Reproductive performance and the effect of non-genetic factors
The main traits used to assessing reproductive performance of dairy cows are age at first service (AFS), age at first calving (AFC), calving interval (CI), days open (DO) and number of services per conception (NSPC). The overall least-squares mean for AFS, AFC, CI, DO and NSPC were 22.93±0.22 months, 32.95±0.22 months, 494.16±3.68 days, 221.09±3.73 days and 1.99±0.03, respectively.
Age at first service (AFS)
The least-square means and standard errors for AFS are summarized in Table 1. The AFS in this study is comparable with the report of
Tadesse (2006) for pure Jersey breed. However, it is lower than the report of
Lemma et al., (2010) for Jersey breed,
Eshetu (2015) for Jersey x Horro and
Dinka (2013) for Horro x Jersey F1. The early AFS in this study could be due to the selection of replacement heifers and special care given for them to make a replacement of the nucleus breeding herd.
Year and season of birth had a significant (P<0.0001) effect on AFS. This result is in line with the finding of
Getahun et al., (2019). The lowest AFS was recorded for animal born during 2015-2016 followed by 1985-1987 and 1994-1996 (Table 1). Cows that born during 2006-2008 require 11 more months to attain AFS than those during 1985-1987 period. This might be due to selection, environmental effect and inconsistency of management level across the years. Birth season exerted a significant (p<0.0001) influence on AFS. On the contrary, a non-significant influence of birth season on AFS was reported elsewhere (Tadesse, 2006;
Haile et al., 2009; Lemma et al., 2010; Getahun et al., 2019). In this study, cows born during the dry and main rainy seasons attained AFS of 2.34 and 1.45 months earlier than those born during the short rainy season, respectively (Table 1). This could be explained by the environmental effect and inconsistency of management (variability of forage quality and quantity across the season as natural pasture is the main source of feed).
Age at first calving (AFC)
The overall least squares mean AFC was 32.95±0.22 months (Table 1). The result obtained in this study is comparable with the report of
Tadesse (2006) for Jersey breed. However, it is lower than the report of
Lemma et al., (2010) for Jersey,
Tadesse (2006) for Holstein Frisian (HF),
Mengistu et al., (2016) for HF and Goshu
et al.,
(2014) for HF. This variation could be due to breed differences, variability in feeding management, heat detection and the time of insemination especially for heifers, health control and climate variation. According to Kumar and
Tuki (2014), the desirable AFC in local breeds is 3 years and 2 years in crossbred cattle. The AFC in this study is prolonged than this standard and this could influence both the productive and reproductive life of the female due to less number of calving.
Age at first calving (AFC) was significantly (p<0.0001) affected year and season. This result is in agreement with the finding of
Hunde et al., (2015) and
Getahun et al., (2019). The earlier AFC was recorded for animals born during 2015-2017 followed by 1985-1987 and 1994-1996. In this study, cows born during the dry and main rainy seasons attained AFC of 2.67 and 2.03 months earlier than those born during short rain season, respectively (Table 1). However, a non-significant influence of birth season on AFC was reported in previous studies
(Haile et al., 2009; Lemma et al., 2010; Tadesse et al., 2010; Yalew et al., 2011; Belay and Chakravarty, 2014;
Getahun et al., 2019).The influence of year and season in this study could be due to management fluctuation among years and seasons.
Calving interval (CI)
The least-squares mean of CI for the levels of fixed effects are summarized in Table 2. The CI in this study is lower as compared to the report of
Tadesse (2006) for Jersey breed,
Hunde et al., (2015) for Jersey breed,
Getahun et al., (2019) for 50% F2 Friesian x Boran and
Effa et al., (2006) for 75% Jersey crossbreds. However, this result is higher than the finding of
Lemma et al., (2010) for Jersey breed,
Haile et al., (2009) for Holstein Friesian x Boran and
Getahun et al., (2019) for Holstein Friesian x Boran. In general, the average CI in this study is higher than the optimum values (356 days) desirable for profitable milk production. The differences could be explained by breed genetic potential, seasonal availability and quality of feed, climate and other management factors like heat detection, the skill of AI technicians and the quality of semen used for insemination.
Farm-bred cows had a shorter calving interval (466.68±3.66 days) than the imported cows (521.64±8.42 days) (Table 2). This result is in agreement with
Lemma et al., (2010) for Jersey breed. The superiority of farm-bred cows could be attributed to the sample size used, genetic to environmental interaction and adaptation problem as the animals were exposed to a new environment which may affect the expression of their genetic potential.
The significant (p<0.0001) influence of the calving period on CI observed in this study is in agreement with
Tadesse (2006) and
Hunde et al., (2015) for Jersey breed. The lowest CI (414.97±8.64 days) was observed for cows calved during 1989-1991 followed by cows calved during 2001-2003 (463.47±8.43 days). An extended CI (568.15±8.71 days) was observed for cows calved during 1992-1994 than those other years. The calving season was a significant (p<0.05) sources of variation for CI. This result is in line with the report of
Birhanu (2014) and
Getahun et al., (2019) for Holstein Friesian x Boran. However, a non-significant influence of calving season on CI was noted in literature (
Tadesse, 2006;
Tadesse et al., 2010; Hunde, 2012). Parity did not have a significant effect on CI. The differences across season and year could be attributed to differences in management practices and climatic variables.
Days open (DO)
The overall least square mean of DO was 221.09±3.73 days (Table 2). The DO in this study is lower than the report of
Getahun et al., (2019) for 75% F1 Friesian crossbreds (244±14.0 days), but higher than
Mengistu et al., (2016) for HF (139.58±7.91 days) and report of
Haile et al., (2009) for Boran and its crossbred with Friesian in Ethiopia (141±19 days). In general, the DO in this study is higher than the optimum values (85 days) desirable for profitable milk production. Feed shortage, lack of proper heat detection and silent estrus could be the possible reason for longer days open.
Days open was significantly affected by animal group. Farm bred cows had shorter days open (187.50±3.74 days) than the imported animals (254.69±8.52 days). This could be attributed to the sample size used, genetic to environmental interaction and adaptation problem as the animals were exposed to a new environment which could create more difficulty to express their genetic potential.
The significant effect of the calving period observed in this study is in agreement with
Hunde et al., (2015) for Jersey breed and
Getahun et al., (2019) for HF x Boran. Animals calved during 1989-1991 had shortest DO followed by 2001-2003 and 2007-2009 (Table 3). The longest DO was observed for cows calved during 1992-1994. This is due to inconsistent management across calving years. The calving season did not have a significant effect on days open. This result is agreed with other findings (
Tadesse, 2006;
Haile et al., 2009; Tadesse et al., 2010; Hunde, 2012;
Mengistu et al., 2016). The significant effect of parity on DO observed in the present study is consistent with the study of
Tadesse (2006),
Lemma et al., (2010) and
Hunde (2012) for Jersey breed. Days open in the sixth parity was significantly shorter than the second, third, fourth and fifth parities.
Number of service per conception (NSPC)
According to Mukasa-Mugerewa (1989), the NSPC higher than 2 is considered poor. The mean NSPC for Jersey cattle in this study was 1.99±0.03 (Table 3). This result is slightly higher than the report of
Mengistu et al., (2016) for HF,
Teketay et al., (2015) for HF x Arsi and the report of
Lemma et al., (2010) for Jersey. However, it was lower than the report of
Tadesse (2006) for Jersey breed and
Haile et al., (2009) for Boran and Boran x Friesian. Herd management and performance of cows are associated with lower services per conception and could be the possible reason for the inconsistency of results across studies.
The NSPC was significantly affected by service period (p<0.0001), parity (p<0.0001) and service season (p<0.01). The lowest NSPC was observed in animals served in during 1987-1989 followed by those served in during 2017-2018. However, the highest service per conception recorded was during 1993-1995. The difference across periods of service could be due to variation of heat detection, skill of inseminator, time of insemination, inconsistent management and environmental variability.
Cows inseminated during the main rain season required more service than dry and short rain seasons. The significant effect of service season on NSPC was agreed with the report of
Deberga et al., (2009) and
Teketay et al., (2015). Shortage of feed might be the main reason for repeat breeding of cows during the main rain season. The significant effect of parity on NSPC observed in the present study is in agreement with the finding of
Hunde (2012) for Jersey breed. The lowest NSPC was observed in the first parity (1.71±0.06) and the highest NSPC was recorded on 2
nd parity (2.16±0.06). However, statistically there were no significant differences between 3
rd, 5
th and ≥ 6
th parity (Table 3).
