The results show that during the hot-dry season, the afternoon and evening hours were most thermally stressful to the laying hens. The ambient temperatures and correspondingly the heat load rose progressively from daily nadirs (minimum) at early hours of the day, beginning from 06:00 h and attaining the peak (maximum) at the end of the day, at 18:00 h. Diurnal variations show that the overriding environmental factor affecting the hens was the ambient temperature. The finding was similar to the reports of Vathana et al. (2002), Ayo et al. (2007) and Minka and Ayo (2010) that ambient temperature is characterised by diurnal variations and it is the principal determinant of the metabolic parameters. The recorded trend in environmental temperature was similar to that obtained during the hot-dry season by Dzenda et al. (2013, 2015) that heat stress is prevalent throughout the day in the zone. Thus, the hot-dry season in the zone may be thermally stressful to poultry, starting from the early hours of the day, at the rising of the sun, to the end of daylight.
The upper limit of DBT (36.71 ± 0.6°C), which ranging from 35.0–39.0°C and recorded at 18:00 h, corresponded to a THI of 33.0°C (31.40–35°C) shows that the value was remarkably higher than the DBT upper limit of 24 or 26°C of the thermoneutral (comfort) zone for poultry in the temperate environment (Holik, 2009; Kingori, 2011). The value was also outside the range of 18–26°C, reported for poultry, reared under tropical environmental conditions (Ayo et al. 2010; Oluyemi and Roberts 2000; Fernandes et al. 2013). At 6:00 h the lowest DBT of 24.3 ± 0.5°C, corresponding to THI of 23.1°C fell within the range of the upper limit reported for poultry, and it was the only DBT value recorded that was strictly within the comfort (thermoneutral) zone (Tables 2 and 3). Thus, the laying hens during the hot-dry season in the zone were reared under the DBT values that were predominantly outside the thermoneutral (comfort) zone, indicating that the thermoregulatory mechanisms of the laying hens may be overtasked to near exhaustion in an attempt to ensure homeothermy (Altan et al. 2003; Minka and Ayo (2010). Such impairment results in a down-turn in physiological and performance parameters of the laying hens, indicating that the ambient temperature may have negative influence on the well-being and productivity of the laying hens (Ayo et al. 2011; Leinonen et al. 2014b).
The heat stress observed from the study period corroborates the reports of Ayo et al. (2007), Aluwong et al. (2017) and Makeri et al. (2017), who demonstrated the prevalence of heat stress in the zone during the hot-dry season. Oladele et al. (2003) reported the deterrent effects of heat stress on biochemical parameters in avian species in the zone, and Ayo et al. (2014a) documented some detrimental effects of heat stress on CT and performance parameters of pullets in the zone.
The present study showed that high ambient temperatures are inimical to egg production and supports the reports of Felva-Grant et al. (2012) and Leinonen et al. (2014), who demonstrated that high ambient temperature is a deterrent on fecundity in poultry. Obidi et al. (2008a, b) have demonstrated the deleterious effects of the hot-dry season on fertility and hatchability in breeder hens, and that the season depresses sperm concentration, motility and fertility in breeder cocks in the zone.
Panting recorded shortly after 9:00 h was heightened between 15:00 h and 18:00 h of the day due to sustained heat load. In some cases during the study, the laying hens were visibly going-off their feeders and concentrating activity around the drinkers, where they were observed to drink more intensively and rested under their feed augers. The observations corroborate the findings of Minka and Ayo (2010) and Mack et al. (2010, 2013), who reported stressful behavioural responses in birds exposed to heat stress.
The present study has shown that high ambient temperature, resulting in increase in thermal load in poultry, induced panting which is an indication of behavioural thermoregulation in the laying hens (Felver-Gant et al. 2012). Such behavioural response in domestic birds to heat stress has been reported by Mack et al. (2010, 2013), who demonstrated that laying hens subjected to heat stress spend less time feeding, but more time drinking and panting. They also spend more time with their wings elevated, less time moving or walking, and more time resting.
The results of the present study show that supplemental co-administration or individual inclusion of lycopene and vitamin E significantly supported fecundity in hens during the hot-dry season in the zone, when heat stress was prevalent. Similar results were obtained by Sahin et al. (2002), Olson et al. (2008) and Turk et al. (2010), who demonstrated that lycopene and vitamin E enhance ovarian function during heat stress. Although there was no significant difference between the treatment groups, antioxidant-administered laying hens recorded a relatively hen-day egg production than the control hens. The result, for the first time, has provided evidence on the beneficial role of the carotenoid lycopene and α-tocopherol in ovarian function in avian fertility in the zone. Few studies have demonstrated the significant potential of lycopene in enhancing immunity and reproductive efficiency in poultry (Olson et al. 2008; Turk et al. 2010). The co-supplementation of lycopene and vitamin E to laying hens during the hot-dry season resulted in the highest hen-day egg production throughout the experimental period. The result shows that lycopene- and/or α-tocopherol-treated laying hens maintained consistently hen-day production, amounting to over 6% weekly mean egg production, compared with the controls.
The the rate of decline in hen-day egg production was mitigated in lycopene- and vitamin E-administered laying hens in comparison with the control hens. This was, apparently, irrespective of the deterrent associated with old age of the laying hens and the thermal load acting on them during the hot-dry season. This finding supports the previous report on the antioxidant and/or antistress activities of vitamin E and corroborated by Vazquez et al. (2018), Lewis et al. (2019) and Liu et al. (2019), who reported the antistress and antiinflammatory effects of vitamin E in hens exposed to heat stress-induced conditions. Olson et al. (2008) and Yardibi and Turkay (2008) demonstrated the ameliorative effects of vitamin E on egg production, egg quality and nutrient utilisation in laying hens exposed to heat stress, thereby improving fertility and fecundity. The present study was conducted on laying hens, aged 41 weeks at the beginning of the study, when vital indices of fertility, including the rate of egg production and quality in hens normally start to decline due to age-related gonadal dysfunction and metabolic alterations, mediated by oxidative stress (Surai et al. 2001; Romero-Sanchez et al. 2007; Surai 2009). Such age-related ovarian functional depreciation is significantly associated with stress-mediated compromise in mitochondrial function due to the susceptibility of the ovarian structural lipoproteins to oxidative stress (Burstein et al. 2009; Bentov et al. 2014; Ben-Meir et al. 2015).
The result shows that lycopene ameliorated the effect of heat stress on laying hens and improved hen-day egg production. Although the mechanism of action of lycopene was not elucidated in the present study, the reports of Sahin et al. (2002) and Yardibi and Turkay (2008) demonstrated lycopene’s antioxidant activity in avian species and its beneficial biological intervention in stress-induced ovarian failure. Bakker et al. (2010) showed the anti-inflammatory and antioxidative roles of a dietary mixture of vitamin E and lycopene. Since most physiological anomalies, including disease conditions and reproductive (gonadal) dysfunctions, are mediated by inflammatory and oxidative stress mechanisms, the findings of the present study on lycopene and α-tocopherol in alleviating heat-induced stress and reproductive losses during the hot-dry season may also be due to their anti-inflamatory and antioxidative properties (Liu et al. 2019; Pitargue et al. 2019).
The remarkable decrease in eggshell thickness and eggshell weight in control laying hens, suggests a decrease in calcium mobilisation and utilisation during the heat stress. The results corroborate previous reports (Lin et al. 2004; Periera et al. 2008; Melesse et al. 2010; Mack et al. 2013) that heat-induced stress adversely affects qualitative indices of eggshell in laying hens. Eggshell quality is an important index of overall egg quality, associated with the efficiency of calcium mobilisation and deposition on eggshell in the process of egg formation. This physiological aptitude of the laying hens reduces during unfavuorable conditions, including excessive increase in environmental temperatures (Hafeez et al. 2016; Lee and Han, 2018; Klasing and Korver, 2020). The results show that lycopene and vitamin E ameliorated the deleterious effects of heat stress on the laying hens in the zone, which suggests that the antioxidants may improve qualitative indices of the egg in the laying hens, exposed to environmental heat stress in the hot-dry season in the zone. The observation supports the reports of Olson et al. (2008) and Sahin et al. (2008, 2016) that lycopene supplementation alleviates heat-induced stress on performance parameters of domestic birds, including egg qualitative indices. Similar report by Scheideler et al. (2010) demonstrated the beneficial effect of vitamin E on egg production, qualitative eggshell indices and integrity in breeder hens, subjected to high ambient temperature. The combined effect of vitamin E and lycopene in alleviation of stress indices in domestic chickens exposed to high oxidant conditions has been demonstrated by Lu et al. (2014).
In conclusion, thermal environment conditions during the hot-dry season in the Northern Guinea Savannah zone induced heat stress, adversely affecting domestic laying hens. The thermal environmental conditions exerted negative effects on the laying hens via impairment in egg qualitative indices. Lycopene and vitamin E alleviated the risk of adverse effects of heat stress on their productivity during the hot-dry season by increasing hen-day egg production and improving eggshell qualitative indices.