Effect of Qualitative Feed Restriction Duration and Monensin Ionophore on Compensatory Growth and Fattening Performance of Kabudeh Shirazi Male Lambs

Introduction
In extensive and semi-intensive small ruminant production systems across arid and semi-arid regions, feed availability is subjected to sharp seasonal fluctuations. During lean periods, animals experience qualitative and quantitative nutrient shortages, curtailing their genetic growth potential. However, upon restoration of nutrient-dense feeding (realimentation), animals frequently exhibit compensatory growth—a physiological surge characterized by an accelerated growth velocity that exceeds that of continuously fed counterparts (Hornick et al., 2000; Kamalzadeh et al., 2009). Exploiting compensatory growth holds substantial biological and economic significance, as it enhances nutrient utilization efficiency and reduces feeding costs in feedlots (Abouheif et al., 2015; Mahjoubi et al., 2015).
The magnitude of the compensatory response is modulated by initial body composition, animal maturity, genetic background, and, most crucially, the duration and severity of the restriction phase (Kamalzadeh et al., 1997; Morris et al., 2020. While compensatory feeding and monensin independently improve growth parameters, their potential metabolic synergy remains unelucidated in fat-tailed sheep breeds such as Kabudeh Shirazi. Therefore, this study evaluated the effects of qualitative feed restriction duration (0, 30, and 60 days) and monensin supplementation (0 vs. 30 mg/kg DM), along with their interaction, on feed intake dynamics, daily gain, and feed conversion efficiency in growing male Kabudeh Shirazi lambs.
Method
Animals, Experimental Design, and Diets
The experiment was conducted at the Agricultural and Natural Resources Research and Education Center of Fars Province (Aliabad Kamin Station, Iran). Thirty single-born Kabudeh Shirazi male lambs, with an average initial live body weight of 25 ± 1 and an average age of 150 ± 10 days, were allocated to a completely randomized design with a factorial 2×3 arrangement. The first factor was the duration of qualitative feed restriction (0, 30, and 60 days), and the second factor was dietary monensin level (0 and 30 mg/kg DM), yielding six distinct experimental treatments with five replicates each (n=5 individually penned lambs per treatment). The experimental protocol consisted of two consecutive nutritional phases: a qualitative restriction phase and a subsequent realimentation (finishing) phase. Before initiation, a 7-day adaptation period was implemented to acclimate the lambs to housing and baseline diets; all lambs were vaccinated against enterotoxemia and treated with albendazole and ivermectin for internal and external parasites. The qualitative restriction diet was formulated using low-density forage components (70% wheat straw and 30% alfalfa hay on a dry matter basis), supplying 7.62% crude protein (CP), 1,650 kcal/kg metabolizable energy (ME), 62.8% neutral detergent fiber (NDF), and 44.0% acid detergent fiber (ADF). This diet was offered ad libitum to ensure physical satiety while inducing a severe nutrient deficit (a 52% reduction in CP and a 48% reduction in ME relative to the finishing diet).
The realimentation (compensatory finishing) diet was formulated according to NRC (2007) guidelines using a 75:25 concentrate-to-forage ratio (39% barley, 15% corn grain, 12% wheat bran, 7% soybean meal, 25% alfalfa hay, 1% vitamin-mineral premix, 0.5% salt, and 0.5% dicalcium phosphate). The finishing diet provided 15.85% CP, 2,665 kcal/kg ME, 20.0% NDF, and 15.6% ADF. For monensin-supplemented groups, monensin sodium was thoroughly mixed at 30 mg/kg DM across both phases. Diets were offered in two equal daily meals (at 07:00 and 18:00 h), allowing for 10%-15% refusals to maintain ad libitum intake. Fresh water was freely accessible.
 Measurements and Statistical Analysis
Data were analyzed via two-way Analysis of Variance (ANOVA) using Jamovi software (version 2.7.37.0) based on the linear model:
Y_ijk=μ+R_i+M_j+(RM)_ij+e_ijk
where Yijk is the observed dependent variable, μ is the overall mean, R is the fixed effect of restriction duration (i=0,30,60), Mj is the fixed effect of monensin supplementation (j=0,30 mg/kg), (RM)ij is the interaction effect between restriction and monensin, and eijk is the random residual error. Residual normality and homoscedasticity were verified before ANOVA. Treatment means were compared using Tukey’s post-hoc test at a statistical significance threshold of P<0.05.
Results
Dry Matter Intake
Restriction Phase: Dry matter intake was significantly affected by restriction duration (P=0.003) and the restriction duration × monensin interaction (P=0.014), whereas the main effect of monensin was non-significant (P=0.150). Lambs restricted for 60 days consumed 24% less DM than those restricted for 30 days (416 vs. 548 g/d, SEM=34.2). Monensin inclusion reduced intake during restriction (469 vs. 627 g/d in the 30-day group and 439 vs. 392 g/d in the 60-day group).
Realimentation Phase: DMI was significantly increased by feeding strategy (P=0.029) and monensin supplementation (P=0.007), while their interaction was not significant (P=0.119). The 30-day restricted lambs exhibited higher intake than the 60-day cohort (1,513 vs. 1,316 g/d, SEM = 45.7). Across treatment combinations, the 30-day restriction with monensin group achieved the highest realimentation intake (1,672 g/d), significantly surpassing the unsupplemented control (1,312 g/d) and the unsupplemented 60-day group (1,242 g/ d).
Overall Trial: Both restriction duration (P<0.001) and monensin (P<0.001) significantly affected cumulative DMI (P=0.170for interaction). Control lambs (1,389 g/d) and 30-day restricted lambs (1,290 g/d) had higher total intake compared to the 60-day restricted group (979 g/d, SEM=40.5).
Growth Performance and Compensatory Gain
Restriction Phase: Lambs entered a negative energy balance with daily body weight loss (105.2 g/d in 30-day vs. 68.3 g/d in 60-day restriction, SEM=13.2). Daily weight loss was not significantly affected by main effects (P=0.069 for duration; P=0.132 for monensin).
Realimentation Phase: Average daily gain (ADG) was significantly improved by previous restriction duration (P<0.001), monensin supplementation (P=0.010), and their interaction (P=0.026). Restricted lambs grew faster than control lambs (271 and 239 vs. 209 g/d, SEM=12.6). The 60-day restricted group with monensin recorded the highest ADG (291 g/d), whereas the unsupplemented control grew the slowest (178 g/d).
Final Slaughter Weight: No significant differences were observed for the main effects of restriction duration (P=0.326) or monensin (P=0.532), but their interaction was significant (P=0.017). Final weights were 45,420  g (control), 47,158  g (30-day restriction), and 45,769  g (60-day restriction, SEM=868), with the 30-day restriction + monensin treatment reaching the highest numerical weight (48,956 g).
 Feed Conversion Ratio (FCR)
Realimentation Phase: FCR was significantly improved by feed restriction (P<0.001) and monensin (P=0.023), with no interaction effect (P=0.228). FCR improved from 6.82 in controls to 5.95 in 30-day and 5.33 in 60-day restricted lambs (SEM=0.231). Monensin independently improved realimentation FCR from 6.34.
Overall Trial: Feeding strategy did not negatively impact cumulative FCR (P=0.223), while monensin significantly improved overall FCR (6.32 vs. 6.93, SEM=0.251  ; P=0.017). Monensin resulted in a 15% FCR improvement in the 30-day restricted group (6.04 vs. 7.16; P<0.05), compared to only 5% in controls (6.65 vs. 7.08) and 3% in the 60-day restricted group (6.25 vs. 6.43).
The lower DMI observed during the restriction phase, particularly in the 60-day group, is primarily attributable to the high physical fill of the fibrous straw diet and physiological downregulation under prolonged energy dilution. During subsequent realimentation, the marked hyperphagia displayed by restricted lambs, most notably the 30-day restricted group supplemented with monensin (1,672 g/d). reflects an active metabolic drive to restore body mass. Monensin supported this transition by modulating ruminal fermentation kinetics, preventing subclinical acidosis, and stabilizing intake on high-grain diets.
The deceleration of daily weight loss over time during the restriction phase indicates rapid metabolic adaptation. Ruminants subjected to nutrient deprivation lower their basal maintenance energy requirements (Nem) by reducing fasting heat production, decreasing circulating thyroid hormone levels, and down-sizing high-metabolic visceral organs such as the liver and gut mucosa.
Upon realimentation, the dramatic acceleration in growth rate (ADG) observed in restricted lambs demonstrates the classic compensatory response. Because visceral re-growth lags behind realimentation intake, a greater portion of absorbed nutrients is partitioned directly into lean tissue synthesis and skeletal accretion rather than basal maintenance. Monensin further augmented this response by shifting the rumen volatile fatty acid profile toward propionate, providing an abundant gluconeogenic substrate for protein retention.
The absence of significant differences in final slaughter weights confirms complete compensatory growth recovery across all restricted treatments. Restricted lambs fully compensated for earlier weight loss within the finishing period. Importantly, the 30-day restriction combined with monensin proved to be the most advantageous nutritional protocol; it avoided the potential digestive mucosal damage associated with prolonged (60-day) restriction, while providing a sufficiently long finishing phase to fully capitalize on monensin-driven energetic efficiency, yielding the lowest overall FCR (6.04) and the highest final slaughter weight (48.96 kg).
Conclusions
Qualitative feed restriction induced complete compensatory growth in Kabudeh Shirazi male lambs, allowing them to match or exceed the final slaughter weights of continuously fed control lambs without compromising growth capacity. Monensin supplementation (30 mg/kg DM) acted synergistically with realimentation, enhancing average daily gain and dry matter intake while stabilizing ruminal fermentation. The 30-day feed restriction regimen combined with monensin supplementation achieved the best overall feed conversion ratio (6.04) and the highest final slaughter weight (48.96 kg). Under the conditions of this study, a 30-day qualitative feed restriction followed by realimentation with monensin supplementation is recommended as an effective nutritional and management strategy for fattening Kabudeh Shirazi male lambs.