Feeding behavior of grow-finish swine and the impacts of heat stress

Abstract

Heat stress has negative impacts on pork production, particularly in the grow-finish phase. During heat stress events, the feeding behavior of pigs is altered to reduce heat production. Several different systems have been developed to study feeding behavior. Most systems are not accurate representations of grow-finish commercial production as feed intake is monitored for only one pig at a time. The objective of this study was to utilize a feed monitoring system, representative of commercial conditions, to determine feeding behavior patterns of grow-finish pigs throughout the year and to identify changes that occurred during heat stress events. Feeder visit data were collected on barrows and gilts (n = 932) from three different sire breeds, Landrace, Yorkshire, and Duroc, between May 2014 and April 2016. Days in the study were partitioned into groups based on their maximum temperature–humidity index (THI), where a THI less than 23.33 °C was classified as “Normal”, a THI between 23.33 and 26.11 °C was classified as “Alert”, a THI between 26.11 and 28.88 °C was classified as “Danger”, and a THI greater than 28.88 °C was classified as “Emergency”. Feeding behavioral differences among breeds and sex were observed across all THI categories. Landrace-sired pigs had fewer feeder visits compared to Duroc- and Yorkshire-sired pigs. Gilts had fewer feeder visits than barrows in all THI categories. Differences in feeding behavior patterns between THI categories demonstrated that heat stress reduced the feeding duration of Landrace-sired pigs without any dramatic effects on the other pigs in the study. During elevated temperatures, all pigs tended to increase feeding events during the early (03:00–05:59) and late (18:00–20:59) periods of the day. Utilizing a feed monitoring system that is a more accurate representation of commercial conditions will lead to a greater understanding of feeding behavior among breed types and sexes during heat stress, allowing producers to enhance their ability to properly care for their pigs during both normal and heat stress events.

INTRODUCTION

Swine feeding behavior monitoring enables producers and researchers to better understand the factors that influence feed intake. Several factors influence feeding behavior in pigs, including but not limited to breed, gender, season, and stressors. Stressors are stimuli disrupting physiological equilibrium or homeostasis (Khansari et al., 1990). During warm months, pigs are subject to heat stress. Due to their limited capacity to use water evaporation to lose heat (Ingram, 1965), pigs decrease heat production during times of elevated ambient temperature by decreasing activity, decreasing feed consumption, and increasing respiration rate (Nienaber and Hahn, 1982; Nienaber et al., 1999; Collin et al., 2001; Quiniou et al., 2001; Huynh et al., 2005b). Renaudeau et al. (2011) documented that signs of heat stress are noticed at lower temperatures as pigs get heavier and that modern genetics are more susceptible to reduced performance during elevated ambient temperatures. Kerr et al. (2003) and Morales et al. (2018) demonstrated that the negative effects of heat stress can be reduced by supplementing diets with crystalline amino acids. Many advances have been made in production management and barn cooling systems; however, production efficiency continues to decline during warm months. Economic losses for the United States pork industry due to heat stress were estimated at $300 million a year (St-Pierre et al., 2003), with $200 million associated with grow-finish production losses.

Feeding behavior has been studied using several different systems that measure feed intake, but most of these systems only allow one pig to feed at a time (Brown-Brandl et al., 2013b). This is not an accurate representation of grow-finish commercial production, where animals are fed from group feeders. Brown-Brandl and Eigenberg (2011) created a monitoring system representative of a typical grow-finish commercial setting. This system consists of a five-slot feeder fitted with a multiplexor and antennas for each feed slot, which records meal length, meal interval, number of meals per day, and total time spent eating.

In order to gain a better understanding of feeding behavior changes during heat stress events, it is important to understand normal feeding patterns throughout the year. The objective of this study was to determine feeding behavior patterns of different breeds and sexes of pigs throughout the year and to identify changes that occurred during heat stress events using a feed monitoring system representative of feeding conditions in commercial grow-finish operations.

MATERIAL AND METHODS

All measurements recorded were approved by the U.S. Meat Animal Research Center’s Animal Care Guidelines and conformed to the Guide for Care and Use of Agricultural Animals in Research and Teaching (Federation of Animal Science Societies, 2010). Data were collected on four groups of grow-finish pigs from May 2014 to April 2016 (Table 1). At approximately 8–10 wk of age, barrows and gilts were placed in a grow-finish barn equipped with sprinkle cooling. Feeding behavior was monitored over a 4-month grow-out period. Grow-finish groups (n = 240) were distributed across six pens with 40 pigs per pen resulting in ~0.8 m²/pig of pen space. Supplemental heat was provided when the temperature in the center of the building was below 18.5 °C. Each pen had nearly equal representations of each sex and breed. Sire lines included Duroc, Landrace, and Yorkshire. All pigs were produced from Landrace–Yorkshire composite sows. Upon entry, pigs were tagged with electronic identification tags. Animals that were removed from pens due to illness and animals whose radio frequency electronic identification (RFID) quit working were not included in analyses, resulting in 932 pigs being studied. Data collection stopped once the heaviest animals were removed to be marketed.

Table 1.

Barn entry and exit dates for each grow-finish group and the number of days recorded in each THI category

Group	Barn entry	Barn exit	Normal	Alert	Danger	Emergency
G1	May 2014	October 2014	64	51	27	3
G2	December 2014	April 2015	120	0	0	0
G3	June 2015	October 2015	39	43	29	5
G4	December 2015	April 2016	137	0	0	0

Pens were fitted with an electronic feeding system containing five slots to monitor feeding behavior (Brown-Brandl et al., 2011). A corn–soybean meal diet, balanced with crystalline forms of lysine, methionine, and threonine, was provided ad libitum to all pigs. The feed was formulated to meet or exceed all nutritional requirements of the growing pig. Feeders were fitted with a multiplexor and antennas for each feed slot, allowing the animals’ low-frequency RFID tags to be read while the animals were at the feeder. Data were collected every 20 s from all antennas during the test period. Feeder visits were summarized using two different methods. First, the total time a pig was at a feeder each day was determined to analyze the impact of fixed effects on an animal’s daily feeding behavior. Next, the average amount of time an animal spent at the feeder was determined for eight periods throughout the day. Here, feeder visits in time period 1 were recorded between 00:00:00 and 02:59:59, those in time period 2 were recorded between 03:00:00 and 05:59:59, and so on to create eight 3-h periods per day.

Outside temperature (°C) and relative humidity (RH) were obtained from the National Weather Station located 3 miles northwest of the grow-finish barn and were used to calculate temperature–humidity index (THI; National Oceanic and Atmospheric Administration, 1976):

$${\displaystyle \begin{array}{l}{\displaystyle \mathrm{T}\mathrm{H}\mathrm{I}\left({}^{\circ }\mathrm{C}\right)=\mathrm{T}\left({}^{\circ }\mathrm{C}\right)\left[0.55(0.0055\times \mathrm{R}\mathrm{H})\right]\times [\mathrm{T}\left({}^{\circ }\mathrm{C}\right)14.5]}\end{array}}$$

THI was calculated in 1-h increments because outside temperatures were reported every hour. The outline of Brown-Brandl et al. (2013a) for THI categories, which were based on feedlot cattle, were used as the temperature breakpoints for this study as they reflected changes in biological parameters observed in pigs by Huynh et al. (2005a, 2005b). Days were partitioned into temperature groups (Table 2) according to their maximum observed THI to examine feeding behavior during extreme heat events. A THI less than 23.33 °C was classified as Normal, THI between 23.33 and 26.11 °C were classified as Alert, THI between 26.11 and 28.88 °C were classified as Danger, and THI greater than 28.88 °C were classified as Emergency. Feeding behavior data were then summarized based on each animal’s sex, breed of the sire, time of day, and the maximum THI category for the day.

Table 2.

Estimates for fixed effects on average daily time at the feeder

Fixed effect	Level	Estimated effect (min/d)	SE
Sex	Barrow	0.00	—
	Gilt	−6.21	1.04
Sire breed	Duroc	0.00	—
	Yorkshire	3.56	1.28
	Landrace	−11.37	1.28
THI category	Normal	0.18	0.19
	Alert	0.00	—
	Danger	0.11	0.20
	Emergency	−4.16	0.43

Analyses were conducted in R, fitting a mixed-effects model. The model fit for total daily feeding time included fixed effects of sex, sire breed, contemporary group, and THI category, while animal was fit as a random effect. Significance of differences in estimated feeder visit activity effects for fixed effects was determined using a paired t-test for breed, THI category, and sex.

RESULTS

Of the 932 grow-finish pigs used in this analysis: 309 were Duroc sired, 312 were Landrace sired, and 311 were Yorkshire sired. There were 459 barrows and 473 gilts. Maximum outside daily THI ranged from −8.36 to 30.52 °C for groups G1–G4 and the number of days in each category for each group of pigs is presented in Table 1. Estimated effects of sire breed, sex, and THI category on average daily feeding time are shown in Table 2. Significant differences in feeding behavior were observed between breeds. Yorkshire-sired pigs visited the feeder more often throughout the day than Duroc-sired pigs, which spent more time at the feeder than Landrace-sired pigs. Barrows spent 6.2 min longer at the feeder than gilts each day. The average time at the feeder was not different between Normal, Alert, or Danger categories, but pigs did reduce their time at the feeder by approximately 4 min when the THI reached Emergency status.

Daily feeding behavior patterns for each of the three breeds of sire is shown in Fig. 1. When THI was in the Normal category, most feeder activities took place during a broad window of 06:00 and 17:59. However, when ambient temperatures exceeded Normal, animals altered their feeding pattern with peak feeding activity occurring during 06:00–08:59, decreased activity during mid-day, and then another increase of activity during 18:00–21:59. Feeding behavior plots for Duroc-, Landrace-, and Yorkshire-sired pigs displayed similar changes to eating patterns when THI exceeded Normal and these changes in behavior were similar across sexes (Fig. 2). The greatest reduction in feeding activity from Normal THI during Emergency THI was seen in the 12:00–14:59 time period for all breeds of sire and sexes.

Figure 1.

Feeding behavior patterns for Normal (x < 23.33 °C) THI, Alert (23.33 ≤ x < 26.11 °C), Danger (26.11 ≤ x < 28.88 °C), and Emergency (x ≥ 28.88°C) by sire breed. Average time (min) feeder per pig is plotted where time of day is pooled into 3-h periods beginning at 00:00–02:59, 03:00–05:59, etc.

Figure 2.

Feeding behavior patterns for Normal (x < 23.33 °C) THI, Alert (23.33 ≤ x < 26.11 °C), Danger (26.11 ≤ x < 28.88 °C), and Emergency (x ≥ 28.88 °C) by sex. Average time (min) feeder per pig is plotted where time of day is pooled into 3-h periods beginning at 00:00–02:59, 03:00–05:59, etc.

DISCUSSION

Environmental temperatures are known to affect swine feeding behavior and animal performance. Outside environmental conditions from a nearby weather station were used to compute THI as it approximated the thermal conditions inside the barn due to its strong statistical relationship with barn temperature (R² = 0.848; Cross et al., 2018). Outside temperatures seemed relevant as the only cooling method in summer was sprinklers and fans. The barn was heated during cold weather (barn temperature < 18.5 °C) and air quality in extremely cold periods likely changed as less fresh air was introduced and supplemental heat was provided. Preliminary analyses attempted to identify changes in feeding behavior during extremely cold periods relative to moderate conditions but no effect was found.

Heat production in pigs comes from feed consumption, maintenance, and physical activity (Kerr et al., 2003). Pigs decrease physical activity to reduce heat production when exposed to high temperatures (Kerr et al., 2003) and, therefore, pigs spend more time laying down and less time eating during high ambient temperatures (Hicks et al., 1998; Brown-Brandl et al., 2001). Previous research has determined that, when daily temperatures exceed comfortable thresholds, pigs shift meal times to early morning or evening hours when temperatures are lower (Xin and DeShazer, 1992; Nienaber et al., 1996; Quiniou et al., 2000). In the current study, maximum feeder visits were observed towards mid-day for Normal THI but shifted toward early morning and late afternoon at higher THI categories. This pattern was similar across all breeds of sire and was also observed in gilts and barrows (Figs. 1 and 2). Feeding activity peaked during 06:00–08:59 and again in the early evening (time period 18:00–20:59) for all animals when THI exceeds normal. Increased feeding behavior to cooler times of the day is likely a coping mechanism that pigs implement to avoid consuming large amounts of feed during the heat of the day when THI exceeds Normal. This shift in feeder visit activity allows pigs to adapt to warm environments by decreasing heat production, reducing the amount of heat that needs to be dissipated into the environment (Nienaber and Hahn, 1982; Collin et al., 2001; Quiniou et al., 2001).

Despite changes in feeding behavior during elevated ambient temperatures, Quiniou et al. (2000) reported no effect on the number of daily meals when temperatures exceeded thermal-neutral zones due to pigs shifting the timing of meals. The current study also found no difference in the total time spent at feeders for most THI categories except the most extreme category, Emergency, where a reduction in approximately 4 min/d was observed. Surprisingly, THI category did not affect the average length of a meal in the current study, where each meal tended to last approximately 12.5–15.5 min, depending on the breed of the sire, sex of the pig, and time of the day.

Sex differences have been observed in feeder activity. Brown-Brandl et al. (2013b) reported that barrows spent more time at the feeders than gilts. However, Hyun et al. (1997) reported no difference between sex and time spent at feeders. These conflicting results may be due to differences in feed monitoring systems. Hyun et al. (1997) used an electronic feeding system that allowed only one pig to eat, while Brown-Brandl et al. (2013b) used an electronic feeding system consisting of one feeder with five feeding spaces. In the present study, barrows had higher feeder visit activity than gilts totaling 6.2 min/d more time at feeders (P < 0.01). Additional time at the feeder likely resulted in more feed consumed as barrows had greater weight gains on test.

General differences observed in feeder visit activity between sexes or breed of sire could be a result of normal feeding behavior or competition. Gilts followed the same feeding behavior profile as barrows but spent slightly less time at a feeder each day. This may be because gilts do not visit the feeder as frequently as barrows or because there was competition at the feeder. Thus, competition could explain the different results from Hyun et al. (1997) indicating that sex is not a significant factor in feeding time per day. Observed behaviors of Duroc- and Yorkshire-sired pigs when THI categories exceeded Normal were similar, but peak activity of Duroc-sired pigs was in the late afternoon, whereas Yorkshire-sired pigs peaked in the morning period. Feeders were occupied at ~50% capacity during these times and a pig’s choice of which time to eat may have been associated with competition of the other pigs at the feeder. Landrace-sired pigs spent less time at the feeder each day compared with Yorkshire- and Duroc-sired pigs (P < 0.01) for all THI categories. If this choice was strictly due to competition, a dramatic increase in nighttime feeding might be expected; however, this was not observed. Studies to separate competitive differences due to sex and/or breed of sire could be conducted but would require a different penning strategy than used in the current study. Future studies will focus on altering the number of feeding spaces per pen to assess competition at the feeder between barrows and gilts.

Feed intake has been shown to be reduced in pigs that experience long-term heat stress (Song et al., 2011; Renaudeau et al., 2013). Increasing consecutive days of extreme heat has a more drastic impact on feeding behavior and growth; however, pigs will tend to have compensatory growth once the ambient temperature returns to their thermal-neutral zone (Zumbach et al., 2008). The current study lacked long-term heat events and is unable to confirm those results. The categories used in this study were based on suggestions for feedlot cattle (Brown-Brandl et al., 2013a). The temperature threshold between Normal and Alert (23 °C) agrees with Huynh et al. (2005a, 2005b), where heat stress begins to affect pig behavior and metabolism; however, they did not see a second impact temperature in the pig until temperatures reached 29–30 °C. Based on the current study, future studies with pigs should partition THI into three unique categories: Normal, Alert/Danger, and Emergency as there were few differences between the performance of animals during Alert and Danger THI. Combining these categories would improve the distribution of animal days in this new category with the Normal category. Unfortunately, increasing the number of animal days in the Emergency category without artificially modifying the barn temperature would be difficult.

In summary, while subtle breed differences exist for feeding behavior during elevated ambient temperatures, all animals tended to shift eating patterns to cooler periods of the day. Understanding these altered behaviors will empower the management to modify production systems to improve animal performance. Utilization of individual animal variation within breeds may be the industry’s best approach to create pig populations that are less affected by excessive THI as genomic variation exists and genetic markers have been detected (Cross et al., 2018).