Age at puberty and sexual maturity are important economic traits. Many factors are directly or indirectly related to age at puberty in dairy animals. Huge economic losses have been related to delayed puberty. Among the all factors affecting age at puberty, nutritional is most important. Higher correlations have been observed between age at puberty and body weight. This can be achieved by proper nutrition than other factors. For early puberty, heifer must be fed properly to get a sufficient weight for puberty at recommended time. So nutritional management should be given priority for reducing age at puberty. Use of green fodders along with mineral mixture reduces the age of puberty and maintains the proper growth. The Ca to P should be supplemented at the ratio of 1:1 to 2.5. Long-day photoperiod hastens growth and sexual maturity in the dairy animals. Diet containing balanced energy and protein concentration helps in proper growth and puberty. The use of herbal preparation like Shatavari @ 150mg/kg BW/day has significant effect on growth and sexual maturity. Application of new knowledge and modern technologies are necessary for growth and reproductive management for dairy animals.
Puberty is the period when the sexual organs are functionally developed. Puberty in heifers is characterized by first ovulation and plasma progesterone concentrations above 1 ng/ml . Sexual maturity is the stage when the animal is able to express its full reproductive potential. Sexual maturity in males is characterized by sperm ejaculate that contains minimum 50 million spermatozoa with minimum of 10% motile sperm . The onset of puberty is the result of a series of complex events that occur within the reproductive endocrine system . A cow maturing at early age will produce more milk in her whole life time. Delayed puberty in cattle and buffalo is a major problem in the dairy industry. Heifers calved at 540 kg were more economical than those at 620 kg . Rauw et al. (1998) reported that modern reproduction and DNA-techniques in animal breeding programme may result in physiological, behavioural, and immunological problems in animals. Many factors like species, genetic potentiality, plane of nutrition, growth, body weight, role of different hormones, health and other management conditions have a direct or indirect effect on growth, puberty and sexual maturity in animals . Among all factors, body weight at early age has an important role on life time performance of animals including production and reproduction.
Age of the animal :
Average age of puberty in cow heifers is between 37 and 34 months but Sahiwal attain puberty at 46 months . In the tropical condition the age at puberty in Bos indicus range between 16 and 40 months . The Indian buffalo (Bubalus bubalis) attain puberty between 16 to 40 month but the average time is over 2.5 years of age (CIRB Annual Report, 1999–2000). Murrah attain puberty as late as an average age of 33 months (NDRI Annual Report, 1995–1996), Nilli Ravi at 32.5 months and Surti at 45.5 months of age . Postnatal growth plays an important role in the performance of the dairy animals.
Genetics and Breed :
Genetic selection is not an effective tool as other environment managemental approaches are necessary for early puberty. Age of puberty is moderately heritable and in cattle it ranges between 0.16 to 0.57 . Nogueira (2004) suggested that selection of heifers should be based on age at first calving and recommended crossbreeding between Zebu and Bos taurus cattle for early puberty in zebu cattle. He also suggested for consideration of environmental effect for early puberty in zebu cattle. Zebu cattle are famous for disease resistance and better feed utilization with low management requirement than exotic animals. Naz and Ahmad. (2006) reported a low correlation between maturity age and weight at first conception in Nili-Ravi buffalo. He also reported a genetic correlation for age at maturity with weight at maturity, age at first conception were 0.49 and 0.88, respectively. Thyroid hormones have a great role in the metabolism and cell growth in the body. Fernandez et al. (2014) reported a significant association (P < 0.05) between single nucleotide polymorphisms (SNPs) markers and puberty in Angus bulls. So this marker is good indicator for puberty in animals. In case of males scrotal circumference is an important criterion for selection because it is highly heritable and related to reproductive performance . During the under nutrition Neuropeptide Y (NPY) is responsible for low secretion of luteinizing hormone (LH). Leptin has an important role in suppression of Neuropeptide Y (NPY). Vaiciunas et al. (2008) reported a lower NPY-Y1 and NPY-Y4 expression had a regulating role for puberty in early-maturing Bos indicus heifers. Heifer selection should based on good health condition, structurally large body size and puberty at early age.
Growth and body weight:
Maturity of the heifer depends on the body weight rather than age. Lower growth rate occurs due to underfeeding or imbalanced feed composition. Birth weight also affects growth rate and age at puberty. Many genetic and non-genetic factors are responsible for body growth in animals. Genetic factors along with nutrition, hormones, animals’s individuality and many other factors determine the growth of animals. Feeding high energy or high concentrate diets not only reduce the age of sexual maturity but also lowers the time period for attaining the age of first calving. Buffalo heifers reared on seasonal green forages and crop residues resulting in poor growth rates and delayed onset of puberty (Bhatti et al., 2007). The body weight gain may have a greater influence on onset of puberty . About 60 to 65% of mature body weight may be a standard during the starting breeding season in heifers . Da Luz et al. (2012) reported that Murrah buffalo reached sexual maturity at 2 year of age and at this time sperm production is 13 million sperm per gram of testis. The monthly weight gain was faster upto 3 month of age and slower from 3 to 6 month of age in case of swamp buffalo (Das, 2004) and Murrah buffalo. A particular body weight has a role in attainment of puberty and a low body weight causes delay in onset of puberty . During the early phase of life, body’s cells and parenchyma cells of the mammary gland develop very fast than the late phase of life. However, over conditioned animals presently facings many health related problems resulted in poor productive and reproductive performance
Balanced feeding and improved management can be helpful in better growth and early sexual maturity. The first important factor that affects age of maturity is the plan of nutrition . Delay in puberty also occurs due to inadequate supply of feed and essential nutrition during the early growing period. But some authors reported that a predetermined body size at which puberty will occur within each specific breed (Oyedipe et al., 1982). The onset of puberty at the early stage (between 4 and 6.5 months of age) occurs due to high plan of feeding . Colostrum is essential at early age of life to provide the natural immunity to the newborn calves. At the starting stage colostrum feeding through the bottle leads to higher intake rate than the pail feeding . A good quality calf starter should contain 18% crude protein and 3.0 Mcal/Kg metabolizable energy (NRC, 2001). Khan et al. (1992) worked on the Sahiwal calves and reported that the restricted suckling calves gained live weight 49% faster with 80% improvement in efficiency of converting milk into live weight. Similar observations were reported by the Bwire et al. (1996) in Zebu heifers, they reported that restricted suckling tended to gain more body weight (g/day) than the than those on artificial rearing system. Contrary to this report Crabtree. (1967) reported that pre-weaning management system had no significant effect on growth performance. Nanda et al. (2003) observed that better nutrition reduces the age of maturity in buffalo heifers. Shatavari (Asparagus racemosus) can be used as a feed supplement for growth and puberty in dairy animals. It has anti-stress properties (Kumar et al., 2008) and causes early puberty and increase in weight of ovaries, uterus and teats in female (Sharma, 2011). Feeding of Shatavari @150mg/kg BW/day has significant effect on attaining higher average daily gain, early attainment of puberty and age at first service in Sahiwal heifers (Jamara et al., 2014). Proteins and energy are most critical nutrients influencing the growth of calves. High level of nutrition is essential during the growth period in the buffalo calves but the cattle calves require low level of protein than the buffalo calves (Basra et al., 2003). Contrary to this report Fluharty and Loerch. (1995) reported that higher protein concentrate mixture supplementation did not increase the growth rate. According to NRC (2001), heifers fed with dietary level of ME 124% gained higher growth rate than on other diets. Anjum et al. (2014) worked on the Sahiwal heifers with StairStep Feeding (consists of two rations having 20% below or 20% above of NRC energy levels) and reported that 100 % of animal came in puberty at 22 months of age than control (83%). Fiaz et al. (2012) reported that high dietary energy level (ME 124% of NRC) enhanced the growth parameter but adequate performance of Sahiwal heifers in terms of age of puberty was achieved even at low lower dietary energy level (ME 88% of NRC). The feed conversion efficiency was higher when the Sahiwal heifers were fed with the extra dietary energy than the recommendations of NRC (2001). He also reported that age of sexual maturity, age at conception and serum progesterone level were not influenced by the different dietary energy level. The supplementation of different energy levels in the diet of heifers is an effective key for the optimum growth rate from 13 to 18 month of age. The diet containing 16% protein and 3.0Mcal/kg energy is sufficient for the growth of red Sindhi calves (Javaid et al., 2014). The diet with proper concentration of nutrients reduces the age of maturity in buffalo heifers. The animals fed with green fodder along with of 2.0 Kg concentrate ration reach maturity earlier (727.77 ± 44.17 days) than the control group (993.33 ± 68.78 days) (Rafiq et al., 2008). Animals require specific minerals for the growth and skeleton development. The supplementation of minerals (Chaudary et al., 1991) and UMMB (urea molasses mineral blocks) (Garg et al., 1990) may be associated with early maturity. Phosphorus is involved in the many metabolic process and cellular metabolism in the body (Rasby et al., 1998), VFAs concentrations and bacterial population in the rumen (Zain et al., 2010). Previous studies revealed that optimal growth rates can be achieved when diets containing P level from 0.20% to 0.22% of DM (Tillman et al., 1959), 0.33% to 0.40% of dietary P concentration (Ferguson and Sklan, 2005) and Ca to P from 1:1 to 7:1 (Wise et al., 1963). Anjum et al. (1996) reported that the ration containing 0.75% Ca and 0.31% P on dry matter basis with Ca: P ratio 2.5:1 is more suitable for better weight gain in Nili-Ravi buffalo. Niacin plays a critical role in mitochondria respiration and the metabolism of carbohydrate, lipids and amino acids. Oral administration of niacin has resulted in increased microbial protein synthesis and higher weight gain in growing animals (Flachowsky, 1993). Contrary to this observation, Kumar et al. (2006) reported that supplementation of niacin at 100 and 200 ppm in the diet of buffalo calves had no significant effect on their growth and nutrient utilization. Use of higher levels of vitamin E in the diet improves the growth and skeleton development in the calves.
Key factor for puberty: The fundamental requirement for the onset of puberty is the secretion of a gonadotropin releasing hormone (GnRH) from the hypothalamus which stimulates release of gonadotropin hormone i.e. luteinizing hormone (LH). GnRH plays an important role in the regulating secreation of LH, follicular development, and secretion of steroid hormones (Figure 1). Madgwick et al. (1995) worked to know the effect of GnRH on sexual puberty in heifer calves from 4 to 8 weeks of age and concluded that GnRH treated heifers reached puberty earlier than control heifers (56.8 ± 1.7 vs. 62.8 ± 2.4 weeks) with high level of LH hormone (0.58 ±0.06 ng/ml vs. 0.41± 0.02 ng/ml) and greater number of LH pulses (2.0± 0.19 vs. 1.32± 0.12 pulses per 10 h.) than the control. The changes in the metabolic status cause changes in metabolic hormones leading to the onset of puberty. In case of bull calves increased level of LH causes early age of puberty (Evans et al., 1995), testicular development (Chandolia et al., 1997) and increased spermatogenesis cycle (Rawlings and Evans, 1995). Growth hormone (GH) has important role in growth and development during postnatal life. Growth hormone releasing factor (GRF) has an important role for activity of hypothalamus–pituitary–gonadal axis. Haldar and Prakash. (2006) worked on the Murrah buffalo with administration of 10 µg/100 kg body bGRF (Bovine gonadotropin releasing factor) to each animal and reported that GRF has a significant effect on the body weight, plasma progesterone concentrations and onset of puberty. Effect of GRF on GH and LH concentration and age at puberty are presented in Table 2. Buffalo heifers treated with bovine growth hormonereleasing factor (bGRF) showed puberty onset at an age of 887.5 ± 17.5 days (Mondal and Prakash, 2004). Plasma Insulin like growth factor I (IGF-I) has important role in regulation of cell growth, cell differentiation, cell function and immune function. Many authors reported about the role of IGF- I in growth of the cattle (Ortega et al., 2008; Lancaster et al., 2008). Laxmi et al. (2014) used fermented yeast culture (Saccharomyces cerevisiae) which stimulated IGF- I for growth of ruminal bacteria in low body weight Murrah buffalo calves. This is due to increasing FCE (feed conversion efficiency) and digestibility of essential nutrients. Long term administration of GRF causes faster growth in buffalo calves resulted in higher body weight due to increase plasma LH level (Mondal and Prakash, 2004). Progestogens have a role in initiation of oestrus and ovulation in prepubetral heifer. It is due to enhanced luteal function and stimulation of endocrine system. Polat. (2009) reported that PRID (progesterone releasing intravaginal device) 1.55 g of progesterone and 10 mg of oestradiol benzoate is effective on delayed pubertal heifer. Gulia et al. (2010) reported that the animals secreted high level of testosterone during the early growth period for attaining the early sexual maturity. So maintenance of testosterone during early phase of life is important. Leptin hormone produced mainly by adipose tissue has a role on onset of puberty, energy balance and feed intake. Vaiciunas. (2008) reported a higher leptin level regulates puberty in early-maturing Bos indicus heifers. Hormonal supplementation is helpful to reduce the age of sexual maturity in the Indigenous cattle and buffalo
Photoperiod can alter long-term physiological processes, particularly reproduction and production. Long day photoperiod hasten puberty and accelerates lean growth in dairy heifers. Perera et al. (1989) reported that light has no effect on growth (16.2 kg. vs. 20.8 kg.) but higher progesterone (0.41ng/ml vs. 0.19) and high prolactin level was observed in case of Surti buffalo. Long day photoperiod (LDPP) treated calves tended to have higher mean concentrations of PRL relative to SDPP (short day photoperiod) animals (11 ng/ml vs 5 ng/ml) (Rius et al., 2005). LDPP causes decline in the levels of melatonin which is important for the reproductive performance in the animals (Walker et al., 1996). Kassim et al. (2008) worked on the buffalo heifers, divided animals into two groups as G1 for long photoperiod in which heifers were exposed daily 16 hours of light and 8 hours darkness per day. The second group (G2) consisted of natural photoperiod of 8 hours light and 16 hours darkness daily. They reported that animals in G1 had higher values in live body weight than G2 at puberty and first ovulation
Variation of the season has an important role in the body weight of the animals. A positive relationship was observed between season and onset of puberty. Winter season is favorable for early puberty in dairy animals. The non-significant effect of season was observed in swamp buffalo at 6 month of age but significant effect was observed from 7 to 12 month of age (Das, 2004). Zaman (1996) also found a non significant effect of season on the body weight. During initial six month of life autumn and winter season has a positive effect for early weight gain and puberty. Effect of season is related with managemental and nutritional practices. Penchev et al. (2014) reported lower calving age in the heifers born in summer and autumn in Bulgarian Murrah heifers.
Exposure to male:
Biostimulation or male effect is the stimulus provoked by the presence of males, which induces estrus and ovulation through genital stimulation, pheromones or other external cues (Tiwari et al., 2014). Biostimulation is helpful for early puberty in males and females. Heifers exposed to bulls attained puberty at an earlier age than heifers that were not exposed to bulls. Presence of a vasectomised bull has been reported to hasten the onset of puberty in heifers (Rekwot et al., 2001). Roberson et al. (1991) reported that more number of heifers exposed to bulls came in puberty earlier than the non exposed heifers (61.8 % vs. 45.4%). Izard et al. (1982) reported that the urine treated heifers came in puberty earlier than the control group (222 ± 4.9 days vs. 277 ± 5.0 days). Biostimulation may be effective and economic tool to boost sexual maturity in animals. The knowledge of effectiveness, conditions and procedure are important for successful implementation.