Diets with high roughage content limit ruminant performance mainly by increasing ruminal fill and reducing energy density, thereby restricting voluntary intake and nutrient utilization. However, the extrusion process improves ruminal microorganism access to nutrients because the combined action of heat, pressure, and shear forces alters the physical structure of the feed, breaking down cell walls and modifying fiber and starch organization, which enhances digestibility, nutritional value, and facilitates forage handling and storage(Assis et al., 2022);(Oliveira et al., 2020), thus promoting this technique as a promising strategy for fiber utilization in ruminant nutrition.

In modern and intensive sheep production systems, the inclusion of high levels of concentrates has been a common practice to maximize energy intake and daily weight gain(Li et al., 2021). Nevertheless, this approach may lead to selective feeding behavior by animals that increases labor and management costs. In this context, the extrusion of the aerial parts of grasses emerges as an innovative strategy to enable the inclusion of higher proportions of roughage in the diet without compromising animal performance.

Studies indicate that extrusion can significantly improve nutrient digestibility(Abadi et al., 2023);(Amirteymoori et al., 2021), increase intake(Silva et al., 2020);(Silva et al., 2021);(Siqueira et al., 2022), and enhance the efficiency in animal production(Li et al., 2021);(Zhang et al., 2019). Consequently, the inclusion of extruded roughage in ruminant diets allows a more efficient adjustment of the roughage-to-concentrate ratio while meeting energy requirements and ruminal physical constraints(Oliveira et al., 2019);(Oliveira et al., 2019). Despite these advances, limited information is available regarding the maximal level of roughage inclusion in fully extruded diets for growing lambs. To date, most studies evaluating extruded feeds in sheep have focused on partial inclusion of extruded ingredients or on performance outcomes alone, without simultaneously addressing ingestive behavior and carcass traits under fully extruded dietary systems(Assis et al., 2022);(Oliveira et al., 2025);(Ribeiro et al., 2025);(Silva et al., 2023).

This study investigated the effects of different roughage levels in fully extruded diets for growing sheep by evaluating the diet’s influence on animal performance, nutrient utilization efficiency, ingestive behavior, and carcass traits. We hypothesized that diet extrusion would allow higher roughage inclusion without impairing nutrient intake, growth performance, or carcass traits while simultaneously increasing chewing and rumination time through physical modification of fiber structure during extrusion.

Dietary dry matter (DM) decreased progressively from 92.04% to 91.59% as the forage proportion increased from 30% to 60% (Table 2). A similar pattern was observed for nutritional attributes, including mineral matter (MM; 5.52% to 5.11%), crude protein (CP; 15.45% to 12.13%), and total digestible nutrients (TDN; 75.83% to 72.04%). Consequently, a higher proportion of dietary forage increased the contents of organic matter (94.58% to 94.89%), neutral detergent fiber (NDF; 23.41% to 33.86%), acid detergent fiber (ADF; 16.59% to 24.56%), and ether extract (EE; 1.57% to 1.76%).

There was no effect (p>0.05) of R:C ratio on BW, BCS, or body conformation (in vivo biometric measure-ments) of the animals (Table 3). However, a quadratic effect of evaluation day was observed (p<0.01) for BW, BC, TC, CW, and BL. Despite the exponential growth pattern, these measurements became stabilized toward the end of the experimental period. In contrast, BCS, RH, and WH showed a positive linear effect (p<0.01) throughout the entire experimental period (Table 4).

The ADG of lambs was not affected by the R:C ratio over the total evaluation period (p>0.05). However, a quadratic effect was observed during the 15–30-d period (p=0.03). ADG increased as the concentrate proportion decreased (241.33, 263.33, and 315.00 for ratios 30:70, 40:60, and 50:50, respectively), then declined at 40% forage inclusion in the diet (86.66). A negative linear effect was observed during the 30–45-d period (p=0.01), with ADG increasing concomitantly with the reduction of concentrate in the R:C ratio. Conversely, a positive linear effect was observed during the 75–90-d period (p<0.01), in which ADG decreased concomitantly with an increase in forage proportion in the R:C ratio (Table 5).

No effect of the different R:C ratios on the carcass characteristics evaluated by in vivo ultrasonography was observed (p>0.05; Table 6). There was a significant (p<0.05) increase in subcutaneous fat thickness (SGT) and in base and height measurements (used to calculate LEA) on day 84. At the end of the study, base, height, and SGT were 5.73, 5.10, and 0.80 mm greater, respectively, compared with the beginning of the experiment.

There was an interaction between the R:C ratios and the evaluation period for LEA, with LEA being higher on day 84 than on day 0 (baseline) across all diets (p<0.01; Table 7). At the end of the study, the 30:70, 40:60, 50:50, and 60:40 ratios showed increases of 3.62, 2.69, 1.87, and 2.59 mm, respectively, compared to values at the beginning of the experiment. However, there was no effect of increasing dietary forage proportion on LEA across evaluation days.

Increased forage levels linearly increased (p<0.01) the time spent in TCT, which rose from 391 to 544 min/d as the R:C ratios increased from 30:70 to 60:40 (Table 8). In contrast, an opposite pattern was observed for idle time, which decreased linearly (p<0.01) as dietary forage increased. Idle time ranged from 1,049 to 896 min/d when the forage proportion in the diets doubled from 30% to 60%.

The increase in dietary roughage associated with reduced idle time and similar body performance among all diets reinforces the role of extrusion in modifying fiber structure and stability during digestion, indicating that this processing technique enables greater fiber inclusion in diets for growing lambs without impairing productive performance, as previously reported for meal-type diets(Parente et al., 2016). Physically processed, pelleted, or extruded diets markedly reduce particle selection compared with non-processed diets(Abadi et al., 2023), which is corroborated by our observations.

Extrusion reduces particle selection by altering fiber length, density, and the cohesion among dietary components, resulting in a more homogeneous feed matrix and limiting the animals’ ability to consume concentrate fractions selectively(Oliveira et al., 2019);(Oliveira et al., 2025);(Zhang et al., 2019). This reduction in sorting behavior has been directly quantified in sheep through lower refusals of fine or coarse particles and reduced variability in nutrient intake among animals(Li et al., 2021);(Nascimento et al., 2020). This uniformity of intake is beneficial for animal performance, as it ensures a balanced distribution of essential nutrients for growth and body development(Ding et al., 2016);(Dougherty et al., 2022). Moreover, by maintaining fiber integrity and preventing selective intake, extrusion contributes to a more stable ruminal environment, improving digestive efficiency and nutrient absorption, as evidenced by studies reporting improved feed efficiency and ruminal fermentation profiles in lambs fed extruded diets(Li et al., 2021);(Oliveira et al., 2020);(Zhang et al., 2019).

Thus, the results of our study demonstrated the relationships among the proportion of roughage in the lambs’ diets, fiber content (NDF and ADF), and TCT. Initially, increasing the proportion of roughage resulted in a significant increase in fiber content, with higher NDF and ADF values due to the greater proportion of structured plant material rich in fibrous components. The higher fiber content in the diets directly affected the feeding behavior of the lambs, particularly TCT, since fiber requires more chewing time to be adequately broken down and prepared for ruminal digestion.

Despite significant changes in fiber content and chewing behavior, BC, although related and potentially indicative of intake capacity and abdominal expansion(Oliveira et al., 2020), did not follow the same pattern. Fiber in larger particles leads to ruminal wall distension and, consequently, an increase in BC. However, the fiber particle size of approximately 2 mm resulting from extrusion appears to have prevented excessive ruminal fill, which is important for increasing dry matter intake(Oliveira et al., 2024);(Rodrigues et al., 2022);(Siqueira et al., 2022).

This result suggests that variation in diet composition did not perceptibly affect the animals’ body conformation. The maintenance of BC may indicate that, although fiber content and chewing time increased, total nutrient and energy intake were sufficient to sustain uniform body growth(Dougherty et al., 2022);(Oliveira et al., 2020). Thus, even with variation in the R:C ratio, the diets were nutritionally adequate to support the lamb’s body development.

BCS followed a positive linear trend throughout the study, as the animals stabilized muscle growth and began to deposit fat. Differences in the growth rates of muscle, bone, and adipose tissues are observed due to physiological maturity(Ponnampalam et al., 2024). Studies such as that by (Hicks et al., 2021) show that bone tissue exhibits early growth, followed by intermediate muscle growth, and finally by later adipose tissue growth. In summary, muscles tend to develop more rapidly in young animals, whereas fat deposition becomes more pronounced after complete body development. In addition to physiological maturity, diet composition can influence the efficiency and extent of fat deposition in growing sheep(Dougherty et al., 2022). Diets with higher energy density, particularly those with increased levels of digestible carbohydrates or lipids, have been associated with enhanced adipose tissue deposition and different carcass fat patterns(Dougherty et al., 2022);(Ponnampalam et al., 2024). Conversely, diets with higher fiber content but lower energy density may limit the efficiency of energy partitioning toward fat storage, even as animals reach later growth stages(Ding et al., 2016). Therefore, the energy concentration and nutrient balance of the diet play important roles in determining the pattern and efficiency of fat deposition during the finishing phase of lamb growth.

Lambs follow a sigmoidal growth curve, characterized by a slow initial phase, acceleration until puberty, and stabilization at maturity(Silva et al., 2024). During early growth, muscle tissue deposition predominates, but as animals mature, fat deposition becomes more prominent. This shift reflects the progressive approach to physiological maturity in which hormonal regulation redirects nutrient partitioning from lean tissue accretion toward adipose deposition, a process that is largely independent of short-term dietary variations(Bell et al., 1987). This transition is crucial for subcutaneous fat thickness, which is fundamental to producing high-quality meat(Shao et al., 2024). Therefore, animals subjected to the different treatments in our study exhibited consistent growth, suggesting that although diet may play a role in tissue deposition efficiency, the overall growth pattern is strongly influenced by intrinsic biological factors, such as genetics and maturity, as well as feeding management.

The observed increase in SFT and length and height of the longissimus dorsi muscle, as well as BCS and overall body development of the animals during the experimental period, can plausibly be attributed to the transition to the pubertal phase. The significant increase in the length of the longissimus dorsi muscle recorded on day 84 reflects homogeneous growth in both length and height, indicating overall body development of the animals. The increase in muscle height may be related to adjacent bone growth, which provides structural support for the growing muscle, particularly at its base(Silva et al., 2024). These findings highlight the relevance of puberty in the morphological development of the animals and its potential influence on body composition and muscle development during the evaluation period.

(Oliveira et al., 2019) demonstrated that increasing dietary energy concentration is directly associated with increases in LEA and, consequently, with the amount of muscle in the carcass, showing a positive correlation between these traits. Therefore, LEA is frequently used as an indicator of carcass composition(McManus et al., 2013). Thus, it was expected that the animals had a larger LEA at the end of the experimental period since the extruded feed consumed is characterized by higher digestibility and fermentability of nutrients(Oliveira et al., 2020), including proteins, carbohydrates, and fibers, with TDN values above 70%, indicating high energy availability (Table 2).

Short-term fluctuations in ADG throughout the experimental period were likely associated with dietary adaptation and inherent physiological growth phases. During the initial stages of exposure to fully extruded diets with varying roughage levels, animals may undergo transient adjustments in intake patterns and ruminal function, which can temporarily influence growth rates(Oliveira et al., 2019);(Oliveira et al., 2025). As adaptation progresses, ADG tends to stabilize and increasingly reflects the natural growth trajectory associated with advancing age and proximity to maturity, rather than solely direct dietary effects.

Throughout the experiment, we observed that even with an increase in the proportion of roughage in the diet, from 30% to 60%, the animals maintained a similar LEA. This suggests that, in addition to the extrusion process of the roughage improving fiber digestion, diets with higher roughage proportions can achieve similar gains due to greater energy availability from the diet’s higher TDN values (Table 2). These results highlight the advantage of including extruded roughage in animal diets, as it allows increasing the proportion of this component without compromising LEA or, consequently, carcass composition.

Despite the consistency of responses across treatments, our study had limitations. The experimental duration and sample size, although sufficient to detect differences in ingestive behavior and carcass-related traits, may have limited the detection of subtle treatment effects on short-term growth dynamics, particularly during early dietary adaptation. In addition, the evaluation period did not extend to slaughter, which restricts inferences regarding final carcass yield and commercial cut composition.

From a practical perspective, the results indicate that fully extruded diets with roughage inclusion levels of up to 60% can be successfully applied in feedlot or semi-intensive sheep production systems without compromising animal performance or carcass development. The reduction in idle time and maintenance of carcass traits suggests improved feeding efficiency and potential benefits for labor management and feed utilization. These findings support the adoption of extruded roughage as a strategy to increase dietary fiber intake while maintaining productive efficiency, thereby enabling more flexible, potentially cost-effective feeding programs for growing sheep.