The feeding behavior of growing-finishing pigs may influence nutrient digestibility, performance and carcass traits (de Haer & de Vries, 1993a; Andretta et al., 2016; Carcò et al., 2018). Feeding behavior may be defined by variables such as average daily feed intake (ADFI), total number of feeder visits per day (TV), time spent eating per day (TD), visit size (VS) and feeding rate (FR) among others (de Haer & Merks, 1992). It is known that the feeding behavior of pigs is affected by many factors such as housing conditions (de Haer & de Vries, 1993a; Bornett et al., 2000), diet composition (Brouns et al., 1994), feed form (Li et al., 2017), ambient temperature (Quiniou et al., 2000), sex (Andretta et al., 2016) or age (Carcò et al., 2018). For instance, as pigs grow, ADFI, meal size (MS) and FR increase, whereas small variations or decreases in TV and TD have been reported with differences in the average and the evolution of those feeding behavior patterns among studies (Fornós et al., 2022).

On the other hand, it has been demonstrated that providing pelleted feed to pigs enhances pig performance compared to feeding the same diet in mash owing to an improvement in nutrient digestibility or a reduction in feed wastage (De Jong et al., 2016; Rojas & Stein, 2017; O’Meara et al., 2020). Moreover, an increase in the TD and feeder occupancy rate together with a reduction in the FR in pigs fed mash diets compared with pigs fed pelleted diets has been reported in the literature (Laitat et al., 2004; Li et al., 2017).

Pigs are highly susceptible to heat stress due to their limited number of sweat glands and high quantity of adipose tissue (Baumgard & Rhoads, 2013). Under heat stress, metabolic mechanisms to dissipate heat are activated and ADFI and pigs´ activity decrease (Quiniou et al., 2000; Collin et al., 2001; Le Bellego et al., 2002; Hyun, 2005). The ADFI reduction due to heat stress can increase with age and pig’s body weight (BW) (Quiniou et al., 2000; Renaudeau et al., 2011). In fact, Le Bellego et al. (2002) reported a reduction of the ADFI by 260 and 540 g/d in growing and finishing pigs, respectively, when temperature increased from 22 to 29°C, and Dos Santos et al. (2018) reported a reduction of the ADFI by 640 and 700 g/d in growing and finishing pigs, respectively, when temperature increased from 23 to 30°C. Moreover, changes in the feeding behavior due to hot conditions have been observed in young and growing-finishing pigs that could explain part of the negative impact of heat stress on pig performance (Quiniou et al., 2000; Collin et al., 2001; Nyachoti et al., 2004). A reduction in the TD and MS have been reported in finishing pigs under heat stress (Quiniou et al., 2000; Serviento et al., 2020). Young pigs under high temperatures reduced their TD by 34% and their MS by 32% (Collin et al., 2001), whereas heavier pigs reduced their TD by 28% and their TV by 20% (Quiniou et al., 2000). However, few studies have been published regarding the effect of heat stress on the feeding behavior of growing-finishing pigs during a long period of time and under commercial conditions.

Therefore, the present work investigates the combined effect of environmental conditions and feed forms on the performance and the evolution of the feeding behavior of group-housed growing-finishing pigs.

It is known that environmental conditions and feed form are factors that affect pig performance and feeding behavior (Quiniou et al., 2000; Collin et al., 2001; Renaudeau et al., 2011; Rojas & Stein, 2017; Vukmirovic et al., 2017). In the present study, these two factors (environmental conditions and feed form) are mixed, with only one feed form per environmental condition. This is a weakness of the study that does not allow to clearly differentiate both effects. Therefore, the present study analyses the combined effect of both factors, which is still interesting, specially under commercial conditions.

In the present study, differences in pig performance were obtained with pigs from HT-P trial presenting a greater ADFI and ADG and a lower FCR than pigs from TH-M trial from the beginning to the end of the growing-finishing period. The literature describes that growing-finishing pigs fed with pelleted feeds, compared to mash, usually result in lower ADFI with improved ADG thanks to a better nutrient digestibility and lower feed waste (Rojas & Stein, 2017; Vukmirovic et al., 2017). The characteristics of the feeders used in the present study minimized feed waste for both feed forms; in fact, a lower ADFI was obtained with pigs fed in mash (TH-M) compared to pigs fed in pelleted feed (HT-P). The lower ADFI in TH-M pigs compared to HT-P pigs may be mostly explained by that TH-M pigs were under hot conditions (high THI) during the last 42 days of the finishing period, the stage with the highest amount of feed intake of all the growing-finishing period, and not due to that the feed was provided in mash form. We would expect a higher ADFI in pigs fed in mash than in pigs fed in pelleted form under thermoneutral conditions. Those results indicate that hot conditions have a greater negative impact on finishing pigs than on growing pigs as similar THI conditions measured during the growing period of HT-P trial did not affect the ADFI of younger pigs. In this regard, the decrease in feed intake is a widely known effect under heat stress, with a more pronounced effect on heavier than on lighter pigs (Renaudeau et al., 2011; Baumgard & Rhoads, 2013; da Fonseca de Oliveira et al., 2018).

Feed intake increases body temperature after eating, which would explain why heat stressed growing-finishing pigs reduce their feed intake (Cervantes et al., 2018). Dos Santos et al. (2018), comparing the performance of pigs housed at 30°C vs 23°C, observed a reduction of 640 and 700 g/d in the ADFI in growing pigs and finishing pigs, respectively. Le Bellego et al. (2002) also found, when comparing pigs housed at 29°C vs 22°C, a reduction in ADFI of 260 and 540 g/d, for growing (27 to 65 kg BW) and finishing pigs (65 to 100 kg BW), respectively, associated with a decrease in the ADG (110 and 180 g BW/d, respectively), but with no effect on feed efficiency or energy cost of gain. Under heat stress pigs activate mechanisms to dissipate the body heat such as an increase in the respiration rate implying a higher energy maintenance cost (Gonzalez-Rivas et al., 2020) and, in consequence, FCR may be penalized (Olczak et al., 2015; Ross et al., 2015; Anderson et al., 2020; Serviento et al., 2020). In fact, in the present study TH-M pigs had a lower ADG than HT-P pigs penalizing FCR by 200 g. This result could be explained by the combined effect of high THI in heavier pigs and the mash feed form, two factors known to penalize pigs´ growth performances (Li et al., 2017; Rojas & Stein, 2017).

Under heat stress, protein deposition is lower than in thermoneutral conditions due to the higher cost of protein deposition compared to lipid deposition (Brown-Brandl et al., 2000; Kouba et al., 2001; Le Bellego et al., 2002; Qu & Ajuwon, 2018). However, in the present study, HT-P pigs grew more, obtained higher backfat thickness and loin depth together with a lower lean percentage of carcasses than TH-M pigs. Therefore, the results obtained suggest that the differences obtained in carcass quality between both trials were more influenced by the growth rate than by the environmental conditions. In fact, our results are in accordance with the findings of Correa et al. (2006), who reported that fast growing pigs were fatter than the slow growing ones.

Several pig feeding behavior changes have been reported in the literature, both under heat stress conditions (Quiniou et al., 2000; Collin et al., 2001) and under different feed forms (Laitat et al., 2004; Li et al., 2017). Changes in the feeding behavior could affect pigs’ performance (Rinaldo & Maurot, 2001; Kerr et al., 2003; Serviento et al., 2020) and modify carcass traits (Rinaldo & Maurot, 2001; Pearce et al., 2013). In the present study, sex affected FR but not the rest of feeding behaviors studied, thus, this discussion is focused on the combined effects of environmental conditions and feed forms on feeding behavior. In general, pigs need more time to achieve the same feed intake when feeding a diet in mash form than in pelleted form due to the lower FR (Laitat et al., 2004). In fact, Mac Donald & Gonyou (2000) reported that finishing pigs fed a mash diet spent a 20% more of TD than pigs fed a pelleted diet. Also, Li et al. (2017) reported a higher TD together with a lower FR and a higher feeder occupancy rate in growing-finishing pigs fed with mash than with pelleted feed. Those results coincide with the ones obtained in the present work in which pigs fed with mash (TH-M pigs) spent a longer TD and had lower FR during all the periods studied, regardless of the environmental conditions, than pigs fed with pelleted feed (HT-P pigs). The average TD obtained in HT-P pigs was close to the values obtained under thermoneutral conditions by Labroue et al. (1994), who reported 63.7 and 49.6 daily minutes spent eating with 40 and 90 kg BW pigs, respectively. However, our TD results are lower than those obtained by Rauw et al. (2006), Li et al. (2017) and Carcò et al. (2018) in group-housed pelleted fed pigs under thermoneutral conditions. Discrepancies could be explained by the genotype and housing conditions, factors which are known to affect the TD of growing-finishing pigs (de Haer & de Vries, 1993a,b; Labroue et al., 1999; Bornett et al., 2000). In this regard, Carcò et al. (2018) used 96 Topigs Talent × PIC pigs with a stocking density of 1.8 m²/pig fed with a Compident Pig - MLP (Schauer Agrotonic, Austria); Rauw et al. (2006) used Duroc pigs fed with a Hokofarm, IVO-G (Marknesse, the Netherlands) feeder; and Li et al. (2017) used PIC Canada Ltd pigs with a stocking density of 1.0 m²/pig and fed by crystal spring feeders (St Agatha, Manitroba, Canada). On the other hand, the average TD of TH-M pigs at 40 kg BW was similar to the TD reported by Li et al. (2017) in growing pigs fed mash feed under thermoneutral conditions (100 vs 106.9 min/d for TH-M pigs at 40 kg BW and Li et al., 2017, respectively). However, Li et al. (2017) reported a longer TD (106.5 min/d) in heavier pigs (60 to 100 kg BW) compared to the 79 min/d registered under heat stress conditions in the present study (TH-M pigs during the finishing period). Hence, those results indicate that hot conditions reduce the TD of finishing pigs fed in mash. Furthermore, in the present study HT-P pigs had similar FR values throughout the experimental period (22.1 and 41.2 g/min at 40 and 80 kg BW) than the ones reported by Rauw et al. (2006) under thermoneutral conditions but lower than values reported by Labroue et al. (1994) and Li et al. (2017) in group-housed pelleted fed pigs. On the other hand, in the present study, TH-M pigs had a FR of 15.5 and 27.7 g/min at 40 and 80 kg BW, respectively. This is a similar result to the one obtained by Li et al. (2017) in mash-fed pigs under thermoneutral conditions (19.7 and 25.6 g/min in growing and finishing pigs, respectively). Quiniou et al. (2000) and Collin et al. (2001) also reported no effect of hot conditions on FR in young and heavier pigs. As expected, and in concordance with the literature cited (Labroue et al., 1994; Hyun et al., 1997; Rauw et al., 2006; Carcò et al., 2018), in the present study, TD decreased and FR increased linearly as pigs grew; however, the decrease in TD was more pronounced when hot conditions occurred during the last 42 days of the finishing period.

On the other hand, in general, increasing the temperature from 23 to 33°C decreases TV and MS in pigs (Collin et al., 2001). Quiniou et al. (2000) also reported a reduction in TV but in terms of MS only a numerical decrease was reported when the temperature increased from 19 to 29°C in heavier pigs. Those results are partially in agreement with those obtained in the present study in which young pigs under high THI values (HT-P pigs) had a greater number of TV and a lower VS; while in heavier pigs (TH-M pigs), the number of TV was not affected, but VS was reduced. Since no literature has been found regarding the effect of feed form on the TV and VS, it is hypothesized that under heat stress, pigs reduce their VS as a mechanism to reduce the body heat production after eating (Cervantes et al., 2018). As pigs grew, in both trials (HT-P and TH-M) and independently of the temperature, the TV was reduced and VS increased in agreement with the literature (Labroue et al., 1994; Hyun et al., 1997; Andretta et al., 2016; Carcò et al., 2018). However, our results suggest that due to the environmental conditions, the reduction in the TV and the increase in VS were less pronounced in the TH-M trial than in the HT-P trial being by -38 vs -67% in TV and by 144 vs 419% in VS, respectively. Then, feeding behavior may be modified by both, feed form and environmental conditions, although it appears that environmental conditions mainly affected the TV and VS, and feed form modified the TD and the FR.

The present study analyzed the combined effect of environmental conditions and feed form over feeding behavior and performance of growing-finishing pigs. Although the experimental design did not allow us to fully differentiate between both factors, hot environmental conditions during the finishing period decreased the ADFI of pigs fed in mash form, while similar hot conditions during the growing period in pigs fed in pellet form hardly affected feed intake. This was due to the fact that growing pigs reduced the VS and increased the TV as a strategy to achieve the desired ADFI. On the other hand, pigs fed with a mash diet spent more time eating per day to achieve the desired ADFI due to a lower FR compared to pelleted feed, independently of the environmental conditions. Finally, from a practical point of view THI and pigs’ BW should be considered to analyze the information from feeding stations to establish alerts and/or implement feeding strategies to mitigate heat stress effects on performance.