Introduction

Restriction of dietary nutrient intake during periods of rapid growth can be used to induce compensatory growth without negatively affecting overall growth performance. The phenomenon of compensatory growth occurs naturally in almost all domestic animals especially after weaning or after elimination of internal parasites. Restriction of daily DM intake (50 % of intake) during post-weaning has particularly been used to reduce the proliferation of hemolytic Escherichia coli and the occurrence of diarrheal infections (Rantzer et al. 1996). However, induced compensatory growth caused by re-alimentation of dietary nutrients following a period of restriction can also be used as a management strategy by farmers to increase growth rate (Heyer and Lebret 2007). The strategy is based on the fact that nutrient restriction during the period of rapid growth, usually after weaning, increases the rate of growth by increasing protein turnover through an increase in protein accretion and a decrease in protein degradation (Heyer and Lebret 2007). Even though the phenomenon of compensatory growth has been investigated in many farm animals, factors such as genotype, the level and nature of dietary restriction, and the length of restriction can influence the phenomenon.

The Ashanti Black pig is the commonest local breed in Ghana. They are highly prolific and adapted to the utilization poor quality feed (Barnes and Fleischer 1998), but their smaller matured size has warranted breeding them with the Large White to increase matured weights of the crossbred. The major problem of pig production is the continuous supply of feed from weaning to market weight. Recurrent feed cost constitutes ∼65 % of total production cost, and pig farmers who want to remain economically competitive must adopt feeding strategies that increase feed efficiency while reducing cost. Strategic restricted feeding is one of the management strategies by which feed cost may be reduced during post-weaning. It involves restricting dietary nutrient intake followed by an increased intake of these nutrients thereby inducing compensatory growth. In pig production, growth is defined as an increase in tissue mass which occurs by hyperplasia (cell multiplication) early in life and then by hypertrophy (cell enlargement) later in life (Danfaer and Strathe 2012). Consequently, a period of restricted growth through restricted nutrient intake followed by an increase in nutrient intake has been used to increase slaughter weight of finishing pigs (Edmonds and Baker 2010). The rate of protein accretion can be increased by increasing protein synthesis, or decreasing protein degradation, with the rate of synthesis being greater than the rate of protein degradation when positive growth occurs (Kristensen et al. 2002). If pig farmers can increase or achieve markets weights similar to those fed ad libitum by inducing compensatory growth through restrictive feeding, they could increase productivity without negatively affecting their profit margins. Hence, the present study investigated the effects of increasing dietary protein or energy following a period of being fed a restricted or maintenance diet on growth performance of weaner pigs.

Materials and methods

Animals, diets, and feeding

A total of 18 Ashanti Black × Large White crossbred weaner pigs (7.5 ± 0.30 kg; mean ± SD) were randomly assigned to nine pens with two pigs per pen. The pens were randomly assigned to three dietary treatments in a completely randomized design resulting in three replicate pens per treatment. Prior to the start of the study, pigs were identified with plastic identification tags (Fearing Int. Ltd, Northampton, UK) and drenched with oral suspension of albendazole.

The pigs were accustomed to their dietary treatments and housing conditions for 7 days prior to the start of the study. In the first treatment, pigs were fed a nutritionally deficient diet [14.4 % crude protein (CP); 12.0 MJ/kg metabolizable energy (ME)] formulated to allow for maintenance of body weight at ad libitum intake for 56 days (maintenance diet). In the second and third treatments, dietary CP and ME were increased to 17 % (protein diet) and 14.0 MJ/kg DM (energy diet), respectively, for additional 28 days to meet the requirements of these animals for growth (Barnes and Fleischer 1998). The re-alimented diets were formulated supply CP and ME values that were intermediate between the requirements for the breed (Barnes and Fleischer 1998) and NRC (1998) values. The ingredient, protein, and energy content of the diets are indicated in Table 1. Pigs were allotted to nine concrete-floor pens (3.0 m × 5.0 m) at the University for Development Studies, Tamale, Ghana. Each pen consisted of a concrete wallow (2.0 × 1.5 m) and a concrete feeding trough (1.0 × 1.5 m). Pigs were weighed on two consecutive days at the beginning and end of the study with the average of the consecutive weights used as the initial and final weights, respectively.

Table 1 Ingredient and chemical composition of diets
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The major components of the restricted diet included corn chaff and millers waste. Corn chaff has a lower density of CP and energy. It was, therefore, possible to formulate diets with higher density of either protein or energy by varying the proportion of these ingredients.

The amount of feed offered was recorded daily. Orts were also collected daily, weighed, sampled, and discarded. Samples of feed and orts collected were pooled weekly and subsampled and stored (−40 °C) for subsequent proximate analysis. Pigs were weighed every 14 days. The DM of each dietary ingredient was also monitored weekly throughout the experiment for adjustments in the amount ingredients included in the diet, whenever necessary. Dry matter intake by each pig was calculated based on the feed DM offered minus DM of the orts. DM intake, average daily gain (ADG), and feed efficiency (expressed as ADG/daily DM intake) were estimated for each period or the entire 56-day feeding period.

Laboratory analysis of diets

Dry matter of ingredients, feed, and orts was determined at 60 °C for 48 h in a forced air oven.

Proximate analysis was carried out according to the official methods of analysis described by Association of Official Analytical Chemist (AOAC 2005). All analyses were done in duplicates. The protocol was used in determining (%DM) CP (calculated as total N × 6 · 25), ash, ether extract (EE), crude fiber (CF), and nitrogen-free extract (NFE). Nitrogen-free extract was computed using the formula: % NFE = 100 − (%CP + %CF + %EE + %Ash).

Metabolizable energy and net energy of gain of diets were calculated from the equations of Crampton and Harris (1969) and Noblet et al. (1994). The net energy (NE)g equation of Noblet et al. (1994) was slightly modified; NFE instead of starch was used in the present method of estimation because starch in particular was not determined in the present chemical analysis.

Statistical analyses

Data were analyzed using the PROC MIXED procedure of SAS (1991; SAS Inst. Inc., Cary, NC).

All data on DM intake and growth performance of pigs (weight gain, ADG, and feed efficiency) were analyzed for the fixed effects of dietary treatments as a completely randomized design with pen as the experimental unit in the statistical model below:

$$ {Y}_{\mathrm{i}\mathrm{j}}=\mu +{T}_{\mathrm{i}}+{e}_{\mathrm{i}\mathrm{j}} $$

where Y ij is the observation (DM intake, weight gain, ADG, and feed efficiency); μ is the overall mean effect; T i is the effect of dietary treatments; and e ij is the residual error effect.

Data on body weights of pigs for the entire 8-week period and growth performance (DM intake, weight gain, ADG, and feed efficiency) for the restricted and re-alimented periods were analyzed with the BY option of SAS for each week and each period, respectively.

Differences in least-square means were compared using the PDIFF option of LSMEANS. Least square means that showed significant differences were separated by Fisher’s pair-wise t test and were declared statistically significant at P ≤ 0.05 whereas P ≤ 0.10 were reported and discussed as trends.

Results

Table 2 shows DM intake and growth performance (weight gain, ADG, and feed efficiency) of pigs separately during the restricted and re-alimented periods. During the first 28 days, all pigs were fed a maintenance diet, hence, there were no differences (P ≥ 0.512) in DM intake and growth performance among all the treatments. However, in the second 28-day period when the pigs were switched from the maintenance diet to an energy or protein diet (re-alimentation period), DM intake, live weight gain, ADG, and feed efficiency were increased (P = 0.004) for pigs fed the protein diet than for those fed the maintenance and energy diets. During the re-alimented period, DM intake of pigs fed the protein diet increased (P ≤ 0.009) in weeks 6, 7, and 8 compared to those on the maintenance and energy diet, but the difference between the maintenance and energy diet was not significant (P ≥ 0.76) at weeks 7 and 8 (Fig. 1). These differences in DM intake during the re-alimented period did not however affect overall DM intake as it only tended (P = 0.06) to be greater for pigs fed the protein diet than for those fed the maintenance and energy diets (Table 3).

Table 2 Growth of Ashanti Black × Large White weaner pigs fed a maintenance diet (restricted period) for 28 days and re-alimented with a high protein (protein) or high energy (energy) diet for another 28 days in a 56-day feeding experiment
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Fig. 1
figure 1

Changes in DM intake of Ashanti Black × Large White weaner pigs fed a maintenance, energy, and protein diet. During the first 4 weeks, all pigs were fed a maintenance diet for 4 weeks. Thereafter, pigs were continually fed the restricted diet (maintenance diet) or switched to a high protein (protein) or high energy (energy) diet for additional 4 weeks. P values indicate weeks in which DM intake of pigs differed between treatments

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Table 3 Growth of Ashanti Black × Large White weaner pigs fed a deficient diet for 56 days (maintenance) or a deficient diet for 28 days and followed by a high protein (protein) or high energy (energy) diet for another 28 days in 56-day feeding experiment
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Table 3 shows DM intake and growth performance of pigs for the entire duration (56 days) of the study. There was a trend (P = 0.06) toward an overall greater DM intake for pigs on the protein diet compared to those on the other treatments; however, total live weight gain and ADG (P = 0.01) and feed efficiency (P = 0.04) differed among treatments; live weight gain and ADG were greater for pigs re-alimented with protein than for those re-alimented with energy. Re-alimentation with dietary protein increased (P = 0.01) the efficiency of feed utilization compared to re-alimentation with dietary energy, but this did not differ (P = 0.13) from those fed the maintenance diet. Cumulative weights of pigs during the period when the pigs were fed the maintenance diet (weeks 1–4) were similar (P > 0.05); however, body weights reached a difference (P = 0.04) at the 8th week of the experiment (Fig. 2) with pigs re-alimented with the protein diet achieving heavier weights than those re-alimented with energy. The difference in weight between pigs on the protein diet and those on the maintenance diet was not significant (P = 0.20).

Fig. 2
figure 2

Weights of Ashanti Black × Large White weaner pigs fed a maintenance diet for 56 days or a maintenance diet for 28 days followed by a high protein (protein) or high energy (energy) diet for another 28 days in 56-day feeding experiment. P = 0.04 indicates week in which pigs differed in their body weights

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Discussion

During the restricted period, DM intake and growth performance were similar among all treatments. When dietary CP for pigs was increased from 14.4 % during the restricted period to 17.4 % during the re-alimented period, DM intake of pigs fed the protein diet was increased by 0.7 kg. During the period of restriction when all pigs were fed the maintenance diet, neither DM intake nor growth performance was altered, but following re-alimentation, DM intake increased by 18 % whereas ADG and feed efficiency increased by twofold, for pigs fed the protein diet compared to those fed the energy diet (Table 2). Similarly, DM intake, ADG, and feed efficiency increased by 14, 41, and 20 %, respectively, for pigs re-alimented with the protein diet than for those fed the maintenance diet. The lower DM intake observed for pigs fed the maintenance and energy diets during the re-alimented period may explain why the growth rate of pigs fed these diets were lower than those fed the protein diet. Indeed, there was a trend toward higher DM intake for pigs fed the protein diet than for those fed the other diets during the entire 56-day period. Some authors (Ryan et al. 1993) have hypothesized that reduced maintenance requirement and greater deposition of protein were responsible for compensatory growth during the initial stages of re-alimentation, but compensatory growth in the later stages of re-alimentation was attributed greater DM intake. This hypothesis is consistent with the pattern of DM intake observed in Fig. 2.

An improvement in growth rate following periods of restricted nutrient intake is a feeding management strategy used to maximize returns on livestock production. The strategy exploits the phenomenon of compensatory growth and is based on the fact that nutrient restriction during the period of rapid growth, usually after weaning, increases the rate of growth by increasing protein turnover through an increase in protein accretion and a decrease in protein degradation (Heyer and Lebret 2007). The results from this study suggest that in Ashanti Black × Large White weaner pigs, the phenomenon may be enhanced by re-alimentation of dietary protein than dietary energy. Protein accretion during growth involves a balance of protein synthesis and degradation. The rate of protein accretion can be increased by increasing protein synthesis or decreasing protein degradation, with the rate of synthesis being greater than the rate degradation when positive or compensatory growth occurs (Kristensen et al. 2002). At their weight, the pigs used in the present study required a minimum of 18.0 % CP and 13.0 MJ/kg ME to meet their requirements for growth (Barnes and Fleischer 1998); however, the maintenance diet contained only 14.4 % CP and 12.0 MJ/kg of metabolizable energy (Table 1). Even though these values were lower than the NRC (1998) requirements for maintenance for other breeds, the dietary restrictions imposed in this breed appeared to have stimulated the expected growth restriction with body weights remaining statistically similar among treatments until the re-alimented period (Fig. 2). Edmonds and Baker (2010) observed that pigs fed CP-deficient diets (11–16 %) for 14 days and re-alimented to higher CP diets (15–20 %) diet for 21 days can recover and achieve growth performance equal to that achieved with a conventional protein feeding regimen in finishing pigs. In contrast, Critser et al. (1995) earlier found out that compensatory responses do not appear to depend on dietary protein concentration when pigs were fed a maintenance diet (14.4 % CP) similar to that used in this experiment and re-alimented with diets containing dietary CP that ranged from 13 to 18 %.

The DM intake of pigs on the protein diet increased in weeks 6–8 of the re-alimented period whereas those on the maintenance and energy diets decreased but were similar during the same period. Considering that the NE content of a diet offers the best assessment of the true energy value of a feed for more accurately predicting the growth performance of pigs (Noblet et al. 1994), the increased DM intake and the similarity in NEg and yet differences in ADG between pigs fed the protein and energy diets suggest the significant role of CP intake compared to energy on protein accretion and growth. It is possible to therefore hypothesize that the greater ADG and efficiency of gain observed for pigs on the protein diet could be due to increased intake of dietary CP leading to greater protein accretion rate. Whereas compensatory growth may have economic benefits through the reduction in feed cost without adverse effects on growth performance, the type of tissue gained can have implication on the market value of such body weight gains because weight gains during the re-alimented period have been associated with increased growth of internal organs (Fischer et al. 2001; Chaosap et al. 2011) and fat deposition (Hornick et al. 2000) rather than protein or skeletal muscle accretion which is the primary interest of commercial pig farmers.

Contrasting the present results with the literature (Chaosap et al. 2011; Hornick et al. 2000), it can be hypothesized that reduced maintenance requirements during restriction combined with increased protein deposition were particularly more important in the phenomenon of compensatory growth. Indeed, the improvements in the efficiency of gain observed for pigs fed the protein compared to energy diet during the re-alimented period could best be described as efficiency of compensatory growth since efficiency of gain was not significant during the period of restriction.

The occurrence of the phenomenon of compensatory growth in pigs depends on the age at which restriction is imposed. In some studies, ADG was reduced during the growing period but increased during the finishing period when pigs were restricted up to 65 % of their ad libitum intake (Heyer and Lebret 2007). The ADG of the pigs in the current study were similar to those observed for Ashanti Black weaner pigs under on-station (0.03–0.13 kg/day; Baffour-Awuah et al. 2005) and on-farm (0.16 kg/day; Tengan et al. 2015) conditions.

It can be concluded from this study that re-alimentation of dietary protein following a 28-day restriction of nutrient intake can improve growth rate of Ashanti Black × Large White weaner pigs compared to pigs fed a maintenance diet or re-alimented with dietary energy. Compared to the energy diet, re-alimentation of protein also improved the efficiency of feed utilization, but this was not different from pigs fed the maintenance diet. Further studies to determine how the growth observed in this study could affect carcass, and gained tissue composition of Ashanti Black × Large White weaner pigs is warranted.