Kolbroek pigs are popular among local pig breeders. It forms part of national assets, which are being conserved to maintain the breed genetically pure. It contributes to poverty alleviation, as a protein source. Kolbroek pigs are a South African indigenous pig breed that possesses valuable traits, such as longevity, disease tolerance and adaptability to harsh environmental conditions in addition to its contributions to food security [1]. They are small sized, weighing about 0.5 kg at birth and about 45.5 kg at one year of age [2]. They also have been recommended as good alternative sources of protein that can provide additional income especially for local rural farmers [3]. Current literature has shown that indigenous pigs tend to have thicker back-fat compared to exotic breeds, which prevents them from receiving good carcass grades [4]. However, there are observations that Kolbroek pigs are less efficient as protein sources because of their tendency to put on excess back fat [5]. These may be because the standardised protein requirement for Kolbroek boars has not been ascertained [3]. These pigs are typical fatty pig breeds with 28% to 35% of meat quality [6] [7]. Due to economic reasons and customer demands, pig producers use nutrition to improve their carcass quality with low fat content [6] [8]. Moreover, the body weight of a boar is an important indicator of its growth, health and readiness to go to market [9].

Low nutrient levels can easily be met through use of locally available feed resources, such as leguminous leaf meals, groundnut hulls, sunflower cakes and other fibrous protein sources [10]. These nutrients influence the growth performance and carcass quality in boars. Many farms feed boars on sow diets, but the feeding program may neglect some of the unique nutrient requirements that must be met to optimize breeding sire reproductive performance [11]. In addition, low protein diets have been shown to further delay attainment of puberty and reduce boar libido [12].

Back fat thickness (BFT) is one of the important economic traits that are used to assess the carcass quality [13]. It is generally presumed that back fat depth positively correlates with growth and reproductive traits in male animals [14]. It is thus conceivable that selection based primarily on productive characteristics, especially for lean growth, leads to reproductive problems, such as low spermatozoa production and delays in libido expressions after weaning in boars [15]. Diet supplementation with crude protein (CP) could promote lean growth accompanied by decreased BFT and age of attainment of puberty.

The current study was conducted to assess the effect of dietary protein supplementation on growth performance, puberty attainment, BFT, visceral organs and carcass traits of Kolbroek boars. We hypothesized that dietary protein levels have minimal effects on growth performance, puberty attainment, BFT, visceral organs and carcass traits of Kolbroek pigs.

Growth performance and age of puberty

Our data revealed that there was an increase in final body weight with increase in supplemented protein diet with a body gain of 6.2 kg at 13% diet. In addition, there was an increase in total body weight gain as the protein supplementation was increased. Average daily gain was increased from 0.29 kg/day to 0.33 kg/day as the protein diet was increased. The BFT was decreased by 0.2 cm at 16% protein supplementation. Interestingly, the age of attainment of puberty was decreased by 3 days accompanied by loss of weight at puberty albeit insignificantly. The effects of dietary protein inclusion levels on growth performance of pigs are presented (Table 2).

This study revealed that the carcass length increased by 2 cm as the protein supplementation was increased. Warm and cold carcass weight also increased by 5.48 kg and 6.24 kg at 13% and 16% protein supplementation. Accompanying this was the lean meat percentage that increased by only 1% while the drip loss percentage increased by 0.1%. Additionally, meat colour lightness improved accompanied by a decline in meat redness and yellowness (Table 4).

The effects of crude protein supplementation on visceral organ of the Kolbroek boars are shown (Table 5).

Pearson’s correlation coefficients between growth performances of Kolbroek boars are summarized (Table 3). There was a high positive correlation between ADFI and ADG (r = 0.78). Moreover, a positive correlation existed between ADG and BFT (r = 0.12) accompanied by a high negative correlation between FCR and ADG (r = −0.94). Furthermore, negative correlations existed between feed intake and FCR (r = −0.57), feed intake and BFT (r = −0.08) and FCR and BFT (r = −0.02), SEM - Standard Error of the Mean.

ADG- average daily gain; ADFI- average daily feed intake; FCR-feed conversion ratio.

^a-bValues with different superscript differs significantly (P < 0.05) L* (lightness), a* (redness), and b* (yellowness), p-Value and SEM- standard error of the mean.

Improvement of carcass quality is important for the economy and consumer satisfaction. Kolbroek pigs are a good source of protein and are popular with our local farmers because of its desirable traits. In this study, we have shown that by improving the carcass quality and growth performance through dietary crude protein supplementation, we can reduce BFT while improving carcass body weight and length. In addition, the age of puberty decreased accompanied by loss of weight at that age. According to Heyer and Lebret (2007), final body weight has no influence on carcass traits of Large White 70 and Large White 110 and did not modify the effects of feeding strategy on carcass traits [21]. However, Kerr et al. (1995) reported increases in carcass weight, BFT, lean meat thickness and lean percentage in Du × LW and CH × LW pigs when dietary protein levels were 17.3% high and 15.4% low [22]. In Yorkshire male pigs, increases in carcass traits with the increasing dietary protein supplementation were reported [23]. In contrast, Pharm et al. (2010) noted that carcass traits were significantly affected by dietary crude protein supplementation for BFT, longissimuss muscle length, dressing percentage, lean percentage and fat percentage, with the exception of carcass length [24]. Moreover, the ADG increased as was the ADFI. In this study, these parameters had high correlation as the protein supplementation was increased. Additionally, the FCR showed opposing relationships with ADFI and ADG. It has been reported that the optimal dietary crude protein content for maximum ADG was lowest for the Mong Cai boars (14.5%) and medium for the Large White × Mong Cai boars (15.9%) and highest for the Landrace × Mong Cai × Large White boars (16.4%) [24]. Carpenter et al. (2004) and Nyachoti et al. (2000) reported a significant reduction in ADG and ADFI for diets containing a CP level of 19% or less while Madrid et al. (2013) reported that there were no significant effects of dietary treatment on performance traits measured in growing and the finishing period (LW × LR) pigs [25] [26] [27]. Similarly, it has been reported that a strong correlation exists between ADFI and ADG [28] [29]. In addition, it has been indicated that dietary protein supplementation increased the ADG and lean percentage and subsequently decreased the FCR and BFT [30]. Others have shown that increasing the dietary protein level accelerated the increase in ADG in Kadon pigs [31]. In addition, feed intake increased as dietary protein levels were increased in growing to finishing pigs [32].

In this study, WHC appears to decrease as the protein supplementation was increased. In addition, Hong et al. (2016) reported that WHC was decreased by the effect of phase feeding through dietary protein levels in growing pig ([Yorkshire × Landrace] × Duroc) crosses and WHC was decreased and cooking loss was also increased as dietary protein level reduced [33]. Cromwell et al. (1993) reported that increasing the dietary protein or lysine level resulted in improved rates of gain and increased carcass of (Landrace × Mong Cai) × Large White [34]. Interestingly, pigs fed low protein diets have been shown to have fatter carcasses compared with those fed high protein diets [22] [35]. However, in our case, crude protein supplementation decreased BFT.

We also found that lungs, liver, small and large intestine and stomach were all increased as the protein supplementation was increased. Despite others reporting that the decreasing dietary CP level decreased the relative organ weights of liver, heart and pancreas, we have made opposing observation [22] [36] [37]. Heo et al. (2010) reported that reduction in pancreas weight of pigs fed low-CP diets suggested a reduced amount of pancreatic protease, which could be required to digest the smaller quantity of consumed protein [37]. It has been reported that the weights of small and large intestine and stomach were higher in pigs fed the high protein than those fed the low protein diets in agreement with our study [38] [39] [40]. Shortcomings and limitations of this study include the fact that the differences obtained due to various protein diets are very small and often insignificant. However, the overall benefits per carcass could account for greater profits for farmers.

We have shown in this study that we can improve Kolbroek carcass quality by using CP dietary supplementation. These improvements were shown by increases in body weight, length, visceral organs and decreases in BFT and the days of attainment of puberty. This means that inclusion of protein in the diet of Kolbroek will have returns for the farmer, but it also means that the farmers have to spend more in the diet to have these returns. Therefore, this model might not be viable for poor farmers and thus prompt for more research on improving the diets for Kolbroek boars.

This work would not have been possible without the financial support of Agricultural Research Council-Number: P0100935-02-36. In addition, I would like to extend thanks to all of those with whom I had the pleasure to work with throughout the duration of this project.

The authors declare no conflicts of interest regarding the publication of this paper.

Netshirovha, T.R., Umesiobi, D.O., Matabane, M.B., Mphaphathi, M.L., Pilane, C.M., Thomas, S.R., Sebothoma, P., Kanengoni, A.T. and Nedambale, T.L. (2020) Improvement of Kolbroek Boar Growth Performance and Carcass Quality through Dietary Crude Protein Supplementation. Open Journal of Animal Sciences, 10, 502-513. https://doi.org/10.4236/ojas.2020.103031