Variations of BF and production traits
At pre-selective stage, difference between the thickest BF gilts (20 mm) and the lowest one (7 mm) was almost three times (range 13 mm). Similarly, the heaviest gilts (146 kg) had almost two times difference in BW with the lightest one (77 kg). The wide ranges of BF and BW implied the difference in energy reserves and body conditions of gilts in this herd. Due to both traits associated with environmental condition and management regarding to the significant effect of contemporary group (P<0.0001) and their heritability estimates (0.23 ± 0.08 for BF and 0.47 ± 0.08 for BW), thus maintaining suitable environment and monitoring nutrient supply and feed intake throughout the herd are needed to optimize body condition of the gilt. The variations for BF and BW in this population should be reduced. The uniformity of replacement gilt would be helpful for breeding herd management and production planning. To optimize gilt’s body condition and ensure energy reserve for their production, daily feeding should be done by considering BF of an individual gilt. Providing feed as they need, would help each gilt accumulate fatty tissue properly that would be positive for BW, AFF and WSI. Besides, for economic reasons, it could also prevent an unnecessary feed cost from overfeeding and increased expense for extended management time due to underfeeding that caused thin gilt, delayed puberty, and prolonged non-productive days.
Landrace, originated from the temperate country, has widely been using as the dam line for commercial pig production in Thailand. A high ambient temperature in tropical country might affect well-being and production efficiency of the sow. Even the ancestor of gilts in this population has adapted to the tropical environment over two decades, the environmental condition in the housing still significantly affected fat accumulation, growth, fertility, losing piglet at birth and returning to service of primiparous Landrace sows. During the study period (2010 to 2017), although there was only one degree change in average ambient temperature around the study area (26 to 27°C) but the temperature in summer tended to increase by year from 27°C in 2011 to 29°C in 2017 and reached to 30°C in 2016 with the average daily maximum temperature at 37°C (Thai Meteorological Department 2019). High temperature in tropical condition especially during summer could lead to heat stress, resulted in decrease feed intake (Quiniou et al. 2000) and depress the de novo fatty acid synthesis in adipose tissues (Wu et al. 2016). However, at similar level of feed intake, raising pigs under high ambient temperature could lead to increasing proportion of lipid content of backfat (Kouba et al. 2001) that may result from metabolic response to reduce metabolic heat production from protein metabolism, consequently, more energy was obtained for fat deposition (Le Bellego et al. 2002).
As the current result, gilts born in summer tented to have high BF than gilts born in other seasons, particularly winter (average ambient temperature ranged from 23 to 25°C). Nevertheless, BF of gilt at pre-selective stage might not depend only environment and management at the beginning of their life, it might be varied by environment and management along their growing phase, feed and feeding program and selection strategy. In this population, gilts born in 2014 and 2015 had a greater BF than gilts born between 2010 and 2013. This result implied that BF might become an important indicator for replacement gilt selection. Then they received more proper feed management to adequate energy reserve to improve their fertility, therefore AFF tended to reduce, paralleled with increased BF in the last two years. The sufficient diet would also support the gilts to accumulate subcutaneous fat in growing phase, especially in winter. They achieved BF at 14 mm in winter 2014 and 2015 while had BF at 12 mm in summer of the same year. There was 5 degrees different between summer and winter seasons. High ambient temperature might affect voluntary feed intake of the gilts which is a metabolic response to decrease heat production (Baumgard and Rhoads 2013).
An average BF of Landrace gilts in this population was close to other reports in tropical country such as Landrace gilts in Thailand (Wongsakajornkit and Imboonta 2015) and Landrace, Large White and crossbred gilts in Philippines (Lego and Bondoc 2020) that had BF of 11.80 and 12.29 mm at 22 weeks old, respectively. Compared to temperate country, our gilts had a lower BF than Landrace (13.50 mm for BF at 100 kg) and Yorkshire (13.30 mm for BF at 100 kg) gilts in China (Hu et al. 2016) and crossbred gilts (13.26 mm for BF at 29 weeks old) in Slovenia (Flisar et al. 2012). Although gilts raised under cold whether seemed to have a higher BF than gilts raised under tropical climate, a high BF (15.60 mm for BF at 77 kg) was also performed in Landrace pigs in South Africa (Dube et al. 2011), even in Thailand by Imboonta et al. (2007) who reported 13.90 mm for BF at 22 weeks old in Landrace gilts. The various BF in different populations might cause by the difference in genetics, breed groups, ages and site at measurement, feed formular, feeding management and environmental conditions (i.e., open-house system or evaporative cooling system) including factors relating feed intake of the pigs.
However, gilts in our population (CV 18%) had a slightly lower variation of BF than gilts in Philippines (Lego and Bondoc 2020), South Africa (Dube et al. 2011), China (Hu et al. 2016) and Korea (Alam et al. 2021) which had CV ranged from 18.52 to 19.75% but other Landrace populations in Thailand, they had much higher variation of BF with CV of 25.90% (Imboonta et al. 2007) and 28.81% (Wongsakajornkit and Imboonta 2015). The variation of BF in this population was in the range of BF as found in both tropical and temperate countries but more uniformity that implied to the efficiency of feeding program and well management under tropical condition.
For AFF, tPL and tWSI, their variations highly depended on environmental variation (87.75 to 99.24%), indicated the need of monitoring and maintaining suitable environment and management to improve these traits. According to Bertoldo et al. (2009), high ambient temperature could lead to low farrowing rates and delay onset of puberty. But in our results, AFF seemed to not relate to an ambient temperature, its variation would associate with gilt’s body condition (BF and BW) and other factors such as heat detection skill, semen quality and farm owner’s decision. Thus, the farm owners should focus on feed and feeding program to support growth performance and provide sufficient body fat reserve that is needed for puberty onset. High ambient temperature during summer could also increase the occurrence of stillborn piglets in both tropical (Imboonta et al. 2007) and temperate conditions (Rangstrup-Christensen et al. 2017). Although there was no obvious pattern of PL in the particular season in our research, the highest PL estimates (18.18 ± 0.78%) was observed in summer 2015 which had an average ambient temperature of 29°C and reached to 36°C at peak. However, it did not occur in the same pattern every year. Thus, it could be possible that the variation of tPL might not relate to only the ambient temperature at farrowing. Identifying possible risk factors for increased PL in summer 2015 would be helpful to prevent piglet loss in the future.
Association between BF and production traits
A high positive genetic correlation between BF and BW (0.70 ± 0.13) and negative genetic correlation between these two traits and AFF (-0.42 ± 0.28 for BF and AFF, and -0.62 ± 0.20 for BW and AFF) would benefit the genetic improvement program to enhance genetic ability for growth and fertility of the replacement gilt when considering BF at pre-selective stage for selection. Although BF and BW could not be a direct indicator for tWSI due to their correlations but highly positive genetic correlation between AFF and tWSI (0.78 ± 0.36) revealed the possibility of indirect selection for tWSI by focusing BF or BW of replacement gilt. Therefore, it could be possible to use selection index to improve BF, BW, AFF and tWSI simultaneously. In this population, about 19.35% of sires, 26.34% of dams and 25.81% of sows had a great genetic ability (EBV above the population mean) for BF, BW, AFF and WSI. Those animals should be a target for replacement gilt selection. Selection those sows would make more profit in the next production cycle by improving growth, puberty, and fertility of gilt at the same time. On the contrary, there were 16.94% of sires, 20.97% of dams and 17.49% of sows that needed to be culled due to poor genetic ability (EBV below the population mean) for BF, BW, AFF and WSI.
However, due to the moderate phenotypic correlation between BF and BW (0.49 ± 0.03), pig producers could use BF record at pre-selective stage as an indicator for growth performance of the replacement gilt that would be useful for routine feed management. Unfortunately, it could not indicate the first productive performance of those gilts due to low phenotypic correlation (closed to zero for BF and tPL and tWSI). A meaningful level of genetic correlation estimate among BF, BW and AFF revealed the need of genetic parameter estimation for setting the target trait and selection plan in genetic improvement program. Considering phenotypic correlation for selection might lead an inappropriate decision to the farm owner which could negatively affect the production efficiency of the sow in the long term.
At pre-selective stage, the gilts are not fully mature. If the diet is imbalanced or feed intake exceeds the requirement for maintenance, physiological necessity for growth and production, the excess energy will be stored as depot fat (Kyriazakis and Whittemore 2006). Therefore, a thicker BF could imply a heavier BW of gilt at pre-selective stage that allows them passing into the breeding herd, thus, they would produce their first litter early. This result was confirmed by the negative genetic correlation between BF and AFF as the result of BW and AFF. Backfat is the reservoir for several important metabolic hormones associated with puberty attainment of gilts such as leptin and insulin-like growth factor I (Roongsitthichai and Tummaruk 2014). Besides, at the same age, the heavier weight of gilt implied the faster growth rate in the rearing phase and the possibility to be physiologically more mature (Flower 2005). The onset of puberty may be related to the critical body weight and minimum percentage of body fat (Frisch 1984). Thus, gilt that reached the threshold weight or fat percentage early, would show the first estrus and give birth at a young age. Giving first litter early would benefit pig producers by reducing non-productive days and the cost of production. Hence, BF assessment of individual gilt should be practiced routinely for proper feeding suitable to the body condition of the gilts that would help them perform greater fertility.
The genetic correlation between BF and tPL was estimated with high standard error as found between tPL and other production traits. There was a small genetic change in PL when the replacement gilt was selected for BF. Thus, it could be possible to genetically improve both BF and piglet production of the replacement, independently. However, the high standard error of genetic correlation estimates might cause by the limitation of data structure and small dataset, thus, to confirm the association between BF and PL, the genetic correlation should be reinvestigated in a larger dataset.
Regarding to genetic correlation estimates, BF associated with BW more than other production traits. This result might be due to these two traits were measured at similar time. The level of genetic correlation estimates between BF and production traits seemed to decrease with increased difference in measuring time of the traits. It could imply that BF may indicate body condition and energy reserve of gilt within short span lifetime. Therefore, one possible reason for unclear association between BF at pre-selective stage and tPL might be the long interval between BF measurement (28 weeks old) and first farrowing that varied from 20 to 50 weeks, approximately. Naturally, the BF of female pigs can be changed all the time depending on their status in the production cycle. The maternal fatty tissue will be grown in pregnancy as the energy store for fetal development and their own maturity and will be catabolized for mammary development, milk production and rearing their progenies in the lactation period (Kyriazakis and Whittemore 2006). It could be possible that the variation of tPL at first farrowing of sows might more tightly associate with their body conditions at late gestation (days 35 to 100) and parturition which have been known as the critical point for mummified and stillborn piglets (Botaya et al. 2014), or might relate to other risk factors occurred in the duration between BF measurement and first farrowing. The current result was in disagree with Arango et al. (2005) who reported a negative genetic correlation between BF (at 140 or 170 days of age) and number of piglet born dead at first parity of Large White sows (-0.14) and Imboonta et al. (2007) who found a negative genetic correlation between BF (at 22 weeks old) and number of stillborn piglets in first (−0.04 ± 0.18) and later parities (-0.22 ± 0.11) in Landrace population. The different results of those reports might depend on breed group, data structure, age at BF measurement, characteristics of study trait, statistical model, and environmental condition.
Non-correlation between BF and tWSI might also be due to the difference in measuring time as found in tPL. The tWSI was recorded after weaned that was around 24 to 78 weeks away from BF measurement. The fatty tissue will be accumulated during pregnancy and lost it during lactation (Kyriazakis and Whittemore 2006). Thus, the variations of tWSI in primiparous sows might more closely be correlated with their body conditions at insemination, parturition and post-weaning and amount of BF loss during lactation. As the report of Knecht et al. (2020) in Poland, the significant difference in weaning to service interval was observed between sows with different fatness degree at insemination. Under a well-feeding program, the sows would have an adequate fat store for fetal growth, milk production, rearing their young piglets and replenishing their fatty tissue loss during lactation that would be beneficial for recovering their body to rebreeding condition. Unfortunately, the available data had only BF at pre-selective stage. Promoting pig producers to measure BF of individual female pig as the routine along the production cycle since service, during pregnancy, before parturition and after weaned would provide the supportive information to reveal the association between BF and production traits that would be useful for improving body condition to support productivity of the breeding sows. Nevertheless, selection emphasis on BF should be optimized. The excess BF of breeding sows could negatively affect their productivity (Roongsitthichai et al. 2010; Zhou et al. 2018). Due to an optimum BF of gilt and sow in each production stage could vary by their genetics and environment, thus it should be investigated in the target population before setting breeding program and feeding management.
However, the association estimates between BF and production traits are the population parameters that are specific to this Landrace herd. Considering these parameters should concern the population structure, characteristics of the animal, environmental condition, and management. These results would be useful as a case study for commercial Landrace gilts raised under tropical conditions.