Summer heat impacts negatively on livestock production inflicting heavy losses on farmers. The elevated temperatures during the summer detrimentally affect lactating sows performance, particularly voluntary feed intake . The decline in voluntary feed intake during heat stress has been associated with the negative effects on production parameters . In addition, high ambient temperature has been demonstrated to have a pronounced negative impact on milk production of lactating sows, which is correlated with reduced body weight (BW) of piglets at weaning [3,4]. Studies have shown that milk production is basically dependent on feed consumption to provide the required nutrients for lactogenesis . Not only feed intake depression, but also the reduction in the amount blood flow to the mammary glands are responsible for less delivered nutrients for milk synthesis resulting in reduced milk yield and decreased piglet growth [6,7]. Lactating sows mobilize a high rate of body reserves to compensate the adverse effects of heat stress on milk yield reduction, which lead to increased backfat and BW loss [6,8]. Moreover, the prolonged weaning-to-estrus intervals and increased incidences of anestrus in lactating sows are associated with heat stress .
There are various strategies that are aimed at reducing the negative impacts of high ambient temperature on feed intake during summer in intensive rearing systems such as redesigning the feeding program and changing the feeding time to the cooler phases of the day or at night [2,10,11]. Night feeding allows the heat of digestion to be produced during the cooler hours when it’s more easily dissipated. Previous study showed that the voluntary feed intake increased in cooler hours of the day in sows exposed to high ambient temperature . This facilitates increased uptake of nutrients to maintain milk production and inhibit mobilization of sow’s body reserves, which lead to prevent negative impacts in sow’s reproduction and piglets growth. The current study objectives were to determine the effect of extended feeding into the night in improving feed intake in lactating sows during summer season. We hypothesized that night feeding, when ambient temperatures are low will stimulate increased feed intake, thereby ameliorating the negative impacts of heat stress on sow’s production and reproductive abilities.
Materials and Methods
A total of 60 crossbred sows (Yorkshire × Landrace; average initial BW, 245.2 ± 17.2 kg) was allotted to one of two treatments on the basis of performance and parity (13 second parity, 23 third parity, 17 forth parity, and 7 fifth parity). Sows were divided into two groups, day feeding (2 kg per meal at 06:00, 11:00, and 17:00), and night feeding (1.5 kg per meal at 06:00, 11:00, 17:00, and 2 kg at 22:00). All sows used in the present study were artificially inseminated 2 times after the onset of oestrus, and pregnancy was detected and confirmed at d 30 post-breeding using a Pharvision B-mode ultrasound machine (AV 2,100 V; Ambisea Tech. Corp., Shenzhen, China). Sow backfat thickness (BFT) at the 10th rib, 6.5 cm from one side of the backbone was measured at farrowing, and weaning (d 25 of lactation) by using a medical imaging ultrasound (Loveland, CO; USA). Changes in BFT of sows during lactation were measured by calculating the difference between BFT at d 1 of lactation and BFT at weaning (d 25 of lactation). Standard litter traits such as the total number born and born alive, BW (kg) at birth, and weaning, growth rate (kg/d), and average daily gain (g/piglets) were recorded individually. Feed intake (kg/d) of each sow and weaning-to-oestrus interval (d) were also recorded. All the sows were fed a common diet according to the National Research Council  requirements for lactation (Table 1). Cross‐fostering was performed within 1 day of parturition. From 1 day after weaning, oestrus detection was performed twice-daily (08:00 and 16:30 h) for 10 min by boar exposure. The daily rectal temperature was recorded at 15:00 h during the study. The minimum, average, and maximum ambient temperatures observed in the conventional farrowing rooms (25.8 ± 1.4°C, 28.6 ± 1.9°C, and 31.1 ± 2.4°C respectively).
1) Supplied per kilogram of vitamin premix: vitamin A, 12,000,000 IU; vitamin D3, 2,400,000 IU; vitamin E, 132,000 IU; vitamin K3, 1,500 mg; vitamin B1, 3,000 mg; vitamin B2, 11,250 mg; vitamin B6, 3,000 mg; vitamin B12, 45 mg; pantothenic acid, 36,000 mg; niacin, 30,000 mg; biotin, 600 mg; folic acid, 4,000 mg.
On day 1 (post farrowing), and day 25 (weaning, after piglet removal) of lactation, 10-mL blood samples were collected by ear vein catheter before the morning feeding at 6:00 using a disposable vacutainer tube containing sodium heparin as an anticoagulant (Becton Dickinson, Franklin, NJ). Serum automatic biochemical analyzer (Fuji Dri-chem 3500i, Japan) was applied to measure concentrations of blood urea nitrogen, glucose, and triglyceride. Insulin, follicle stimulating hormone (FSH), luteinizing hormone (LH), and cortisol were measured by using kits (Endocrine Technologies Inc., USA) and their concentrations were determined in duplicate by ELISA using Biolog MicroStation system. The minimal detectable concentrations of insulin, LH and FSH were 0.5, 0.1, and 0.5 ng/ mL, respectively. After centrifugation (3,000 ×g for 20 min), plasma samples were separated and stored at −20°C and later analyzed for blood parameters.
Colostrum and milk were collected (30 mL) just after the birth of the first piglet and 10 days after farrowing respectively. The samples were for protein, fat, and lactose composition. Nutritional composition (fat, protein, and lactose content) was analyzed estimated by Milko-Scan 133B (Type 10911) within 24 hours. IgG content (mg/mL) was measured by Procine IgG ELISA Kit (E101-104, Belthyl Lab., USA).
Data generated in the present experiment were analyzed by the SAS statistical package (SAS 9.1, SAS Institute, Cary, NC, USA). The main effects of feeding types were determined by the Student’s t‐test. Initial BW of sows was considered as a covariate. A p-values ≤ 0.05 were considered statistically significant.
There was no difference in rectal temperature of sows between the treatments (data not shown), and the average rectal temperature was 39.3°C.
The effect of feeding frequency on feed intake and sow performance is shown in Table 2. Body weight of sows was not different among the treatments. However, sows in T2 treatment had lower (p < 0.05) BW change during lactation. The BFT change of sows were decreased in T2 treatment during lactation (p < 0.05). The daily feed intake differed significantly between T1 and T2 treatments with increased feed intake of the T2 treatment at 5.47 kg/d compared with 5.14 kg/d in the T1 treatment (p < 0.05). However, the weaning to estrus interval was not affected.
The effect of frequency of feeding on litter performance of lactating sows during summer season is shown in Table 3. There were no significant effects between T1 and T2 on the initial litter size, number of piglets weaned, survivability, and initial litter weight. There was a significant difference (p < 0.01) between T1 (70.50 kg) and T2 (74.34 kg) in total weight of weaned piglets. However, the average daily gain of piglets was not affected by the treatments.
The blood concentration of urea nitrogen, glucose, triglyceride, and creatinine were not affected (Table 4).
There were no significant effects of feeding time on FSH, LH, cortisol, and insulin level of serum (Table 5).
There were no treatment effects on total solid, protein, fat, and lactose content of colostrum (Table 6). The milk content of total solid, fat, and lactose was not differed between the treatments, however, there was a tendency (p = 0.085) for higher milk protein in sows in T2 treatment.
Both BW change and backfat change were decreased in the night fed group. Research has shown that sows under heat stress experienced a 13% decline in feed intake and a significant BW loss during lactation . This BW change can be attributed to mobilization of body lipids and protein reserves to cater for the increased energy demands of lactation and the net energy deficit due to reduced feed intake. Low nutrient intake in heat-stressed pigs fully explain the reduced gross/live carcass weight, which is in agreement with previous studies on pig  and cattle . In another study, the reduced feed intake resulted in declining backfat deposition during heat stress . This is thought to be an adaptive mechanism to allow for increased dissipation of heat through the skin in hot weather as fats insulate the body against heat loss which is deleterious under conditions of heat stress [3,6].
Heat stress impacts negatively on sows’ production and reproductive performance, particularly during the lactation phase of production. Basically, lactating sows are in a negative energy balance due to the increased energy demand and inability to consume enough feed to maintain the high energy requirements . This reduced appetite during heat stress is presumably a strategy to reduce metabolic heat production avoiding a further increase in body temperature. In a previous study it was demonstrated that feed intake of sows decreases up to 50% during heat stress . Furthermore, a decrease of feed intake in sows is thought to be a physiological mechanism aimed at controlling heat increment.
Various feeding strategies have been designed to ameliorate the effects of heat stress during summer season such as feeding in the cooler hours of the day. In the present study, night feeding improved feed intake in lactating sows during summer season. Our study showed a significant increase in feed intake in the night fed group. It has been suggested that feed intake tends to be increased with a decline in ambient temperature during early morning and late evening periods . In a study on multiparous large white lactating sows, it was confirmed that feed intake at nighttime was higher than daytime . It is hypothesized that night feeding allows the heat of digestion to be shifted to the cooler periods of the day when it is more easily dissipated. Therefore, a reduction in intensity of heat stress may positively affect feed intake.
The total weight gain of piglets was improved by the pattern of feeding in the current study. This may have been due to increased feed intake. Higher feed intake may be accompanied with higher nutrient availability for milk production, and possibly affecting piglet growth. As heat stress increases at daytime, the sows suffer more from depressed appetite, and further decrease may occur in the amount of available nutrient and energy for milk production. Similar results were also obtained by Silva et al. , where sows consumed less feed, produced less milk, and showed a lower litter growth during the hot season as compared to sows in the thermoneutral season. Reduced total weight gain in piglets from the daytime fed group may also be attributed to suckling behavior of these piglets. Due to reduced milk production, the piglets tend to have a more suckling frequency per day with an increased suckling interval than piglets from the night fed sows .
The results from our study did not show any significant difference in blood glucose between the daytime fed and the night fed groups at post farrowing and weaning despite the night fed group having a higher feed intake, whereas it has been reported that heat stress causes a reduction in blood sugar concentration in pigs because of the decline in feed consumption . In agreement, Farmer et al.  found no change in glucose concentration in sows that were exposed to heat stress. Changes in blood metabolites are important indicators during heat stress, however, the only differences were noted for a tendency in blood triglycerides with a numerically higher concentration in the night fed group, and a tendency for higher blood creatinine concentration in the daytime fed group, which may be attributed to the increased mobilization of muscle.
In the current study, no significant differences were obtained in total solid, protein, fat and lactose of colostrum and milk, although there was a tendency for the concentration of milk protein. Sows in the night fed group showed a numerically higher milk protein, likely due to higher feed and amino acid intake. Milk yield was not measured in the present study, however, it has been shown that sows with higher feed intake have greater milk production capacity and greater litter weight gain in comparison to sows with low feed intake .