https://orcid.org/0000-0001-7268-4278
Abstract
Two experiments were conducted in a 14,400 head nursery using 3,087 (experiment 1) and 988 (experiment 2) pigs to determine the effect of gruel feeding (experiment 1) and supplemental oral dextrose (experiment 2) on nursery pig survivability after weaning. Upon arrival to the nursery, for experiment 1, the smallest 10% of pigs were selected and randomly placed in pens with 61 to 108 pigs per pen. Pens of small pigs were assigned to one of two treatments in a completely randomized design. Treatments consisted of gruel feeding two or four times per day for 14 d postplacement. At each gruel feeding, approximately 1.13 kg of solid feed was added to a round bowl (Rotecna S.A., Agramunt, Spain) located at the front of each pen and water added at a decreasing rate over time. In experiment 2, every other pig removed for welfare considerations (lameness, sick, or unthrifty) from the general population or pens of small pigs received a single 10 mL oral dose of a 50% dextrose solution (Vet One, MWI Animal Health, Boise, ID), as a source of glucose, before being placed in a removal pen. All removed pigs were tagged and weighed, body temperature recorded, and blood glucose concentration measured prior to and 30 min after entering removal pens. Overall, gruel feeding small pigs two or four times per day for 14 d postplacement did not influence (P > 0.10) mortality from weaning to the end of gruel feeding (3.78% vs. 4.25%, respectively). Likewise, dextrose administration did not influence (P > 0.10) pig mortality after removal to approximately 38 d postweaning (21.4% vs. 23.4% respectively), even though blood glucose concentration increased (P < 0.001) 30 min after removal for pigs administered dextrose. An interaction was observed for blood glucose concentration and body temperature (P < 0.001) where pigs with blood glucose concentrations less than 70 mg/dL had increased mortality as body temperature increased. In contrast, pigs with a blood glucose concentration of 70 mg/dL or greater had decreased mortality as body temperature increased. Pigs weighing less than 4.5 kg also had increased mortality (P < 0.001) compared with pigs weighing greater than or equal to 4.5 kg at removal. In summary, gruel feeding four times per day vs. two times per day or providing a dextrose supplement to pigs removed from the general population did not improve the survivability of pigs after weaning. Additionally, pigs removed with decreased body weight or with body temperature or blood glucose concentrations below or above the normal range had decreased survivability.
Lay Summary
Pigs reared in modern production systems have limited opportunity to develop natural feeding behaviors, which can lead to decreased feed intake and reduced body weight gain postweaning. Hence, the implementation of on-farm feed management strategies has become increasingly important. Gruel feeding is a strategy commonly used where solid feed and water are combined to create a liquid feed, which is added to troughs or bowls located in nursery or wean-to-finish pens. Furthermore, for pigs that struggle to maintain nutrient intake after weaning or simply fail to thrive, there are few intervention strategies available. We hypothesized that an oral dextrose supplement (source of glucose) would increase circulating glucose concentrations when provided to pigs removed from the general population because of welfare considerations, potentially improving pig livability. Therefore, two experiments were conducted to determine the effect of gruel feeding and oral dextrose on the survivability of pigs postweaning. Overall, gruel feeding four times per day vs. two times per day or providing a dextrose supplement to pigs removed from the general population did not improve the survivability of pigs after weaning. However, pigs removed with decreased body weight or with body temperature or blood glucose concentrations below or above a normal range had decreased survivability.
Introduction
Pigs reared in modern production systems have limited opportunity to develop natural feeding behaviors that improve the transition from an all milk-based diet to solid feed. Postweaning, this can lead to feed neophobia resulting in decreased feed intake and reduced body weight gain (Brooks and Tsourgiannis, 2003). The process of weaning also presents multiple concurrent stressors that lead to a heightened “state of threatened homeostasis” (Nirupama et al., 2018), activating the hypothalamic pituitary adrenal axis. Research in both humans and pigs has shown that activation of the central stress response system can have detrimental effects on intestinal barrier integrity, immune function, and neurodevelopment (McCracken et al., 1999; Hart and Kamm, 2002; Moeser et al., 2007; Kelly et al., 2015), all of which can suppress feed intake, nutrient utilization, and the ability of the pigs’ to fight pathogens (Pelleymounter et al., 2000; Salfen et al., 2003; McLamb et al., 2013; Li et al., 2017). It is also well-established that stress and feed deprivation alter glucose metabolism (Berg et al., 2002; Nirupama et al., 2018).
To minimize weaning stress, the implementation of on-farm feed management strategies has become increasingly important. Gruel feeding is a strategy commonly used where solid feed and water are combined to create a liquid feed, which is added to troughs or bowls located in nursery or wean-to-finish pens. Like suckling, it is believed that gruel feeding may help stimulate group feeding behavior, particularly in populations of small or unthrifty pigs. Despite its practice throughout the industry, there are limited research publications available to validate gruel feeding protocols and the potential impact on pig survivability. Additionally, for pigs that struggle to maintain nutrient intake after weaning or simply fail to thrive, there are few intervention strategies available. Buzzard et al. (2013) observed that pigs characterized as unthrifty had decreased blood glucose concentration compared with pigs categorized as healthy, indicating that unthrifty pigs may be malnourished. Hence, we hypothesized that an oral dextrose supplement (as a source of glucose) would increase circulating glucose concentration when provided to pigs removed from the general population due to welfare considerations, potentially improving pig livability. Limited research is also available examining the physiological differences among pigs removed in a large commercial setting and how their body weight, blood glucose concentration, and body temperature impact survivability. Therefore, the objective of these studies was to determine the effect of gruel feeding and oral dextrose on the survivability of pigs postweaning.
Materials and Methods
The Kansas State University Institutional Animal Care and Use Committee approved the protocol used in these experiments.
Pigs were weaned at approximately 21 d of age and transported to a 14,400 head commercial nursery located in the Midwest and placed in rooms based on sow source. A total of 3,087 (experiment 1) and 988 (experiment 2) pigs (241 × 600, DNA, Columbus, NE) were used in two trials to measure nursery pig survivability. Each pen was equipped with two eight-hole stainless steel feeders and a nipple bar waterer to allow ad libitum access to feed and water. Common corn-soybean meal-based nursery diets were fed in three phases and were based on a feed budget. Phase 1 feed budget was provided at 2.5 kg/pig, phase 2 feed budget was provided at 5.4 kg/pig, and phase 3 feed budget was provided at 16.0 kg/pig. In removal and small pig pens, a special early-wean diet was fed at approximately 1.5 kg/ pig prior to beginning phase 1 feed. All diets were fed in meal form.
Experiment 1
The smallest 10% of pigs were selected upon arrival to the nursery and randomly placed in designated pens. A total of 34 pens across eight rooms (2 to 5 pens per room) were used for the small pigs with 61 to 108 pigs per pen. Pens of small pigs were assigned to one of two treatments in a completely randomized design with average pigs per pen balanced across treatment. Treatments consisted of gruel feeding two- or four-times per day. Gruel feeding started when the first group of pigs were placed in each room and continued for 14 d after the last group of pigs were placed in each room. Fill times ranged from 2 to 10 d. At each gruel feeding, approximately 1.13 kg of solid feed from the feeder was added to a single round bowl measuring 40.6 cm in diameter and 7.0 cm deep (Rotecna S.A.) located at the front of each pen. Water was added to feed at a decreasing rate over time such that from days 0 to 5, 6 to 10, and 11 to 14 the ratio of water to feed was 3:1, 1:1, and 1:3, respectively. Pens of pigs were equipped with a heat lamp and two floor mats. One mat was placed under the heat lamp for zone heating and the other was placed near the feeder for mat feeding. Mat feed was provided to all pigs (small pens and general population pens) up to 2 times per day by taking one to two handfuls (approximately 1.13 kg) of feed from the feeder and spreading it across the mat. Additionally, all small pig pens were monitored daily to track removals and mortalities.
Experiment 2
Pigs removed from the general population or pens of small pigs for welfare considerations (lameness, sick, or unthrifty) over a 4-wk period postplacement were used. At removal, every other pig received a single 10 mL oral dose of a 50% hypertonic dextrose solution (Vet One, MWI Animal Health) before being placed in a designated removal pen equipped with a heat lamp and floor mats. The 10 mL dextrose solution was used as source of glucose and provided 5 g of dextrose, which delivered 17 kcal of gross energy. All removed pigs were tagged and weighed, blood glucose concentration measured prior to and 30 min after entering removal pens, and body temperature recorded. Blood glucose concentration was measured by puncturing an ear auricular vein with a needle and using a handheld Glucometer (AimStrip Plus, Germaine Laboratories Inc., San Antonio, TX). Mortality was then tracked through the end of the nursery for approximately 38 d postplacement.
Data Analysis
Gruel feeding data were analyzed as a completely randomized design with pen as the experimental unit. Treatment was considered a fixed effect and room a random effect. A binomial model was used to determine removal and mortality percentages. Mortalities were calculated by totaling the mortalities out of test pens and the mortalities after removal, and then dividing by initial pen inventory.
Dextrose data were also analyzed as a completely randomized design with room as a random effect and pig as the experimental unit. Least square means was used to assess the effect of dextrose supplementation on blood glucose concentrations at removal, 30-min postremoval, and the change over time. Pigs were removed from the dextrose dataset when not accounted for at the end of the experiment and not recorded as a mortality. Due to random chance, entry blood glucose concentrations were statistically different, therefore entry blood glucose concentration was used as a covariate for 30 min postremoval blood glucose concentration and the change over time. A binomial model was used to determine mortality percentages.
The predictor variables of body weight, body temperature, and blood glucose concentration were categorized into biologically significant categories and least squares means were determined using a binary model with mortality as the outcome. A quadratic equation for the continuous predictor variables of body temperature and blood glucose concentration were also tested to determine the predicted probability of mortality based on body temperature and blood glucose concentration. All statistical models were fit using the GLIMMIX procedure of SAS v. 9.4 (SAS Institute, Inc., Cary, NC). Results were considered significant at P ≤ 0.05.
Results
In experiment 1, gruel feeding small pigs two or four times per day did not influence removal (P = 0.239; Table 1) or mortality (P = 0.437) rates from weaning to the end of the gruel feeding period. In experiment 2, dextrose administration did not influence (P = 0.443; Table 2) the survivability of pigs after removal. However, blood glucose concentrations were increased (P < 0.001) in pigs administered dextrose compared with no dextrose, which confirms that dextrose supplementation at 10 mL/pig increased circulating blood glucose concentrations.
| Gruel application, times/d | SEM | P-value | ||
|---|---|---|---|---|
| Two | Four | |||
| Count day 0 | 1,530 | 1,557 | – | – |
| Count day 14 | 1,253 | 1,262 | – | – |
| Mortality/removal analysis, % | ||||
| Removal | 14.1 | 15.6 | 2.50 | 0.239 |
| Mortality3 | 3.78 | 4.25 | 1.560 | 0.437 |
| Gruel application, times/d | SEM | P-value | ||
|---|---|---|---|---|
| Two | Four | |||
| Count day 0 | 1,530 | 1,557 | – | – |
| Count day 14 | 1,253 | 1,262 | – | – |
| Mortality/removal analysis, % | ||||
| Removal | 14.1 | 15.6 | 2.50 | 0.239 |
| Mortality3 | 3.78 | 4.25 | 1.560 | 0.437 |
1A total of 3,087 mixed sex pigs were used with 61 to 108 pigs per pen and 17 replicates per treatment.
2Gruel feed was offered either two-times per day or four-times per day. Gruel feeding started when the first group of pigs were placed in each room and continued for 14-d after the last group of pigs were placed. Fill times ranging from 2 to 10 d. At each feeding, approximately 1.1 kg of solid feed from the back of the feeder was added to a single round bowl measuring 40.6 cm in diameter and 7.0 cm deep, (Rotecna S.A.) located at the front of the pen. Water was added to feed at a decreasing rate over time such that from days 0 to 5, 6 to 10, and 11 to 14 the ratio of water to feed was 3:1, 1:1, and 1:3, respectively. A heat lamp was also provided in each pen.
3Mortality = (mortality after removal + mortality in pen) ÷ initial pen inventory.
| Gruel application, times/d | SEM | P-value | ||
|---|---|---|---|---|
| Two | Four | |||
| Count day 0 | 1,530 | 1,557 | – | – |
| Count day 14 | 1,253 | 1,262 | – | – |
| Mortality/removal analysis, % | ||||
| Removal | 14.1 | 15.6 | 2.50 | 0.239 |
| Mortality3 | 3.78 | 4.25 | 1.560 | 0.437 |
| Gruel application, times/d | SEM | P-value | ||
|---|---|---|---|---|
| Two | Four | |||
| Count day 0 | 1,530 | 1,557 | – | – |
| Count day 14 | 1,253 | 1,262 | – | – |
| Mortality/removal analysis, % | ||||
| Removal | 14.1 | 15.6 | 2.50 | 0.239 |
| Mortality3 | 3.78 | 4.25 | 1.560 | 0.437 |
1A total of 3,087 mixed sex pigs were used with 61 to 108 pigs per pen and 17 replicates per treatment.
2Gruel feed was offered either two-times per day or four-times per day. Gruel feeding started when the first group of pigs were placed in each room and continued for 14-d after the last group of pigs were placed. Fill times ranging from 2 to 10 d. At each feeding, approximately 1.1 kg of solid feed from the back of the feeder was added to a single round bowl measuring 40.6 cm in diameter and 7.0 cm deep, (Rotecna S.A.) located at the front of the pen. Water was added to feed at a decreasing rate over time such that from days 0 to 5, 6 to 10, and 11 to 14 the ratio of water to feed was 3:1, 1:1, and 1:3, respectively. A heat lamp was also provided in each pen.
3Mortality = (mortality after removal + mortality in pen) ÷ initial pen inventory.
Effect of oral dextrose on the mortality rate of nursery pigs removed from general population or small pig pens1
| Oral dextrose | SEM | P-value | ||
|---|---|---|---|---|
| No | Yes | |||
| Count | 476 | 512 | – | – |
| Blood glucose, mg/dL | ||||
| Entry | 82.1 | 74.1 | 2.03 | 0.002 |
| 30 min2 | 89.8 | 96.8 | 1.40 | <0.001 |
| Change2,3 | 11.8 | 18.7 | 1.40 | <0.001 |
| Mortality, % | 21.4 | 23.4 | 4.02 | 0.443 |
| Oral dextrose | SEM | P-value | ||
|---|---|---|---|---|
| No | Yes | |||
| Count | 476 | 512 | – | – |
| Blood glucose, mg/dL | ||||
| Entry | 82.1 | 74.1 | 2.03 | 0.002 |
| 30 min2 | 89.8 | 96.8 | 1.40 | <0.001 |
| Change2,3 | 11.8 | 18.7 | 1.40 | <0.001 |
| Mortality, % | 21.4 | 23.4 | 4.02 | 0.443 |
1Every other pig that entered the removal pen received a 10 mL oral dose of a 50% hypertonic dextrose solution. The 10 mL dextrose solution provided 5 g of dextrose, which delivered 17 kcal.
2Entry blood glucose used as a covariate.
3Represents the average change in blood glucose over a 30 min period after pigs entered the removal pen.
Effect of oral dextrose on the mortality rate of nursery pigs removed from general population or small pig pens1
| Oral dextrose | SEM | P-value | ||
|---|---|---|---|---|
| No | Yes | |||
| Count | 476 | 512 | – | – |
| Blood glucose, mg/dL | ||||
| Entry | 82.1 | 74.1 | 2.03 | 0.002 |
| 30 min2 | 89.8 | 96.8 | 1.40 | <0.001 |
| Change2,3 | 11.8 | 18.7 | 1.40 | <0.001 |
| Mortality, % | 21.4 | 23.4 | 4.02 | 0.443 |
| Oral dextrose | SEM | P-value | ||
|---|---|---|---|---|
| No | Yes | |||
| Count | 476 | 512 | – | – |
| Blood glucose, mg/dL | ||||
| Entry | 82.1 | 74.1 | 2.03 | 0.002 |
| 30 min2 | 89.8 | 96.8 | 1.40 | <0.001 |
| Change2,3 | 11.8 | 18.7 | 1.40 | <0.001 |
| Mortality, % | 21.4 | 23.4 | 4.02 | 0.443 |
1Every other pig that entered the removal pen received a 10 mL oral dose of a 50% hypertonic dextrose solution. The 10 mL dextrose solution provided 5 g of dextrose, which delivered 17 kcal.
2Entry blood glucose used as a covariate.
3Represents the average change in blood glucose over a 30 min period after pigs entered the removal pen.
For the categorical predictor variables in experiment 2, pigs with a body weight under 4.5 kg at the time of removal had increased mortality compared with pigs with a body weight greater than or equal to 4.5 kg at removal (P < 0.001; Figure 1). For blood glucose concentration and body temperature a significant interaction was observed (P < 0.001; Figure 2). Pigs with a blood glucose concentration less than 70 mg/dL had increased (P < 0.05) mortality when body temperature was greater than 39.7 °C compared with pigs with a body temperature within the normal range of 38.6 to 39.7 °C. The mortality of pigs with a body temperature less than 38.6 °C was intermediate. Additionally, providing an oral dose of dextrose to pigs with a blood glucose concentration less than 70 mg/dL did not improve survivability (P = 0.101; Figure 3), regardless of body temperature. Pigs with normal blood glucose concentration ranging from 70 to 120 mg/dL had increased (P < 0.05) mortality when body temperature was less than 38.6 °C compared with pigs with a body temperature within the normal range of 38.6 °C to 39.7 °C. The mortality of pigs with a body temperature greater than 39.7 °C was intermediate. Pigs with a blood glucose greater than 120 mg/dL and a body temperature less than 38.6 °C had increased (P < 0.05) mortality compared with pigs with a body temperature of 38.6 °C or greater. The combination of increased blood glucose concentration and decreased body temperature resulted in substantially greater percent mortality compared with any other combination of blood glucose concentration and body temperature. However, this represented a small proportion of pigs (approximately 2.6%).
Main effect of body weight at removal on the mortality of nursery pigs after removal. Number listed within each bar represents pig count.
Interactive effect of blood glucose and body temperature at removal on the mortality of nursery pigs after removal. Bars within blood glucose category that lack a common superscript differ by P < 0.05 . Number listed within each bar represents pig count.
Effect of oral dextrose at removal within blood glucose category on the mortality of nursery pigs after removal. Number listed within each bar represents pig count.
Blood glucose concentration did not influence mortality for pigs with a normal body temperature ranging from 38.6 °C to 39.7 °C (P > 0.05). Pigs with a body temperature greater than 39.7 °C and a blood glucose concentration less than 70 mg/dL had increased (P = 0.033) mortality compared with pigs with a high body temperature and a blood glucose concentration between 70 and 120 mg/dL. Additionally, for pigs with a body temperature greater than 39.7 °C, there was no evidence (P > 0.10) that blood glucose concentration greater than 120 mg/dL resulted in mortality rates different from pigs with a decreased blood glucose concentration.
For the continuous predictor variables in experiment 2, a quadratic relationship was observed for body temperature and probability of mortality (P < 0.001; Figure 4). Hence, pigs with a body temperature below or above the normal range had increased predicted mortality. A quadratic relationship was also observed for blood glucose concentration and probability of mortality (P < 0.001; Figure 5), such that pigs with decreased or high blood glucose concentration had increased predicted mortality. Once pigs reached a blood glucose concentration greater than 250 mg/dL, their probability of mortality was nearly 100%.
Effect of body temperature at removal on the predicted probability of mortality of nursery pigs after removal.
Effect of blood glucose at removal on the predicted probability of mortality of nursery pigs after removal.
Discussion
Poor feed intake and body weight gain by pigs after weaning is a challenge for commercial swine producers. Numerous studies have assessed dietary approaches to improve feed palatability and nutrient utilization; however, the responses often observed do not fully prevent performance losses. Hence, it has become increasingly important to consider opportunities beyond diet formulation strategies to help pigs transition after weaning. A review by Brooks and Tsourgiannis (2003) indicated that the development of feeding behavior preweaning has an important role in how pigs begin consuming feed postweaning. Thus, it is important to understand the feeding behavior of pigs prior to weaning. There are three main considerations: 1) at weaning pigs are accustomed to consuming an all-liquid diet, 2) as a species, pigs naturally prefer to eat together, and 3) feeding up to this point has been elicited by the sow when she grunts and calls pigs to nurse.
Gruel feeding is practiced throughout the swine industry because it is thought to provoke similar feeding behaviors to suckling (provides liquid mixed with dry feed, several pigs can eat at once, and caretakers entering the pen to gruel feed encourages pigs to get up), while also helping transition pigs from an all milk based diet to solid feed. However, nearly no published literature is available on gruel feeding. Some researchers have evaluated all-liquid diets after weaning, but the responses observed are not consistent. Kim et al. (2001) showed that pigs fed an all-liquid diet the first 14 d postweaning had improved gain compared with pigs fed an all-dry pelleted diet. Conversely, Lawlor et al. (2002) showed a reduction in ADG and poorer feed efficiency when feeding an all-liquid diet, despite increased dry matter intake. Others have reported no difference in growth performance or morbidity and mortality rates feeding an all-gruel diet (Corrigan et al., 2000). In the current study, no differences in morbidity and mortality were observed when gruel feeding in combination with standard trough feeding. Interestingly, Corrigan et al. (2000) showed that feeding an all-gruel diet decreased eating behavior at the feeder 3 d postweaning, which may emphasize why gruel feeding in combination with standard trough feeding is a more practical and potentially beneficial strategy than feeding all-liquid diets after weaning. Gruel feeding allows for partial liquid feeding, while also encouraging pigs to start eating solid feed; therefore, preventing pigs from having to undergo a second weaning when an all-liquid diet is stopped, and dry feed is started. Because feed intake and gain were not measured in the study reported herein, and due to the limited number of trials conducted on gruel feeding in combination with standard trough feeding, the effect of gruel feeding on wean pig growth performance, morbidity, and mortality cannot be fully elucidated.
While more research is needed to understand if gruel feeding can minimize morbidity and mortality, it is unlikely that any feed management strategy will fully eliminate the challenges associated with poor health and pigs that simply fail to thrive postweaning. Hence, in the current study, once challenged pigs were identified, different biological measures were assessed that may influence survivability, including body weight, body temperature, and blood glucose concentration. It is important to note that each of these factors are not the direct cause of mortality but rather indicators of underlying physiological issues that lead to mortality. The results herein showed that pigs removed from the general population with a body weight below 4.5 kg had increased mortality compared with pigs removed with a heavier body weight. This agrees with earlier reports by Larriestra et al. (2006) who identified that weaning pigs ≤ 3.5 kg was a significant predictor of nursery mortality.
Body temperature at removal also appears to be a predictor of nursery mortality. Normal body temperature ranges from approximately 39.0 °C to 39.6 °C in weaned pigs (Dewey and Straw, 2006), indicating that anything substantially below or above could be considered hypo or hyperthermic, respectively. In humans, hypothermia occurs as core body temperature drops below 35 °C, while hyperthermia occurs as temperature rises above 40 °C (Mayo clinic 2022a, 2022b). It is expected that these reference values would vary slightly for pigs; however, based on the quadratic response observed for body temperature and predicted probability of mortality, it appears these reference values are suitable. In the current study, removed pigs with a body temperature between approximately 38 °C and 40 °C had decreased mortality compared with pigs with a body temperature outside of this range. While the direct cause of low or high body temperature was not elucidated in this experiment, thermogenic regulation has been linked to the bodies central stress response pathway. This pathway can be activated by stressors such as pathogen exposure or feed deprivation (Parrott et al., 1995; Inoue and Luheshi, 2010), both of which can occur around the time of weaning.
One of the mechanisms which can result in elevated body temperature is systemic inflammation due to infection, which is a critical component of the host immune system (Almeida et al., 2006). However, when systemic inflammation becomes severe the body responds with a decline in core body temperature (Almeida et al., 2006). Trammell and Toth (2011) showed that low body temperature in mice used to model infectious disease was a valid marker of eventual predicted mortality. Furthermore, recent data in humans showed increased mortality in sepsis patients that presented with body temperature < 36 ˚C (Inghammar and Sunden-Cullberg, 2020; Baek et al, 2022). If feed deprivation occurs prior to infection, this adds an extra layer of complexity on the ability of the body to mount an effective fever response. Feed deprivation in and of itself leads to hypothermia due to decreased leptin concentrations; however, when pathogen exposure occurs in combination with feed deprivation, a weakened fever response is observed (Inoue et al., 2008; Krall et al., 2010). Under these circumstances, energy conservation becomes a priority rather than increased body temperature (Inoue and Luheshi, 2010). Likewise, decreased intake and thus lower heat production through metabolism may be another contributing factor to hypothermia in feed deprived pigs. However, Case et al. (2011) proposed that under ambient temperature conditions, heat produced through digestion contributes minimally to body temperature maintenance. Collectively, these data may explain the increased mortality observed in pigs with low body temperature.
Blood glucose concentrations can also be affected by pathogen exposure and feed deprivation (Berg et al., 2002; Marik and Bellomo, 2013). In humans, normal blood glucose concentration ranges from approximately 80 to 130 mg/dL, depending on a fasted vs. fed state (Centers for Disease Control and Prevention, 2021). We hypothesized that decreased feed intake after weaning would lead to blood glucose concentrations below normal, therefore increasing mortality rates. Limited research evaluating supplemental glucose has been conducted in wean pigs; however, of the studies available from birth to approximately 30 kg, dosages ranged from 0.30 to 1.75 g/kg body weight (Manell et al., 2016; Kvidera et al., 2017; Engelsmann et al., 2019; Klaaborg and Amdi, 2020). Route of administration (intravenously, subcutaneously, or orally) also varied. Hence, the 10 mL dose used was selected based on an average of the reported literature, ease of application, and an assumed average pig weaning weight of 5.4 kg (approximately 1g/kg body weight). Furthermore, the 30-min blood glucose concentration measured postdextrose administration was selected based on an oral glucose tolerance test in pigs that showed a peak in blood glucose concentration 10 to 30 min after glucose intake (Manell et al., 2016).
While the results herein showed increased blood glucose concentrations when pigs were administered dextrose compared with no dextrose, no differences in survivability were observed. However, a quadratic response was observed for blood glucose concentration and predicted probability of mortality, with low and high blood glucose concentrations resulting in greater mortality. Interestingly, in the current study, high blood glucose concentrations > 250 mg/dL resulted in nearly 100% mortality. While the specific cause of high blood glucose concentration in this population is not known, high concentrations of glucose in circulation are often associated with the adaptive response of the body to acute or chronic illness, known as stress hyperglycemia (Marik and Bellomo, 2013). This is caused by an increased release of cortisol, epinephrine, and norepinephrine in response to hypothalamic pituitary adrenal axis activation, which increases hepatic glucose production. Additionally, cytokines and other inflammatory mediators such as prostaglandins that are released as part of the stress response pathway have been linked to insulin resistance (Parrott et al., 1995; McGuinness, 2005), which is known to increase blood glucose concentrations.
Moderate hyperglycemia in humans ranges from 140 to 220 mg/dL (Marik and Bellomo, 2013). Within this glycemic range, glucose uptake is maximized to provide fuel for the immune system and brain, while also preventing hyperosmolarity (Marik and Bellomo, 2013; Kenny et al., 2016). This is necessary to ensure host survival; however, at blood glucose concentrations > 220 mg/dL, Marik and Bellomo (2013) suggest that hyperglycemia may become more harmful than protective. Furthermore, as energy reserves become depleted due to severe or prolonged inflammation, hypoglycemia may occur following hyperglycemia (McGuinness, 2005). Collectively, these data indicate that the cause of death in pigs removed from the general population because of welfare considerations with low or high blood glucose concentration may have been related to feed deprivation, infection, or an underlying metabolic disorder.
Body temperature and blood glucose concentration also appear to interact, although their relationship is not well-understood. The interaction of blood glucose concentration and body temperature can be interpreted as pigs with a body temperature less than 38.6 °C and a blood glucose concentration greater than 120 mg/dL had increased mortality (P < 0.05) compared with pigs with a body temperature less than 38.6 °C and blood glucose concentration less than 120 mg/dL. In the present study, pigs with a blood glucose concentration less than 70 mg/dL had increased mortality as body temperature at removal increased. Throughout the literature hypoglycemia has been observed in combination with both hypo and hyperthermia (Chochinoz and Daughaday, 1975; Tran et al., 2012; Naseerullah and Murthy, 2018), although hypothermia is generally associated with severe hypoglycemia rather than hyperglycemia. Pigs with a blood glucose concentration greater than or equal to 70 mg/dL had decreased mortality as body temperature increased. A retrospective study looking at outcomes in newborn infants showed that 100% of infants with blood glucose concentrations > 200 mg/dL who were also undergoing therapeutic hypothermia became disabled or did not survive compared with infants with blood glucose concentrations < 200 mg/dL (Chouthai et al., 2015). These data may support the results observed herein, indicating that pigs within the high blood glucose concentration and low body temperature population first experienced hypothermia, which led to hyperglycemia. We also hypothesize that this group of pigs may have already transitioned into organ failure, whereas pigs with both high blood glucose concentration and body temperature may have been mounting a more normal immune response to infectious pathogens, therefore, decreasing mortality. The interaction between blood glucose concentration and body temperature appears to be dependent on if changes in blood glucose concentration precede body temperature or vice versa, and what the underlying cause (feed deprivation, infection, or metabolic disorder) was that initiated these changes.
In summary, pigs removed from the general population with decreased body weight or with body temperature or blood glucose concentration below or above the normal range had decreased survivability. While the mechanism by which body temperature and nutrient homeostasis interact is not fully understood, these data suggest a complex relationship between physiological functions and livability. Unfortunately, no differences in livability were observed in the current study when gruel feeding four times per day vs. two times per day or providing pigs supplemental oral dextrose.
Acknowledgments
Contribution 23-014-J from the Kansas Agricultural Experiment Station, Manhattan 66506-0201. Appreciation is expressed to Pillen Family Farms, (Columbus, NE) for their technical support. This research was supported wholly or in part by funding from The National Pork Board and the Foundation for Food and Agriculture Research, grant #18-147.
Conflict of Interest Statement
None declared.
