A summary of the studies found in the literature review is shown in Table S1 [suppl] (112 references); the oldest found corresponds to Aparicio-Macarro (1977) and the most recent one corresponds to Vázquez-Gómez et al. (2020). Within these references, the finishing is the most studied phase (74 references), while lactation is the least studied one (27 references). Most of the references give information on only one phase (60/112), while 12 references study the pigs whole life.
Figure 1 shows the triage of papers with the application of the inclusion criteria above mentioned. Finally, 18 works (16.07%) about growing and finishing Iberian pigs in montanera system have been selected and included for this meta-analysis. What is more, for the weight-age data from Castellano et al. (2013 and 2014) only a study has been included, because both studies correspond to the same experience.
Therefore, after applying the restriction criteria to the publications, 76 trials from 18 studies were used to complete the database, these including 22,558 animals.
There is a lack of data for Iberian pigs between 120 and 240 days old, which corresponds to the growing and pre-finishing phases. The reason for this information gap could be due to the complexity of these phases, when a strong feed restriction is necessary to maintain slow growth rate to obtain pigs of adequate body composition and weight before the final free-range finishing period or montanera (López-Bote et al., 2001). This feed restriction leads to a wide variety of management procedures to optimize the first phases, from intensive to extensive systems, with different types of growth modulation (Rodríguez-Estévez et al., 2011).
Although several models have been calculated with all the data (76 trials), only the simplest one is presented:
f(x) = -1.7738513 + 0.346855x
(RMSE= 12.66, r2 = 0.98, x ∈ [0, 537]) [Model 1]
where f(x) is weight (kg) and x is age (days).
However, this model is not good for the values near the end points of the interval, with bad fit and large RMSE. In order to improve this, the interval was divided according to the following categories: lactation phase, nursery phase and montanera finishing phase (Table 2).
Firstly, a mathematical model to predict growth rate as a function of weight and age is necessary. For the analysis of this lactation phase, there were available data from piglets weighed at birth and weaned between 21 and 60 days (9814 piglets from 23 trials within 8 studies). Here it is important to indicate that the traditional lactation length of this breed is 60 days.
This is the simplest fitting model to predict the lactation growth rate (Fig. 2):
f(x) = 1.24929 + 0.2025 x
(RMSE= 0.787, r2 = 0.974, x ∈ [0, 60] [Model 2]
where f(x) is weight (kg) and x is age (days).
The average birth BW in the database was 1.36 kg (Table 2), and the fitting model 2 estimated 1.2419 kg. Ayuso et al. (2016) reported a birth weight of purebred Iberian piglets of 1.21 kg, while the birth weight of crossbred Iberian piglets ranged from 1.75 to 1.77 kg (Soto-Caballé, 2006; Ayuso et al., 2016). The maximum weight (1.60 kg) found for purebred animals in this database is reported by Forero et al. (2001), and it is still lower than the average BW in crossbred piglets of the studies previously shown. Soto-Caballé (2006) reported an average weaning BW at 38 days of 8.41 kg in Iberian × Duroc crossbred pigs, which is lower than the BW of 8.94 kg obtained for purebred Iberian pigs at 38 days old with model 2. In this line, Ayuso et al. (2016) reported a lower birth weight of Iberian piglets compared to crossbred piglets and a higher postnatal growth of purebred Iberian pigs in the first weeks.
The ADWG during lactation from 0 to 60 days old obtained with the model 2 is 0.203 kg day-1. Aguinaga et al. (2010) obtained 0.165 kg day-1 in piglets weaned at 34 days and without having received any pre-starter feed (fed only with maternal milk); which could be the reason for that lower ADWG. On the other hand, Aparicio-Macarro (1987) found 0.225 kg day-1 for a weaning age of 60 days in a very traditional system, probably with poorer sow diet but with additional feed for piglets at the end of this period. The ADWG found in the present study was higher than the obtained by Vázquez-Gómez et al. (2020), with Iberian × Duroc crossbred piglets weaned at 24 days with an ADWG from 0.132 to 0.201 kg day-1, which is also consistent with the higher postnatal growth of purebred Iberian pigs previously reported by Ayuso et al. (2016).
There were available data from 9,085 piglets from 19 trials for the analysis of the nursery phase, with an average initial age from 28 to 71 days and average initial weights from 6.45 to 22.2 kg BW, average final age from 50 to 120 days old and average final weight between 11.4 and 33.8 kg and from (Table 2).
The best model to calculate the growth rate at the nursery stage is (Fig. 2):
f(x) = 0.95456 + 0.28285 x
(RMSE= 2.4388, r2= 0.852; x ∈ [28, 120]) [Model 3]
where f(x) is weight (kg) and x is age (days).
According to this model, the ADWG is 0.283 kg day-1, compared to the average of 0.317 kg day-1 resulting from the studies included at this database.
Sánchez-Esquiliche & Rodríguez-Estévez (2011) indicated that a target of 18.2 kg could be set for crossbred Duroc × Iberian piglets at 61.7 days old; which is close to the estimation of 18.4 kg BW with model 3. Vázquez-Gómez et al. (2020) showed from 20.6 to 24.7 kg of BW at 71 days in crossbred piglets; while this model 3 estimated 21.03 kg at that age. Thus, it is shown that the weight of Iberian piglets and Duroc × Iberian crossbred piglets around the 3-month of life was similar.
A new model [4] was calculated to evaluate the weight-age for lactation and nursery phases as a whole, from birth to 120 days:
f(x) = 0.726235 + 0.2680132 x
(RMSE= 2.32, r2= 0.896; x ∈ [0, 120]) [Model 4]
where f(x) is weight (kg) and x is age (days).
At weaning time there were many changes and challenges for the piglets, such is solid diet instead of milk, litter grouping or separation from their mothers. Therefore, it is necessary to know the age at weaning to use one of these previous models [2] or [3]. However, for the first 120 days of life, the model [4] could be applied even if the age at weaning is unknown, because this model simplifies the growth modelling; although, it does not improve the accuracy of the two previous ones (RMSE was 0.787, 2.4388 and 2.32 for models [2], [3] and [4], respectively).
For the analysis of the traditional free-range finishing system based on just grazing natural resources (acorns and grass), there were available data from 3,659 pigs from 34 trials within 11 studies, with data from pigs that were 240 to 537 days old, with BW from 77.38 to 191.3 kg. At the beginning of the finishing phase, the average BW obtained was 114.2 (from 77.38 to 140.3) kg with 356.9 (from 240 to 429) days old (Table 2); and at the end of this phase, pigs were on average 438.80 (from 351 to 537) days old and weighed 162.58 (from 114.25 to 191.3) kg of average body weight (ABW).
A cloud of dots (Fig. 3) was obtained in the finishing age-weight and the mathematical model [5] is:
f(x) = 10.947763 + 0.31643 x
(RMSE= 18.45, r2= 0.75; x ∈ [240, 537]) [Model 5]
where f(x) is weight (kg) and x is age (days).
Finishing phase in montanera does not begin until there are enough acorns to forage only natural resources in a free-range grazing system (Rodríguez-Estévez et al., 2009) and, meanwhile, the pigs are fed restrictively. Montanera starting depends on weather conditions (first autumn rains) and availability of ripen acorns (mast) on the ground for the pigs foraging from October to February.
According to these meta-analysis data, 100-kg BW pigs were obtained between 250 and 425 days old, and 150-kg BW animals ranged from 350 to 525 days old (Fig. 3). This fact demonstrates that the growth rate of free-range fattened Iberian pigs does not only depend on their age, but also on the previous restricted feeding pattern. The pig age to start this finishing period is not fixed because it is determined by the availability of acorns in the ground; therefore, there is a large dispersion for the starting age, with extreme values like 240 to 429 days old and from 77.38 to 140.3 kg BW of initial weight in the reported studies.
Thus, there is an important age difference at the beginning of the montanera. Daza et al. (2007) stratified a group of pigs with a mean of 101.2 kg BW into three groups according to their age at the start of this finishing period. In line to this last cited work, the database obtained in the current work was classified according to the age of the animals at the start of this phase and three mathematical models were obtained for three age intervals (Table 1): 9 trials and 218 animals with a starting age under 334 days [group α and model 6], 17 trials with 2390 pigs and an age of 365±30 days [group β and model 7], and 1075 animals from 8 trials with an age over 396 days [group γ and model 8].
| Characteristics [a] | Growth stages | Finishing age group [b] | |||||
|---|---|---|---|---|---|---|---|
| Total [1] | Lactation [2] | Nursery [3] | Total [5] | α [6] | β [7] | γ [8] | |
| N | 22558 | 9814 | 9085 | 3659 | 218 | 2390 | 1075 |
| Trials | 76 | 23 | 19 | 34 | 9 | 17 | 8 |
| Average initial age (days) | 171 | 0 | 43.0 | 356 | 283 | 364 | 418 |
| Minimum initial age (days) | 0 | 0 | 28 | 240 | 240 | 341 | 413 |
| Maximum initial age (days) | 429 | 0 | 73 | 429 | 323 | 390 | 429 |
| Initial ABW (kg) | 54.3 | 1.36 | 13.3 | 113 | 103 | 117 | 117 |
| Minimum initial BW (kg) | 1.11 | 1.11 | 6.45 | 77.4 | 77.4 | 97.9 | 103 |
| Maximum initial BW (kg) | 140.3 | 1.6 | 22.15 | 140.3 | 119.5 | 140 | 135 |
| Average final age (days) | 228 | 43.2 | 69.1 | 441 | 369 | 460 | 485 |
| Minimum final age (days) | 21 | 21 | 50 | 351 | 351 | 438 | 473 |
| Maximum final age (days) | 537 | 60 | 120 | 537 | 383 | 510 | 537 |
| Final ABW (kg) | 80.3 | 9.89 | 21.1 | 161 | 145 | 168 | 164 |
| Minimum final BW (kg) | 3.9 | 3.9 | 11.4 | 114 | 114 | 150 | 154 |
| Maximum final ABW (kg) | 191 | 15.6 | 33.8 | 191 | 158 | 191 | 174 |
| Average period length (days) | 57.3 | 43.2 | 23.1 | 85.9 | 85.3 | 95.2 | 66.8 |
| Minimum period length (days) | 8 | 21 | 8 | 58 | 60 | 60 | 58 |
| Maximum period length (days) | 120 | 60 | 60 | 120 | 120 | 120 | 117 |
| Average weight gain (kg) | 26.0 | 8.54 | 7.77 | 48 | 42.3 | 51.5 | 46.9 |
| Minimum weight gain (kg) | 0.50 | 2.79 | 0.50 | 31.7 | 31.7 | 33.2 | 32.3 |
| Maximum weight gain (kg) | 62.0 | 14.4 | 20.0 | 62.0 | 62.0 | 61.3 | 57.7 |
| ADWG (kg day-1) | 0.404 | 0.194 | 0.317 | 0.594 | 0.548 | 0.553 | 0.734 |
| Minimum ADWG (kg day-1) | 0.063 | 0.132 | 0.063 | 0.271 | 0.271 | 0.369 | 0.442 |
| Maximum ADWG (kg day-1) | 1.000 | 0.260 | 0.510 | 1.000 | 0.790 | 0.871 | 1.000 |
[a]ABW: average body. BW: body weight. ADWG: average daily weight gain. [b] α [6]: Model for animals until 330 days old. at the beginning of the finishing period. β [7]: Model for animals between 331 to 396 days old at the beginning of the finishing period. γ [8]: Model for animals older than 387 days old at the beginning of the finishing period.
When this stratification was carried out, the mathematical models obtained for each age group had less RMSE than the model that does not consider the age of stratification at the beginning of the finishing period. For these age intervals, the ABW at the beginning of this finishing phase was 102.6, 116.9 and 116.7 kg for α, β and γ age groups, respectively.
It has been already mentioned that the start of free-range finishing is determined by the availability of acorns to be grazed and not by the age of the pigs (from 283.3 to 418.3 days of initial age at the start of finishing phase in this meta-analysis). The initial BW is driven by growth modulation with food restriction in the pre- finishing stages (Freitas, 1998). In this study, initial BW of β and γ groups were similar; however, in the finishing phase a substantial greater ADWG was found in γ group compared with the β group (0.553 and 0.734 kg d-1 for β and γ respectively), probably because, although the animals of both groups were of similar BW, their body fat/protein composition was different. This increase in ADWG is generally accompanied by an improvement in carcass quality characteristics, like an increase in the intramuscular fat (IMF) content. Indeed, Daza et al. (2007) or Rodríguez-Sánchez et al. (2010) compared animals of different ages at the start of this finishing period and found a lower meat quality at lower ages, additionally ADWG also decreased. However, Latorre et al. (2004) analysed commercial pigs (Pietrain × Large White sires mated with Landrace × Large White dams) and concluded that an increase in slaughter weight between 113 to 133 kg did not produce significate changes in IMF.
On the other hand, Oka & Iwamoto (2007) found that a feed restriction and re-alimentation of cattle caused a higher fat deposition and higher marbling scores than when cattle is fed ad libitum. The feed restriction is a common practice during the growing phase of the Iberian pigs to get a minimum increase weight and prepare these for montanera. In adition, a fast growth in this free-range grazing phase increases fat deposition because protein deposition is restricted by the low supply of protein by acorns (Lopez-Bote et al., 2001; Nieto et al., 2002), and the genetic limit for protein deposition in Iberian pigs (Nieto et al., 2012).
Therefore, it could be important to predict weight and age at slaughter when analysing the productive yields in finishing and the quality characteristics of the carcass of the Iberian free-range pigs. To facilitate these analyses, a surface model was applied to the database obtained in the present work, where the ADWG has been evaluated with respect to the age and weight of the pig at the beginning of the finishing period. Considering the age and weight of the pigs at the start of the finishing period a two-variable model was calculated to predict the ADWG during this period of finishing (Fig. 4).
f(x,y) = -0.6199 + 0.1135x – 0.031y – 0.0002x2 – 0.0001xy + 6.85·10-5 y2 (RMSE = 0.1249) [Model 9]
where x = age, and y = weight.
With this mathematical model, it would be possible to obtain an estimate of ADWG and the date or age at which these pigs could be sent to slaughterhouse and their target weight.