. The scrotum of bulls was inspected with respect to size, symmetry and any visible skin diseases. The consistency of tests was assessed on the basis of firmness and resilience on 1-5 scale.
Collection of semen The semen was collected by an artificial vagina18. Before the collection of semen, all parts of the artificial vagina (AV) set were cleaned, sterilized, and assembled. The inner liner was put into the cylinder and both ends of the inner liner were reflected over the cylinder forming a water-like space between them. The cone along with vial was slipped over one of the ends of the cylinder and then tightened with a rubber band. Two third of the outer jacket of the vagina was filled with warm water. The temperature inside the artificial vagina was (110-115) 0 F. An airscrew was used for blowing air between two layers to create desired pressure. The required amount of lubricant was applied inside the artificial vagina with a glass rod. Component of an artificial vagina includes a glass container for semen with a water jacket and a volume scale, a thin flexible latex sleeve, a latex cone joining the end of the vagina with the collection tube, a rubber cylindrical casing with a valve for pouring water and blowing air, a bag and a thermal protector and a mechanical container. When the bull was sufficiently excited to jump over the dummy, the penis of the bull was directed into the artificial vagina by holding the sheath to collect the semen in a vial. After collection, the vial containing semen was put into hot water at 370C for preventing cold shock. It was closed with cotton and labeled. In all of the cases, semen was collected by using the artificial vagina, and only the first ejaculate was used for the study purpose and a total of 30 semen samples were collected and analyzed for physical-morphological analysis.
Data analysis For each parameter the mean value was calculated for weekly values in six bulls (pooled mean) which was then analyzed for its overall mean, median, minimum, maximum and standard error of semen characteristics by using the descriptive statistics and bivariate correlation coefficient of those semen characteristics were analyzed using SPSS (2002) statistical package, and the 95 % confidence interval of semen characteristics were analyzed using STATA (2001) statistical package. Proportion calculation was used to see the proportion of different semen colors and to see the relation between the color of semen and its spermatozoa concentration. A single sample t-test was used to compare the mean values of the present semen parameters with the literature values.