Frozen semen doses (FSD) production centers select semen samples for cryopreservation based on semen volume, sperm concentration, sperm kinetics and morphology. Thus, factors that affect initial semen quality play very deterministic roles on selection of ejaculates for processing, and it subsequently influence freezability, post–thaw motility, FSD production, sperm fertility and ultimately success of AI and embryo production. A wide range of factors that affect seminal attributes have been reported viz. breed, age, body condition score, testicular morphology, vascularity of testicular cones, testicular thermo-regulation, gonadal development, thero-adaptability of bulls etc. (Brito et al. 2002a,b; Mandal et al. 2004, 2009, 2010; Kastelic, 2013; Kastelic et al. 2018). Although problems of cryo-preservability, high reproductive wastage rate etc. are major issues with B. taurus X B. indicus breeding programmes (Mandal et al. 2012; Singh et al. 2013), however, the same has been inadequately addressed in tropical countries. Present investigation provides a detailed account of important factors affecting reproductive ability in crossbred dairy bulls.
Values (mean ± SE) of seminal attributes of HF X Sahiwal dairy bulls observed in this study were within the normal range as reported earlier HF X Sahiwal crossbred bulls (Mandal et al. 2009, 2010) and other HF crossbreds bulls (Chauhan et al. 2010). Present findings indicated that semen quality improved gradually with age (Table 1). In HF X Sahiwal crossbred bulls sperm concentration increased significantly up to 5th year of age, thereafter increase was marginal and non-significant. This was major difference between presently studied HF crossbred bulls and exotic HF bulls, where sperm concentration reported to increase 1–3 years of age (Taylor et al. 1985; Argiris et al. 2018) in pure HF bulls. In zebu bulls maturity occurs at ≥ 4 years (Addass et al. 2011; Bhakat et al. 2011) because of slow body development and gradual attainment of sexual maturity. This might be the same reason for improvement in sperm productivity with age in taurus-zebu crossbred bulls (Mandal et al. 2004; Addass et al. 2011). Improvement in semen quality with age could be attributed to sexual and gonadal development, increase in testicular circumference, numbers of seminiferous tubules, enhancement in size of testes, maturity and adequate development of testes vascular cone and other apparatus responsible for effective thermo-regulation of testes etc. which in combination influences spermatogenesis, sperm production capacity and over all semen quality depending upon breeds and species involved (Brito et al. 2002a, b; Mandal et al. 2004; Sonj et al. 2013). Impact of seasonal heat stress on sperm productivity greatly varied with breeds and it also depends upon individual capacity of heat tolerance. In heat tolerant Frieswal (HF X Sahiwal) bulls the growth rate, body and testicular development, and semen production performance was better than less heat tolerant bulls (Mandal and Tyagi, 2009). Heat tolerance capacities of zebu were more than taurus bulls and produce less abnormal sperms under stressful conditions (Koivisto et al. 2009).
In AI programme number of bulls to be reared in FSD production centers depends upon sperm production ability of bulls and cryo-sustainability of semen. In this study, a large number of crossbred bulls (50.56%; 136 of 269) did not produced any FSD and ultimately rejected from breeding programme although they were physically sound during their induction to collection programme at 15–18 months of age. In purebred bulls, reproductive wastages were comparatively less as compared to crossbred bulls (Chacko, 2005; Singh et al. 2013) and therefore, quality semen productions from crossbred bulls remained as a challenging issue to breed large crossbred cow population in tropical countries. In fact, crossbred cows are the major contributors to total cow milk production in most of the tropical countries and there are steady requirements of quality FSDs of crossbred bulls. However, as revealed in the present study, in some selected FSD bulls (Table 5), improvement and maintenance of quality even beyond 5 years of age (maximum age of bulls was 93 months in this study) indicated existence of potentials for high sperm production in selective HF X Sahiwal dairy crossbred bulls, which needs selection, maintenance and further propagation.
In Non-FSD bulls, although volume and concentration increased with age (Table 5), however, sperm kinematics (MOT) did not improve much with age. In these bulls, the overall SQI showed very marginal improvement (Fig. 1) with age, such type of bulls had no practical breeding utility in FSD production centers. The findings indicated that there was wide difference in SQI even at 2-years of age between FSD (SQI = 52.81) and Non-FSD bulls (SQI = 28.08). In Non-FSD bulls age had no significant impact on subsequent SQI improvement even up to 4 years of age reared in this production stations (Fig. 1). Present results suggested that young bulls having SQI < 50 should be monitored precisely in FSD production centers and need culling as early as possible, even at 24 months age to avoid unnecessary expenditures and economic losses. In an earlier study (Mandal et al. 2010), it was recommended for culling of bulls < 30 months age based on sperm morphology, motility and concentration. However, present study denote for culling bulls at 24 months age provided SQI < 50. Improvement in SQI in few Non-FSD bulls between 4–5 years of age was due to some mediocre bulls that remained in borderlines of semen freezability. In these bulls, it is very difficult to be ascertained about their continuation in FSD production centers, because they produce inconsistent semen quality, ejaculates with incoherent freezability and post-thaw motility. Similar problems also arise on determination of bull fertility either in AI or natural insemination, where differentiation between excellent and poor (infertile) bull is easier than the sub-fertile ones and there exists considerable knowledge gaps in identification of sub-fertile bulls (Kastelic, 2013).
Highly significant (P < 0.01) positive association of age with seminal (Table 7) attributes indicated improvement in semen traits with advancement of age. Significant positive correlation among semen traits indicated for collateral improvement in these traits, if selection pressure is applied for any one of the important traits like age, age of semen donation, early age of puberty, age of freezable quality semen production etc.
Seasons’ effect is the outcome of a very complex interaction of animal system with various environmental variables like temperature, humidity, sunshine, photoperiod, precipitation etc. that directly influence the animals and indirectly via alteration in seasonal fodder and feed quality. The study location had dry hot summer, very hot-humid rainy and moderately cold-humid winter seasons; variations in maximum temperature 9–46°C, minimum temperature 2–31°C and relative humidity 30–100 % (Mandal et al. 2002). Bulls in the present study had the lowest SQI in rainy season and FSD production per ejaculate was also 16–20% less in this season. As the process of spermatogenesis is very lengthy one (60–62 days in bull), the seasonal influence affect both meiotic and spermiogenesis stages of spermatogenesis and its impact is long lasting and carried forward. Qualitative deterioration of semen, less production of frozen semen doses in hot-humid rainy seasons as observed in the present study signifies importance of seasons in bull station and FSD production units. From the regions of temperate climatic conditions also, decrease in sperm quality due to exposure of bulls to natural heat stress has recently been reported (Sabés-Alsina et al. 2017; Luceño et al. 2020)
Because of direct visible impact of seasons on milk production, seasonality on production traits had been extensively studied in lactating cattle (Kadzere et al. 2002; Dash et al. 2016; Mandal et al. 2021a,b). As milk production directly linked with calf production, studies on reproductive physiology of females, oocyte production, embryo development, fertilization etc. had been given more research emphasis and this disparity lead more research focus on female, ignoring males (Roth, 2017; Luceño et al. 2020). The physiological impact of meteorological factors viz. temperature, humidity and seasonality as a whole on the bull has not been established like that of female part and the molecular mechanisms of sperm damage owing to environmental stress have not been entirely characterized (Rahman et al. 2014; Luceño et al. 2020).
As stated earlier, the major aim of any bull station is to produce maximum numbers of quality frozen semen doses (FSD). Present study showed qualitative deterioration in semen characteristics and quantitative reduction (14–20%) of FSD/ejaculate during rainy season and definitely this reduction have substantial financial impacts on FSD production stations. Ejaculates collected in rainy season (July-October) basically had the spermatozoa synthesized and matured at least 60 days before, the period coincides with long duration day length (21st June, longest day in northern hemisphere). Higher duration of photoperiod and longer day length affect semen quality (Sonj et al. 2013; Granados et al. 2014) through melatonin modulated spermatogenesis, maturation and other sperm functions (González-Arto et al. 2014; Fernández-Alegre et al. 2020). Thus, combined effect of high temperature, humidity, photoperiod and other indirect seasonal effects mediated through feed quality might be responsible for semen quality deterioration in rainy season in this study. The present investigation also revealed that in Non-FSD bulls, SQI did not improve after 2 years of age and SQI value remained < 50 (33.40 ± 0.42). Therefore, bulls’ SQI < 50 at 2 years age need immediate culling to avert financial losses because such bulls are non-productive for artificial breeding and inflict an economic burden (estimated to Rupees 73,000 /bull/ year) for maintenance in the present Bull Rearing Unit. Although these bulls are of high genetic merit, received after nominated mating between high yielding dams x proven sires and passed through two stage selection process (at 3–6 months based on body morphometric characteristics and 12–15 month, breeding soundness evaluation), still the high rejection rate (50.56%, 136 of 269 in this study) of crossbred bulls is a very serious issue (Chacko, 2005; Tyagi et al. 2006; Mandal et al. 2012; Singh et al. 2013) in crossbreeding programmes in tropical countries. Mating of B. taurus x B. indicus is commonly termed as crossbreeding, but practically it is an inter-species mating, which might have accentuated the problems of poor reproductive ability in crossbred bulls (Singh et al. 2013). Present finding suggested that age is one the most important factors affecting semen quality in HF X Sahiwal crossbred bulls; sperm productivity in FSD bulls increased even 5 years of age (unlike up to 3 years age in exotic HF bulls) which need selection for improvement in semen quality traits. Summer and winter seasons should be effectively used for optimum FSD production from crossbred bulls maintained under similar climatic conditions. Bulls having SQI < 50 need to be culled at 2 years of age without further expectation on improvement in semen quality. Findings of the present study recommended that thermal stress amelioration measures should be taken up at bull stations from late summer to rainy seasons to minimize negative impacts of environment.