An evaluation of the effects of floor space and lysine deficient diets on the growth rate of finishing pigs and an analysis of post-restriction compensatory gain

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Hagen, Chloe S
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Greiner, Laura
Stalder, Kenneth
Schulz, Lee
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Animal Science
In the spring of 2020, pork producers faced a unique circumstance due to the limited capacity to harvest pigs in processing facilities. This caused a back-log of market weight pigs with no available shackle space to be harvested. Due to this, producers worked to implement strategies to slow the pigs’ growth as they approached market weight. Due to facility limitations within a processing plant, pigs exceeding 136 kg are not easily harvested and therefore have reduced value. For the first time in history, pork producers worked to intentionally slow the pigs’ growth of pigs, whereas traditionally, producers desire pigs that have rapid lean growth. Successful strategies such as using an amino acid deficient diet containing 97% corn showed to be a quick and effective way in slowing growth. Historically, following an amino acid, and specifically lysine deficient diet, compensatory gain has been reported. This is where pigs previously fed a deficient diet grow faster in a recovery period (where pigs are fed a diet meeting its nutrient requirements) than pigs fed an adequate diet the entire period. Additionally, following research performed in 2020, showing negative carcass effects when assessed at the end of the restrictive period, it was questioned whether those negative carcass characteristics continue through market weight when pigs are fed a diet meeting its nutritional needs during the recovery period. Thus, the following study utilized 120 mixed-sex pens with 1680 pigs (n=14 pigs/pen) blocked by starting body weight of the pen (73.5 kg) to evaluate the effects of two restriction lengths of three- or six-weeks for a feed restriction period where the pigs were provided ad libitum access to a 97% corn diet. After the respective restriction period, pigs were allowed one of three lysine concentration levels during their recovery period: either control (100%); +10% of control (110%); or + 20% of control (120%) in terms of SID Lys:ME. Plus, a control that provided 100% of standardized ileal digestible lysine to metabolizable energy based on the predicted body weight (100% SID Lys:ME). It should be noted that pigs provided 100% Lys:ME after restriction were fed the same diet as control for that period, or 10/20% greater than that Lys:ME, and it was not based on the lysine requirements of the actual body weight of the pigs after restriction. Data were analyzed using mixed model methods (SAS 9.4, SAS Inst., Cary, NC) with the fixed effects of restriction length, lysine level within a restriction length, and the random effect of block. Repeated measures data utilized the effect of the phase using the compound symmetry covariance structure with the appropriate interactions. Pigs fed on the 97% dietary treatment were slower to market weight, pigs fed the 97% dietary treatment for 3-weeks took 8 days longer and 6-weeks took 20 days longer to market than the control pigs. Additionally, pigs within the same restriction group fed the 20% greater SID Lys:ME than those fed the same diet as control (100%) made it to market 3 and 2 days faster, for 3- and 6-week restricted groups, respectively. Compensatory gain was evident after both lengths of restriction, with pigs in the recovery periods growing faster (P<0.01) than control pigs. The increase in daily gain (ADG) was not accompanied by an increase in daily feed intake (ADFI), and thus was driven by an improvement (P<0.01) in the gain to feed ratio. With pigs being fed 120% growing greater than those fed the 100% diets within each restriction group within the first 3-weeks of recovery. Pens were marketed when the average weight met 130 kg, therefore, carcass composition was measured as each pig met market weight. Pigs fed 97% corn diets had increased (P<0.01) backfat with the backfat increasing the longer pigs were restricted, as seen with previous research when measuring backfat after restriction. With increased backfat, pigs held on lysine deficient diets had decreased carcass yield and loin depth (P<0.01). The ability to capture individual body weight and phenotypic variables for each pig, allowed for measurement of the standard deviation of bodyweight within a pen. The standard deviation of body weight within a pen increased when pigs were fed 97% corn dietary treatments. Therefore, an observational analysis was conducted (Chapter 3) to understand what may cause pigs to respond differently to periods of nutrient restriction and recovery. From this analysis, we find trends where lower body weight pigs and females become more restricted during a lysine deficient diet. Confirming the biological lysine requirements for pigs, we conclude that pigs having a greater lysine demand respond differently when compared to pigs with a lower lysine demand, and thus may create more variation within a pen at the end of the recovery periods. Another option to slow pigs’ growth was to increase the pen stocking density thereby reducing floor space, where historical research reported that pigs perform poorer when provided a reduced floor space. However, after a literature search, many floor space studies confound group size and floor space by increasing the number of pigs within a pen to decrease the floor space. Decreased resource availability (i.e., feed and water) impacts the ability to differentiate between the effects of group size and floor space. Based on the current understanding that decreasing the pig’s floor space often results in decreased feed intake, and that increasing dietary energy also decreases feed intake, it was questioned whether increasing dietary energy could reduce the negative effects of a lower floor space. Thus, a study (Chapter 4) was conducted to evaluate the effects of reducing floor space within a pen by adjusting pen size rather than group size. Eighty mixed-sex pens were blocked by starting body weight to a 2 x 4 factorial of 4 decreasing floor space allowances from 0.67 to 0.60 m2 per pig and two energy levels within each floor space allowance of low vs. high. Pigs started at an average body weight of 65 kg until the first marketing cut around 120 kg. Data were analyzed using mixed model methods (SAS 9.4, SAS Inst., Cary, NC) with the fixed effects of floor space allowance and dietary energy level, and the interaction between the two main effects. Block was included as a random effect and repeated measures data were analyzed using the spatial power covariance structure to account for unequal spacing between time points. This study found no negative impacts (P>0.1) of reducing floor space beyond the traditional floor space allowance in the facility used. This is hypothesized to be due to the highest floor space treatment being restrictive itself; the group size and feeder space remaining the same across groups; or due to the seasonality effect on the trial. Energy performed as expected, with the high energy diet reducing (P<0.01) feed intake while increasing (P<0.01) G:F with no difference (P>0.1) discovered in ADG between energy. The objective of testing if high dietary energy could compensate for a lower feed intake due to low floor space was insignificant (P>0.1) due to the absence of impacts due to floor space. Thus, it seems that floor space requirements are a function of resource availability rather than the free floor space in the pen.
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