Although no differences were detected among DIM at the start of treatment, the highest DIM was obtained in GV cows with 140.10±6.38 days, while the lowest DIM was obtained 127.30±5.35 days in GI on the contrary. Besides that, parity number of the GII (2.27±0.09) was lower than GV (2.84±0.13). Group I animals had the highest milk yield (32.48±1.10 l/day) and lowest milk yield was obtained in GIII. The milk yield was in the G III lower than in GIV and GV but the BCS was higher in GIII than in both groups (P<0.05; Table 1).
When the fertility parameters of all cows included in the study were examined, regardless of DIM, highest CR was obtained in GIII (50.00%). Although no differences were found in FSPR, P/AI and EL between the groups (Table 2). Besides, it was observed that there was a statistically significant difference in FSPR, P/AI and EL parameters in GI (P<0.01 and P<0.001) when cows were evaluated among DIM (≤150 and >150). Significant differences were found in GII in terms of EL (P<0.001) and in GIII in terms of P/AI (P<0.001) (Table 3). Conception rate in the GIII group was found to be significantly higher (P<0.002 and P<0.05) than both GII and GV groups, with a rate of 57.69% in cows with ≥3 parity. Conception rates in both GII and GIII groups were significantly higher than GV group, with 50.00% and 66.67% in cows with a milk yield of >30 l/day (P<0.05). Furthermore, CR were found to be statistically significantly higher, again with the rates of 44.68% and 50.00%, in the GII and GIII groups than in the GV group in dairy cows with <2.75 BCS (P<0.01; P<0.05). In addition, while a significant difference was found between the CR of dairy cows with <3 and ≥3 parity in GII with 44.44% and 23.53% (P<0.05), it was determined that CR was significantly lower in cows with <2.75 BCS than those with ≥2.75 in GV (Table 4).
It is a well-known strategy to use GnRH and hCG for the improvement of pregnancy rates in cows at the time of AI or after AI. Most of studies are based on improving ovulation rates and luteinization of the follicles to increase the P4 levels for supporting the embryo for a successful pregnancy (Paksoy and Kalkan, 2010;
Khoramian et al., 2011; Nascimento et al., 2013; Mendonca et al., 2017; Cunha et al., 2021).
For this purpose, the gonadotropins are used in the studies were administrated between the 5
th and 7
th days of oestrus or early pregnancy, during the period when the follicles are sensitive to gonadotropins
(Vasconcelos et al., 1997; Santos et al., 2001; Beltran and Vasconcelos, 2008;
Paksoy and Kalkan, 2010;
Shephard et al., 2014; Agarwal et al., 2021). GnRH administrations are effective in the presence of endogenous LH release (
De Rensis et al., 2010) but hCG has a longer half-life
(Rizkallah et al., 1969) and can stimulate luteinization even in follicles smaller than 10 mm
(Sheffel et al., 1982). It also stimulates P4 production by stimulating the transformation of theca and granulosa cells into small and large luteal cells and small luteal cells into large luteal cells
(Farin et al., 1988; Sianangama and Rajamahendran, 1992).
In this study, it was aimed to investigate how the above-mentioned effects of GnRH and hCG would have an effect on fertility parameters in cows with different regimes. Some of the studies stated that GnRH and hCG administrations both on day of insemination and on day 7 after insemination did not affect the pregnancy rates
(Vasconcelos et al., 1997; Buttery et al., 2007; Agarwal et al., 2021) similar to our findings. However, when the data were evaluated according to the DIM, parity, milk yield and BCS of the cows, there were statistical differences between the groups. Especially, when animals with a milk yield of ≥30 /day or ≥3 parity or >150 DIM or <2.75 BCS are evaluated separately, conception rates were significantly higher in GIII (hCG; d0 and d7) than GV (placebo; d0 and d7).
According to
Mendonca et al., (2017) GnRH administration at the time of insemination did not affect the P/AI and the rates of embryonic losses. In our study, P/AI was found to be highest in the GI group given hCG on d0 in ≤150 DIM cows. Also, in the same group P/AI was significantly higher cows with ≤150 DIM than >150 DIM, 0.91 and 0.33 respectively Many researchers reported that hCG administrations after insemination stimulates the formation of primary and accessory corpus luteum and causes an increase in luteal diameter and luteal area, but not affected the pregnancy rates (
Sianangama and Rajamahendran, 1992;
Vasconcelos et al., 1997; Nascimento et al., 2013; Shephard et al., 2014). Similarly, in a study where hCG administrated at day 0 with insemination or on one of post insemination days; 0, 7, 14 or together, an increase in luteal diameter and area was detected, but no difference was reported in pregnancy rates
(Agarwal et al., 2021). increasing the dose of hCG, causes an increase in the stimulation of ovulations, luteal volume and P4 levels
(Cabrera et al., 2021). Considering the long half-life of hCG
(Rizkallah et al., 1969), it is thought that the high conception rates obtained in GIII after may be depending on the higher doses of hCG due to repeated administrations.
Pryce and Løvendahl (1999) reported that relationship between milk yield and BCS is depending on the association between BCS and energy balance and tissue mobilization, furthermore genetically, there is a moderate to strong negative correlation between milk yield and BCS. Cows with <2.75 BCS had higher milk yield in this study. Conception rates obtained among BCS was found higher in animals with BCS <2.75 and in the group in which hCG was administered twice in agreement with
Santos et al., (2001). Contrary to the findings of the same author, a higher rate was found in animals with high milk yield (≥30 l/day) in GIII compared to GnRH (GI) and placebo (GV) treatment.
Rethmeier et al., (2019) reported that conception rates decreased compared to data from Germany between 1996 and 2002 and the USA in 2012 and the target for this rate was below 35%, regardless of milk yield or herd size. When the conception rates obtained in this study were evaluated according to DIM, parity, milk yield and BCS, they were found to be 43.59% and above in GIII and these data suggest that double-dose hCG administration increased CR in dairy cows with ≥3 parity or ≥30 l/day milk yield or <2.75 BCS. The findings of this study showed that hCG administrations in d0 and d7 caused an improvement in conception rates, considering the parity (≥3), milk yield (≥30 l/day) and BCS (<2.75) parameters.
Earlier pregnancy diagnosis after insemination is economically important for shortening service period
(Abdullah et al., 2017). It has been reported that as the day of pregnancy diagnosis increases after insemination, embryonic losses gradually decrease
(Vasconcelos et al., 1997; Santos et al., 2004, Mahajan et al., 2022). hCG is used for synchronisation of the estruses cycle, induction of ovulations or to prevent early embryonic losses
(Buttery et al., 2007, Beltran and Vasconcelos, 2008,
Rahman et al., 2017). It is thought that the pregnancy loss rates of 7.14% and above detected in animals with >150 DIM in GII, GIII and GIV in this study may be related to the fact that pregnancy controls were performed on the 30
th and 45
th days after insemination in this study. In this study, it was found that double-dose hCG administrated dairy cows with ≥3 parity in GIII had higher CR than placebo treatment in contrast with the studies in which the effects of gonadotropins, either at the time of insemination or after insemination, were used seasonally
(Vasconcelos et al., 1997; Beltran and Vasconcelos, 2008;
Mendonca et al., 2017) or in embryo transfer programmes
(Vasconcelos et al., 1997) or only to increase pregnancy rates
(Bartolome et al., 2005; Nascimento et al., 2013; Agarwal et al., 2021), presented that administrations helped to improve the luteal structure and increase P4 synthesis.
