The present study was conducted at Kapila Agro Farm, a commercial dairy farm situated at Timmareddypally village of Kondapak mandal in Siddipet district, Telangana State, located at 17° 58’ 25.7” North and 78° 51’ 56.5” East, at an altitude of 547 meters above the sea level. Analysis of blood and feed sample was carried out at ICAR-CRIDA (ICAR-Central Research Institute for Dryland Agriculture), Santoshnagar, Hyderabad.

Ambient air temperature and humidity were measured for calculation of THI. Dry bulb and wet bulb temperatures were recorded twice a day at 10.00 AM and 3.00 PM using dry and wet bulb thermometers placed about 1.5m above the ground level. The degree of heat stress was determined according to mean THI values measured. THI was estimated for the entire study period as per Bianca (1962). T H I = ( 0.35 × T d b + 0.65 × T w b ) × 1.8 + 32

Where, T_db : Dry bulb temperature and T_wb : Wet bulb temperature (⁰C)

The trial was conducted for a period of 90 days during peak summer season (April 15 to July 15) in the year 2017 on twenty four H.F calves with an average body weight 172.96±4.39 kg and aged 7-8 months. They were randomly distributed into four groups to contain six animals in each group. The group averages of body weights in all the four groups were as uniform as possible. Calves either received a basal diet devoid of any supplementation (T0) or were supplemented with chromium propionate @ 0.5 mg/kg DM (T1); vitamin E @ 500 IU/animal/day and selenium @ 0.3 mg/kg DM (T2) and chromium propionate @ 0.5 mg/kg DM, vitamin E @ 500 IU/animal/day and selenium @ 0.3 mg/kg DM (T3). All the calves were stall fed, reared under standard management conditions and housed in tie stalls with fans.

All the groups were fed total mixed ration (TMR) diet twice daily. The TMR contained chopped paddy straw, maize green (sweet corn with cobs and grains in milk stage) and concentrate mixture with roughage: concentrate ratio of 70:30. Fifty per cent of the daily allocation was provided in the morning feeding at 8.00 am and fifty per cent in the evening at 4.00 pm during the trial period. The basal diet was formulated to contain all the necessary nutrients to meet the nutrient requirements recommended by ICAR (2013). Ingredient composition of TMR for calves consisting of maize, wheat bran and soya bean meal, limestone powder, salt, mineral mixture and sodium bicarbonate is given in Table 1. They had free access to fresh and clean water all the time. The proximate analysis and fiber fractions of feeds was performed as per the procedures described by AOAC (2005) and Van Soest (1991) methods respectively.

Feed offered and feed refusal was monitored thrice weekly for each group of calves and feed intake was calculated. At the beginning of the experiment, all the calves were weighed using an electronic digital balance for three consecutive days in the morning before feeding and watering. Then calves were weighed individually at fortnight intervals before feeding and watering to observe the body weight changes for an experimental period of 90 days.

The total cost of experimental feeds per quintal was calculated on the basis of processing cost (electricity and labour charges) and prevailing market rates of the feed ingredients including paddy straw and maize green.

Samples of TMR were collected, dried overnight in a hot air oven at 100±5^◦C and then ground in laboratory Willey mill and preserved in airtight containers for further chemical analysis. The proximate analysis of feeds was performed as per the procedures described by AOAC (2005). Statistical analysis of the data was carried out according to the procedures suggested by Snedecor and Cochran (1994). Least-square Analysis of variance was used to test the significance of various treatments and the difference between treatments means was tested for significance by Duncan’s new multiple range and F Test (Duncan, 1955).

Temperature humidity index (THI) at the start of experiment (1^st week of April) averaged 75.83±0.22 in the morning (10.00 AM) and 80.44±0.85 in the afternoon (3.00PM) and the THI decreased with the decrease in environmental temperature in July to 72.25±0.31 and 76.69±0.48 in the morning and afternoon, respectively. The mean THI during 90 days of experimental period was 75.10±0.42 in the morning and 80.01±0.64 in the afternoon indicating the animals were under heat stress. Highest THI was found in the month of May with a THI value 83.48±1.41 in the afternoon.

The per cent of crude protein in the TMR fed to the growing calves was 13.90 percent on a dry matter basis (Table 1). In the present study, the CP values for the maize fodder and paddy straw were 8.68 and 3.4 percent, respectively.

The results indicated that the DMI, DMI per 100 kg BW and DMI per kg BW^0.75 were not significantly different among four groups of calves fed experimental rations but relatively higher DMI was found in the supplemented groups compared to control group. DMI increased with the increase in the body weight of the calves in all the groups (Table 2). The variation in environmental variables such as relative humidity and temperature seems to affect the feed intake of the control group calves. DMI per kg weight gain was significantly (p<0.05) higher in control group calves compared to the T3 group and the values were relatively higher than T1 and T2 indicating a positive effect of supplementation on growth performance. A number of reports confirm decreased sensitivity to stress in chromium supplemented animals through a reduced concentration of cortisol in blood (Abdelnour et al., 2019; Kumar et al., 2015). Mousavi et al. (2019) and Kargar et al. (2018b) reported that supplementation of chromium to dairy calves under heat stress resulted in increase in DMI. Similarly Chauhan et al. (2016) also reported increased average daily feed intake with vitamin E and selenium supplementation. Vitamin E and Se are considered crucial nutrients possessing various biological functions as antioxidants to prevent cellular injury triggered by reactive oxygen species (Mahmood et al., 2020). Vitamin E is a lipid-soluble molecule that acts within the lipid membrane to avoid the formation of hydroperoxides while Se is an important component of glutathione peroxidase to detoxify the hydrogen peroxide before damaging cellular molecules (Soliman et al., 2012). Both reduce oxidative stress and thereby increase performance (Kumbhar et al., 2018).

Statistical analysis of data revealed significantly (p<0.05) higher weight gain in calves supplemented with antioxidants compared to the control group. T3 group calves supplemented with chromium, vitamin E and selenium showed significantly (P<0.05) higher weight gain followed by T1 group supplemented with chromium alone and then T2 group supplemented with vitamin E and selenium. The calves fed T1, T2 and T3 ration also showed significantly (p<0.05) higher ADG compared to the control group. These results are in agreement with Soltan et al. (2012) and Abdoun et al. (2015) who reported a significant increase in weight gain in calves supplemented with chromium. Chromium supplementation improves growth rate and body weight by enhancing the absorption and digestion of nutrients (Ghorbani et al., 2012; Kargar et al., 2018a). Contrary to our results Mousavi et al. (2019) did not find significant difference in weight gain in calves supplemented with chromium. Sushma et al. (2015) and Chauhan et al. (2016) also reported an improvement in body weight in kids and lambs, respectively with vitamin E and selenium supplementation. Improvement in body weight in T1, T2 and T3 groups could be attributed to the mitigation of oxidative stress due to supplementation and thereby increase in DMI and feed efficiency.

In the present study supplemented diets were found to be more economical than the control group. Cost of feed per kg weight gain was approximately 6.81, 4.36 and 8.57 percent lower for T1, T2 and T3 ration than the T0 ration which might be attributed to better feed efficiency as a result of low oxidative stress. Similar findings were reported by Abdoun et al. (2015) who conducted a trial on male camel calves divided into three groups with a control group fed on total mixed ration and the other two treatment groups provided basal diet supplemented with 0.5 mg Cr/kg DM and 1.0 mg Cr/kg DM and concluded that chromium supplementation reduced feeding cost of producing a unit of weight by 11 %. Vinu et al. (2012) also found that the feed cost per kilogram body weight gain for the calves supplemented organic selenium was more economical than the control group.