We hypothesised that a longer CR length was associated with improved survival while a shorter AC was associated with lower potential colostrum/milk consumption, and thus compromised survival. On the basis of our data, we can partially accept our hypothesis. As overall piglet size increased (piglet proportions), so did colostrum intake and survival. Proportionate pigs showed increased survival as size increased and had less variation in survival than disproportionate pigs.
The degree of proportion or disproportion could be a better indicator for survival and performance than morphological measures alone.
4.1. Abdominal Circumference
Piglet AC had a linear relationship with body weight at both birth and 24 h, as previously documented for humans [
27,
28]. The high correlation observed between AC and weight may be related to successful suckling and post-natal consumption of colostrum and milk. Human infant AC was negatively correlated with time from the last feeding, which supports the linear relationship but also highlights the possible variability and low correlation with AC across time [
29]. Although not measured in the present study, a similar relationship between time from feeding and AC in piglets may occur. Colostrum intake showed a negligible correlation with birth AC but an improved low correlation with 24 h AC. This correlation may improve if true time from last suckle was adjusted for. The design of this study only included those piglets born under observation, thus allowing for accurate recording at 24 h of age. Further, as the number of milk letdowns and successful suckling bouts were not recorded, the utility of these results is limited. If a strong relationship is present when true time from last suckle and or number of suckles is accounted for, AC change from birth to 24 h after birth could be used as a strong indicator for the successful consumption of sufficient colostrum [
30].
Within this experiment, we assumed a 30 min interval from last suckle for each piglet and found that AC at birth when combined with birth weight was a good indicator of potential colostrum intake. These findings agree with the work of Douglas et al. [
19], who showed that AC and birth weight were good predictors of performance. This is likely due to them being indicative of stomach capacity, which is greater in larger pigs allowing more milk and colostrum to be consumed at each let down [
31,
32]. Further, our second colostrum model supports this assumption as 24 h weight was a good measure of consumed colostrum when modelled with PigProp. As expected, when 24 h weight increased, so did colostrum intake. However, AC only influenced colostrum intake through PigProp and not individually. In light of these results, the true individual time from last suckle may have greater effect on smaller piglets.
In practice, the reproducibility of birth AC may be higher for prediction of colostrum intake than birth weight as it is unlikely to be as affected by factors difficult to account for like umbilicus (and its fluid) weight, although both measures cannot avoid influence by retained birth fluid. This could potentially distort the weight and or presumed empty stomach size; however, we assume this effect would be similar for both measures.
Although birth AC was a good indicator of potential colostrum intake, it could be significantly impacted by successful suckling and, as such, would require greater care for timing when measuring and applying in practice. AC at birth or 24 h alone did not influence survival, although its impact on colostrum intake, which does influence survival, suggests it should not be discounted as an indirect measure of survival as well as potential performance.
4.2. Crown to Rump Length
CR increased linearly with AC and weight at 24 h with a moderate correlation observed. It is commonplace in humans to use CR length during pregnancy and at birth in conjunction with AC, head circumference, and weight to gauge foetal development [
20,
21]. Interestingly, CR alone did not significantly influence weight, colostrum intake, or survival, contrary to our hypothesis. It was assumed that in larger litters with reduced individual uterine space, there would be greater variation in piglet size, and as a result, more piglets with shorter CR would be present and a difference would be observed. The lack of influence of CR preweaning is supported by the work of Douglas et al. [
19], who found CR had no significant predictive value prior to weaning. However, our relatively small sample size may have precluded detection of a CR effect, which would explain the lack of litter impact. Unfortunately, CR was not measured at birth to reduce handling time immediately following piglet expulsion; however, it would have been interesting to compare the survival of piglets with different AC and CR at birth to those at 24 h, the former being a period of relatively higher piglet mortality [
3,
4].
It can be assumed that CR is not influenced by time from the last suckle, supported by its negligible correlation to colostrum intake, making it a more reproducible measure than AC and affected by less circumstantial factors. However, even in humans, CR is rarely used alone, often being combined with a waist to height ratio to diagnose foetal, infant, and childhood conditions such as growth retardation and obesity [
33,
34,
35]. The previous pig morphology paper showed that CR can be used to calculate BMI, which was also a good performance indicator pre- and post-weaning. Therefore, CR is a good indicator of growth but could be a better survival indicator if combined with other morphological measures or used to calculate them.
4.3. Piglet Proportions (PigProp)
Previous studies have demonstrated that piglet body shape rather than birth weight had a greater influence on piglet performance [
16,
17,
19], which was further expanded in this experiment for survival. The AC alone could be significantly affected by colostrum and (or) milk intake, while CR is not influenced by colostrum intake, which is critical to survival outcomes.
When considered together (i.e., PigProp), an interesting pattern of distribution across the groupings and survival was apparent. Proportionate piglets, being those that had the same grouped CR and AC (1,1; 2,2; 3,3 as defined in
Table 1), occurred more frequently (53%) than the disproportionate groupings, with the greatest number being the smallest piglets. It is important to note that disproportion did not occur due to feeding and colostrum intake but was an important factor in determining survival. It could be suggested that the level of disproportion is an indicator of uterine growth performance such as intrauterine growth restriction (IUGR), which is well known to impact piglet viability at birth [
18]. This is supported by the knowledge that as IUGR severity increases, CR decreases; however, it is not known if this effect remains when it is considered relative to AC.
Within the population, 12% of piglets were extremely disproportionate (1,3 and 3,1), resulting in a high error in the analysis. Therefore, we are unable to confidently determine what impact being highly disproportionate has on survival in comparison to more proportionate pigs. However, the remaining disproportionate piglets showed that having a slightly larger AC or CR can improve survival significantly in smaller pigs. As hypothesised, the smallest proportional piglets survived significantly less than did the other proportionate piglets and the disproportionate piglets. Although the influence of PigProp on colostrum intake was known, the survival difference within the smallest piglet grouping when more than 200 g of colostrum was consumed was greater than expected. Although subjective, we observed visually that differentiating between these slightly disproportionate piglets at 24 h was very difficult, lending support to the suggestion that accurate measuring of piglets at birth is valuable in determining viability.
As AC category increased within the same CR grouping, colostrum intake increased. A similar pattern was observed for increasing CR length but was not as definitive. Surprisingly, the piglets from 1,3 had the highest colostrum intake of all PigProp groupings, similar to the largest piglets. Bootstrapping was applied to test if this would remain true in a theoretical larger population, and this was supported. The authors suggest that this may have been due to these piglets having a disproportionately larger stomach capacity, and being able to obtain a full stomach early on provided them greater energy to outcompete their similar-sized poor-performing littermates. However, as stated above, the number of suckling bouts or a controlled allocation/administration of fluids was not tested in this study, and therefore relation to stomach capacity cannot be confirmed. Regardless, PigProp at 24 h may be used as an indirect measure of colostrum intake and continual feeding of the piglet.
The smallest proportionate piglets showed similar weight change to the disproportionate piglets of groups 1,2 and 2,1, and piglets from groups 1,3 showed similar weights to piglets from groupings 2,2 and greater. From these data, we can infer that within visually smaller piglet groupings usually designated as poor performers and survivors, there are piglets that have survival and growth rates similar to those of larger piglets. It would be interesting to further investigate a larger population on the basis of these findings to determine if within the smallest grouping (1,1), there are characteristics which can define the poorest performers once again without having to estimate or monitor colostrum intake. Surprisingly none of the models or the morphological measures showed influence of sex on survival, which is the same as in humans but contrary to previous research in pigs [
36,
37]. Sex has been shown to influence a piglet’s chance of survival, with males tending to die from being overlain by the sow more than do females [
37]. This sex effect was not evident in our study, although the suggested sex effect may be influenced more by the sex ratio of the litter rather than sex per se, as suggested by Seyfang et al. [
38]. A larger investigation into morphological measures involving sex within litter effects would provide further evidence of the applicability of AC and CR as a tool for predicting survival and growth.
Although there is a standard growth chart for humans that is based on recurring measures or multiple measure types, there is not one for pigs, and it is known that there is a large variation in height and weight in humans due to race [
39]. Although speculative, a similar degree of variation may exist between pig breeds for these morphological measures [
21]. It is beyond the scope of this study to compare this; however, in comparison to studies from other countries, our mean CR and AC were smaller. Our mean CR and AC were similar to the means reported in light-weight pigs (AC = 23.3 ± 1.52 cm and CR = 24.0 ± 1.91) in the study by Douglas et al. [
19]. However, we are unable to determine if this was due to uterine capacity as litter size was not reported. Despite this, another study did report greater mean CR (28 ± 0.26 cm) than our study, despite having higher mean litter size (16 ± 1.17) [
40]. It is reasonable to assume that longer piglets need more space and thus with greater litter size, the capacity to reach this would be reduced. The disparity between our and other studies’ findings further strengthen the argument that the variation seen between human races is replicated in pigs. Our findings support the hypothesis that not all small pigs will die, as even within the smallest poor-performing piglet group, there are still piglets that if they obtained sufficient colostrum can survive to weaning without additional intervention.