Effects of an extra-high slaughter weight and a low-lysine diet on growth and meat quality of finishing gilts

INTRODUCTION

The slaughter weight (SW) of market pigs is determined by a number of factors including the genetic lineage and nutrition of the animals, production efficiency, consumers’ demands, carcass grading standards, and others [1–4]. Increasing the SW, in general, causes a decreased daily gain, an increased daily feed intake, and steady fat deposition eventually resulting in a decreased gain-to-feed ratio [5–8]. Most meat-producing pigs are therefore harvested between 100- and 130-kg body weight (BW) worldwide, for which reason only limited information is available as to the effects of heavy market weight on production efficiency and pork quality, especially in pigs slaughtered at an ‘extra-high’ (XH) BW over 140 kg [8].

With increasing SW, the content of the intramuscular fat (IMF), which is generally believed to enhance the eating quality of pork [9–11], also increases at a rate ranging approximately from 0.01%/kg to 0.04%/kg SW in the Longissimus muscle (LM) primarily depending on the genetic background of the pigs [12–16]. In line with this, in our recent study reported by Hwang et al. [14], the LM IMF content, which increased at a rate of 0.04%/kg approximately between 110 and 135 kg of SW, was highly correlated with sensory scores for the juiciness, flavor, and palatability. By contrast, in our earlier studies with lean finisher pigs where the LM IMF content increased only at 0.012%/kg within a similar interval of SW, the eating quality of LM did not change due to the increase of SW [12,13]. It needs to be noted, however, that the relationship between the IMF content and sensory quality traits of meat can be influenced by other factors including the breed, physicochemical properties of the muscle other than IMF, socio-cultural factors, etc. [9,10,17].

Use of a diet having a low lysine (lys) content is known to elicit an increase in IMF deposition accompanied by an increased backfat thickness (BFT) [18–20]. In a companion study of ours preceding the present one [21], gilts fed a low-lysine (0.60%) vs. control (0.80% lysine) diet from 81-kg BW to slaughter at 132 kg exhibited the known consequence of the lysine deficiency indicated by an increased BFT whereas in barrows such a diet effect was not detected. The gilts fed the low-lysine diet, however, did not have a greater IMF content or better eating quality of LM than those fed the control diet. These results were different from those of the studies cited above [18,19], where the LM IMF content increased by as much as 1%–2% depending on the harshness and duration of the lysine deficiency. The present study therefore aimed to find out if it would be feasible to increase the meat quality of finishing gilts by increasing their SW to a 150-kg XH level vs. the 116-kg domestic average (Av) [22] and also by using the Low-lys diet in the production of XH-weight pigs. XH-weight barrows were excluded from the present study because of their over-fatness at 130-kg or greater BW [2,3,5,6], barrows with Av SW being included only as for the gender control of gilts.

MATERIALS AND METHODS

RESULTS

DISCUSSION

The BFT, as expected, was greater for barrows vs. gilts at 116-kg Av SW and also for the 150-kg XH-SW vs. Av-SW group gilts, which was consistent with published results regarding the effects of the gender [2,3,5,12,13] and SW between 100 and 165 kg [6,15]. Moreover, the BFT of the gilts increased between Av and XH SW by 8.9 mm at a rate of 0.26 mm/kg SW, which was much greater than 0.19 mm/kg at 114 ± 6 kg of SW observed in a previous study [20] in gilts having a leanness similar to that of the present ones. Of note, the dressing percentage increased by as much as 5% between Av and XH SW concomitant with the increase of BFT. These results conform to the known fact that with increasing SW, the ratio of the carcass per live weight increases due largely to an increase in subcutaneous and muscle fats [7,8]. It will thus be necessary to watch for over-fattening when producing heavy market pigs.

Regarding the effects of the Low-lys diet, it needs to be noted that ADG for the XHSW-LowLys vs. XHSW group was substantially less during the first 28 days but was slightly greater during the subsequent period to XHSW, albeit not significant statistically. This suggests that the XHSW-LowLys group probably grew faster than normal during the latter experimental period by virtue of the compensatory growth, which refers to a normal biological process whereby the animals previously under nutritional restriction grow at an accelerated rate to achieve a target BW and composition [30–32]. There’s also experimental evidence, if not proven, that in compensatory growth of previously lysine-restricted pigs, excess body fat which has accrued from the lysine deficiency is mobilized during the recovery period to make up for the delayed lean growth incurred by the lysine deficiency [31]. In this regard, the BFT, which was 2.6-mm greater for the low-lys vs. medium-lys diet group at 132-kg SW (p < 0.05) due to a presumptive lysine deficiency in the companion study [21] temporally overlapping with the present one, was equal for both groups at XH SW, suggesting that the disappearance of the BFT gap between the two SW groups probably resulted from lipid mobilization for compensatory growth. Likewise, the lack of effect of the low-lys diet on the IMF content of LL is also thought to be partly related to the presumptive compensatory growth in the XHSW-LowLys group. It is also known that compensatory growth occurs only when the previously restricted energy or amino acid is provided sufficiently during the recovery period [31,32]. In this connection, the calculated standardized ileal digestible (SID) lysine content of the low-lys diet (0.49%) was less than the requirements of 0.56% and 0.51% of dietary SID lysine concentrations for 125–140-kg and 140-160-kg pigs, respectively, estimated by the NRC [23] model [4] and Manini et al. [33], respectively. Nevertheless, the present results suggest that the low-lys diet was adequate in its lysine content to elicit the presumptive compensatory growth of the gilts during the later experimental period, which is not much surprising though, considering that the dietary lysine requirement is variable depending on the assumptions or estimates on the lean gain rate, efficiency of the amino acid utilization, feed intake and wastage of the animals, etc. [23].

In physicochemical properties of LL, the greater a* value for the XHSW vs. AvSW, albeit insignificant, was seemingly reflective of the known correlation between this color variable and SW [2,8]; results of the other color variables L* and b*, as well as those of the drip loss and RW percentages, were within normal ranges [2,8,14] irrespectively of a few detected differences between the experimental groups. The increase of the WBSF value for LL between Av and XH SW was also consistent with published results [14,34,35], but the SW-associated WBSF increase, which has been reported to cause a negative [14] or no [34] effect on the tenderness of cooked pork, apparently exerted no significant influence on the tenderness in the present study. The IMF content of LL increased between Av and XH SW at a rate of 0.014%/kg, which was close to 0.012%/kg between 110 and 133 ± 5 kg of SW obtained from previous studies in lean pigs [12,13]; effects of the SW-associated change of the IMF content, as well as those of the lower pH and greater cooking loss for the XHSW vs. AvSW group, on eating quality of pork muscle will be discussed in the following paragraph. As for the FA composition of LL, the unaltered percentages of major FA between Av and XH SW, including those of palmitic acid, stearic acid, oleic acid, and linoleic acids, were consistent with the results for a composite carcass muscle of finishing pigs between 91 and 127 kg of SW reported by Apple et al. [36]. Moreover, the lower percentages in palmitic acid and stearic acid and the greater oleic acid percentage for the XHSW-LowLys vs. XHSW group, which is presumed to have resulted from a few-percent greater content of animal fat mostly consisting of beef tallow in the Low-lys diet (personal communication with the manufacturer of the diet), were also consistent with the results of Apples et al. [36].

The sensory attributes associated with pork quality are influenced by a number of factors [10,11]. The IMF usually enhances the sensory pork quality attributes including the flavor, juiciness, and tenderness [9,10,11,37]; the pH also influences the sensory attributes of meat through its effects primarily on water holding capacity and myofibril fragmentation, the higher pH between 5.0 and 6.0 being the better in overall pork quality [11,38,39]. As related to the present results, the 0.44% greater IMF content for the XHSW vs. AvSW group, albeit insignificant (p = 0.14), is likely to have contributed, in part, to the increased juiciness and umami for the former group, whereas the 0.17-unit lower pH for the former is likely to have exerted a negative influence on the meat quality indirectly. The lower marbling and flavor scores for the XHSW vs. XHSW-LowLys group, however, were not seemingly related much to either IMF or pH, because differences in these factors between the two groups were relatively small. It was apparently paradoxical that the juiciness of cooked LL, which is known to be negatively correlated with the cooking loss [11], was greater for the XH vs. Av SW group with a greater cooking loss for the former. However, it is also known that the juiciness increases with the increase of SW and IMF [10,11,38], and the cooking loss has been reported to be increased [15], unchanged [14,24] or even decreased [2] by the increase of SW. Moreover, the relationships among SW, cooking loss, and juiciness were not clear in our previous study [14]. It is thus seemingly likely that the difference in cooking loss between the Av and HX SW groups was not significant enough to influence the sensory trait whereas other effects such as those associated with the increased SW and IMF outweighed the negative influence of the cooking loss if any. Obviously, more studies are necessary to elaborate the influences of XH SW on the cooking loss and juiciness of pork.

Linoleic acid and linolenic acid, which are prone to oxidation during storage, can cause off-flavor of meat [40,41], but the increases in these FA percentages in the XHSW-LowLys vs. XHSW group were not big enough to influence the flavor in the present study. Similarly, the increased 18:1 and MUFA percentages and a decreased 18:0 percentage of the LL FA composition for the XHSW-LowLys vs. XHSW group also appear not to have been big enough to influence the meat quality in the present study, although the former FA and 18:0 are reportedly related with good and undesirable eating experiences of beef, respectively [28,42]. With respect to the gender effects, the small differences between barrows and gilts in some quality attributes observed in the present study were similar to the results reported by Trefan et al. [43]. Pork quality is also known to be influenced by a number of water-soluble compounds such as sugars and free amino acids as well as those derived from lipids [10]. However, only limited information is available as to how the contents of those compounds in pork change with increasing SW as related to meat quality [44]. More studies in this area are therefore awaited to better understand the effects of increasing SW of finishing pigs on their meat quality.

CONCLUSION

The meat quality of gilts was improved by increasing their SW from 116 kg to the 150-kg XH level. It will be hence feasible to produce XH-weight market gilts if the increased meat quality can make up for the decrease in production efficiency resulting from the accelerated fat deposition following the increased SW. The low-lys diet, however, neither elicited an increase in the IMF content nor improved the meat quality of the gilts at XH SW. Therefore, use of the low-lys or similar finisher diet for the entire finishing period of the pigs raised to XH SW won’t be effective for increasing their meat quality. Instead, the low-lysine diet may well be a proper choice for heavy pigs near XH SW which have a reduced lysine requirement.