Age at first calving (AFC) is a greatly important economic trait determining the profitability of intensive dairy operations, therefore, one strategy for reducing the costs of milk yield could be the shortening of the rearing period. In zones with intense and prolonged warm weather calves' growth rate is reduced ( López et al. , 2018 ), and therefore, could prolong AFC, an important variable for measuring reproductive behavior in dairy cows ( Oikawa, 2017 ).

The optimum AFC in Holsteins for maximum profit and without negative effects on milk yield and health during their lifetime should be 23 to 24.5 months of age ( Ettema & Santos, 2004 ; Nilforooshan & Edriss, 2004 ). Moreover, reducing AFC can reduce rearing costs and increase the number of calves per cow ( Tozer & Heinrichs, 2001 ). However, a reduction in AFC in first-parity Holsteins cows has been associated with a lower first-lactation milk production ( Ettema & Santos, 2004 ; Curran et al. , 2013 ) and increased dystocia ( Zaborski et al. , 2009 ; Gaafar et al. , 2011 ), which commonly occurs when the heifers are under 22 months old. Thus, dystocia is a negative factor for both reproduction, health and body weight ( Moussavi & Mesgaran, 2008 ). Dystocia is a serious problem in most dairy farms because it reduces milk production by the disorders that result from this reproductive problem ( Mee, 2008 ). Difficult calvings increase uterine infections ( Dubuc et al. , 2010 ) and reduce reproductive efficiency ( Gaafar et al. , 2011 ). Thus, complications at birth result in loss of milk production, reproductive behavior ( Mokhtari et al. , 2016 ), increased days open ( Gaafar et al. , 2011 ), as well as mortality and morbidity in calves ( Bicalho et al. , 2007 ; Lombard et al. , 2007 ).

The combined effect of AFC and duration and intensity of heat stress on periparturient disorders and reproduction performance of heifers has not been studied in depth. Also, until now there have been limited studies to assess the impact of AFC on the reproductive tract health postpartum in dairy heifers. Therefore, it was considered relevant to elucidate the impact of AFC and climatic conditions during parturition on the reproductive performance of Holstein heifers. The objective of this study was to assess the impact of AFC and heat stress at calving on disorders derived from parturition as well as the reproductive performance in first calf Holstein heifers in a warm arid environment.

Table 1 shows the effects of AFC and THI on the occurrence of abortions, dystocia and preterm calves in first-lactation heifers. Regarding abortion rate, no significant differences were found ( p >0.05) between THI levels and AFC. In the area where this study took place, heat stress has been identified as an important risk factor for fetal losses in lactating Holstein cows ( Mellado et al. , 2016 ). The discrepancy between these studies seems to derive from the fact that during heat stress, days in milk (DIM) is barely reduced in heifers ( West, 1999 ) and less metabolic heat is produced com­pared with pluriparous cows ( Baumgard & Rhoads, 2013 ). In fact, in primiparous cows, severe heat stress does not affect milk production ( Avendaño-Reyes et al. , 2009 ). The abortion rate observed in the present study (13.6%) was higher than the 4.8-9.8%% reported for Holstein heifers in temperate climates ( Ettema & Santos, 2004 ; Bach, 2011 ).

Dystocic parturitions increased when parturitions occurred with severe heat stress (>83 units). Conflicting results have been described in the literature regarding the effect of heat stress on difficult parturitions. On one hand, some authors ( Gaafar et al. , 2011 ; Atashi et al. , 2012 ) observed the lowest dystocia frequency with calvings occurring in summer. On the other hand, Neamt et al. (2015) observed the highest incidence of this reproductive disorder in summer. The average incidence of dystocia observed in the present study was 4.6%. This value is much lower than the 7-11% registered by other researchers in Holstein cows ( Berry et al. , 2007 ; Atashi et al. , 2012 ). The fact that the younger heifers at first calving did not increase the odds of dystocia suggest that with a good nutritional level and consequently a good body development at parturition, dystocia can be reduced in heifers calving earlier than 2 years of age. Gardner et al. (1988) also addressed this issue and found that calving difficulty was not different between heifers on an elevated or restricted amount of energy from 6 weeks of age until breeding. Likewise, Berry & Cromie (2009) observed that heifers calving younger for the first time had not a greater likelihood of requiring some assistance at calving.

Preterm births were affected neither by AFC nor by THI >83 at calving. The lack of effect of heat stress at parturition on the occurrence of preterm births came as a surprise because late-gestation heat stress hampers placental growth, which results in fetal malnutrition and hypoxia ( Tao & Dahl, 2013 ) and these are some of the etiologies that contribute to preterm labor ( O'Brien & Lewis, 2016 ).

Premature activation of the fetal hypothalamic-pituitary-adrenal axis may result from fetal phy­siological stress (for example, insufficient utero-pla­cental blood flow; Laburn et al. , 2017 ). The critical mediator of stress-induced premature births appears to be corticotropin-releasing hormone ( Challis et al. , 2001 ), which leads to massive release of prostaglandin F2α (PGF2α), which triggers luteolysis and initiates parturition in the bovine ( Attupuram et al. , 2016 ).

Additionally, heat stress results in higher cortisol concentrations in plasma in cows ( Chen et al. , 2018 ) and cows giving birth prematurely have 100% higher plasma cortisol levels on the day of parturition than cows giving birth at term ( Patel et al. , 1996 ). In fact, exposure of cows to elevated ambient temperature resulted in shorter gestations ( Wright et al. , 2014 ).

Despite the fact that premature calves are smaller and weaker than full-term calves, these calves were viable at birth with few signs of prematurity such as reduced ability to stand or suckle. These results indicate that other factors apart from THI at calving and age of heifers at calving triggers the preterm par­turition. Thus, preterm birth is not a condition in heat-stressed dams, but there are some other causes behind it, particularly infectious diseases ( Kemmerling et al. , 2009 ; McAllister, 2016 ); however, this was not considered in this study.

Reproductive performance of first calf heifers according to AFC and climatic conditions at first parturition is presented in Table 2. Across THI values, heifers with AFC <2 or 2.0-2.2 years had higher ( p <0.01) conception rates at first service than late-calving heifers. These results are partially in line with data of Ettema & Santos (2004) who reported higher conception rates at first AI in heifers calving between 701 and 750 days of age compared to heifers calving outside of this age range. Thus, it seems that heifers in the present study were reared to achieve an adequate body condition score at first calving and were fed a high energetically dense diet in early lactation which allows them to have a better conception rate at first AI compared to older heifers at first calving.

There was an AFC by THI interaction ( p <0.05). The conception rate at first service of <2-year-old first calving heifers and 2-2.2-year-old first calving heifers was higher at THI <77 than >2.2-year-old first calving heifers; whereas conception rate at first service was similar for all age groups at THI >83 at calving. Across AFC groups, conception rate at first service was 17% lower in heifers with severe heat stress at calving compared with heifers with THI <77 units at parturition. There exists a wealth of data describing the negative effect of heat stress on fertility of dairy cows ( De Rensis & Scaramuzzi, 2003 ), particularly there are clear seasonal patterns of conception rate in dairy cows ( Mellado et al. , 2013 ). Some cows in the present study probably were no longer exposed to heat stress by the time they were bred by the first time, but heat stress was present the following months ( Roth, 2008 ) and this explains the drastic reduction in conception rate in heifers calving under severe heat stress.

Across THI at calving, conception rates for all services were 13% lower ( p <0.01) in >2.2-year-old first calf heifers compared with 2-2.2-year-old first-calf heifers. These results are at odds with other researchers who have found a reduced fertility in younger calving heifers as compared with heifers calving for the first time at an older age ( Ettema & Santos, 2004 ; Berry & Cromie, 2009 ). Given that first-calving age, growth rate, and body weight at first calving are generally correlated ( Le Cozler et al. , 2008 ), these results suggest that the younger heifers at parturition did not present smaller live-weight at calving and adequate nutrients were available for the reproductive function. There was a THI by AFC interaction ( p <0.05) for all-service conception rate. All age groups had fairly similar conception rate at THI >83 units. Variation of services per conception was not related to AFC and THI at calving (Table 2). Regardless of AFC and THI at calving the mean ± SD services per conception was 5.3 ± 3.6, which reflects a reduced reproductive performance of this herd during prolonged thermal stress in the study site.

Pregnancy rate of first calf heifers to first service and to all services was not influenced by technician ( p >0.10). Average pregnancy rate after calving to all services achieved per technician ranged from 44 ± 3% to 47 ± 2%. These results emphasize the importance of constantly monitoring the performance of the AI technicians to make sure they deliver the best possible results. Significant ( p <0.05) variation in all-services conception rate was associated with year of service (42.3% for 2016 and 46.7% for 2017), which could be attributed mainly to differences between years in the quality of feed, management practices, and ambient temperature. Variation of services per conception was not related to year of service.

The occurrence of periparturient disorders and culling rate of first calf heifers as a function of THI at calving and AFC is presented in Table 3. Retained placenta was associated neither with THI >83 units at calving nor with AFC. Other reports have found a greater incidence of retained placenta in summer compared to other seasons ( Ahmadi & Mirzaei, 2006 ; Hossein-Zadeh & Ardalan, 2011 ). The occurrence of metritis was not associated with heat stress at calving but heifers calving at younger ages had a higher (11.8%; p <0.01) incidence of this reproductive disorder than heifers calving at 2.0-2.2 years (8.0%) and heifers calving >2.2 years of age (8.3%). The reason for the difference in uterus health between the age groups is not known, but differences in physiology, immune status, or both may be of importance. It is, therefore, possible that immune variables may differ between primiparous cows of different AFC.

The occurrence of ovarian cyst was not associated with thermal stress at calving but this reproductive disorder was two-fold higher ( p <0.01; Table 3) in hei­fers calving at <2.2 years of age compared to heifers calving >2.2 years of age. It is no clear why heifers calving before 2 years of age were more susceptible to ovarian cysts. Probably younger heifers at first calving are more prone to disruption of the hypothalamo-pituitary-gonadal axis, by endogenous and/or exo­genous factors causing cyst formation ( Vanholder et al. , 2006 ).

Culling rate of heifers was associated with both thermal stress at calving and AFC. There was a heat stress by AFC interaction with the higher proportion of culled cows for <24 month old first-calving heifers at THI 77-83 units at calving, whereas heifers other than heifers <2.0 years of age presented the highest culling rates at THI >83 units at calving. In agreement with the present study, other researchers ( Nilforooshan & Edriss; 2004 ; Berry & Cromie, 2009 ) reported a decrease in productive life as AFC increased. Heifers calving at an older age have greater non-esterified fatty acids (NEFA) and β-hydroxybutyric acid (BHBA) blood levels, and lesser insulin, glucose, and urea nitrogen blood values compared with heifers calving at a younger age ( Nyman et al. , 2008 ). Thus, heifers calving at an older age are in more severe negative energy balance in early lactation, due to the greater milk production than cows calving at a younger age ( Nilforooshan et al. , 2004 ).

Higher culling rates in heifers calving under ther­mal stress were more likely to have lower conception rates and to be culled due to infertility. The Kaplan-Meier plot showing the relationship between AFC and lactation length is shown in Fig. 1. This suggests a difference in lactation length between heifers calving <2.0 years, 2.0-2.2 years and >2.2 years of age for the first time. Only heifers calving <2.0 years of age were not culled beyond 1000 days of lacta­tion. On the other hand, some heifers calving for the first time at 2.0-2.2 and >2.2 years of age were culled beyond 1200 days of lactation. Given that non-planed extended lactations are associated with infertile cows ( Mellado et al. , 2016 ), these data suggest that more late-calving heifers were unable to get pregnant and consequently some of them were milked for over 35 months.

These results yield evidence of a substantial nega­ti­ve effect of delayed age at first calving (>2.2 years) in first calf Holstein heifers in a hot environment on dystocic delivery, pregnancy rate at first-service, conception rate at all services and culling rates. Also, these data showed that, overall, early first calving (<2.0 years) was not associated with reduced reproductive performance, although <2.0 year old hei­­fers at first calving presented a higher incidence of metritis and ovarian cysts. Thus, these results support the view that in order to reduce the total costs of rearing, dairy farmers in intensive system can lower their heifers' first-calving age by earlier inseminations, without compromising assistance at calving and reproductive performance. Finally, these results reaffirm the ne­gative association between heat stress at calving on reproductive performance and culling rates of first calf Holstein heifers.