Introduction

The demand for quality lamb meat has aroused the interest of breeders in finishing lambs in the feedlot, which intensifies production systems, and in reducing losses of young animals due to nutritional deficiency and/or parasitic infection. Additionally, feedlotting allows for maintaining a regular meat supply to the market and obtaining a faster return on the invested capital as a result of earlier slaughter and a more uniform end product for the consumer (FRASSON et al., 2016).

In terms of the feeding regime adopted in the feedlot, diets with high levels of energy concentrates offer advantages over those with increased participation of roughage components in that the former are easily standardized, stored, and handled, in addition to providing rapid gains in carcass fat and weight in confined animals. As such, they allow for the intensification of production systems by advancing the time for slaughter and generating better-quality carcasses, in contrast to traditional pasture-based finishing systems (OLIVEIRA, 2013). In this regard, traditional pasture-based sheep meat production systems which usually provide low weight gains and animals slaughtered at an advanced age culminate in the sale of low-quality carcasses, one of the main factors limiting lamb consumption in Brazil (VENTURINI, 2015).

However, major commercial centers have exhibited an increasing consumption of quality lamb with elaborate cuts and carcasses with maximum amounts of muscle, minimum proportions of bone, and a fat content that varies according to the consumer's preference. Moreover, this fat should be sufficient to ensure the juiciness of the meat as well as its appearance and to protect the carcass (PAIM et al., 2011).

On these grounds, the present study was developed to evaluate carcass characteristics of feedlot-finished lambs fed high-grain diets as well as the effect of these diets on the tissue composition of their meat.

Material and Methods

The experiment was carried out at the Sheep Farming Laboratory in the Department of Animal Science of the Federal University of Santa Maria, located in Santa Maria - RS. Brazil, from August to November 2012. Thirty-two castrated male Texel lambs born from single births and weaned at approximately 50 days of age were used.

The animals were allocated to four treatments in a completely randomized design with eight replicates. Treatments consisted of different unprocessed grains, as follows: corn grain (Zea mays), white oat grain (Avena sativa), black oat grain (Avena strigosa), or grain of rice in hulls (Oryza sativa L.). The diet consisted of the whole grain used in the treatment, 15% of a commercial concentrate mixture (89.29% DM, 36.58% CP, 64% TDN, 2.5% Ca, and 0.0009% P), soybean meal, and dicalcium phosphate. Diets were formulated to be isoproteic and meet the requirements of the category used, in accordance with the NRC (2007), to provide a daily weight gain of 200 g (Table 1).

Lambs were confined in individual, fully covered 2.0-m² stalls with slatted floors at approximately 1.0 m above the soil equipped with individual feed and water troughs. The feed was supplied ad libitum twice daily, at 08h00 and 17h00. The amount provided was adjusted according to the leftovers observed daily, which should be 10% of the amount offered on the previous day, so as to ensure maximum voluntary intake. Lambs had ad libitum access to mineral salt in individual containers for that purpose. The chemical composition of the mineral salt was 134 g calcium; 60 g phosphorus; 10 mg magnesium; 110 g sodium; 12 g sulfur; 150 mg cobalt; 60 mg iodine; 2,500 mg iron; 4,500 mg manganese; 30 mg selenium; 6,000 mg zinc; 570 mg fluorine (max.); and 180 g flavoring agent. The mineral salt was free of copper, to prevent possible intoxications by this mineral.

The experimental period was preceded by a 10-day period used for the animals to adapt to the feed, facilities, and management conditions. In this stage, for the animals to accustom to eating solid feed from the trough, ground alfalfa hay was provided as part of the feed plus the grain used in the treatment at an initial roughage-to-concentrate ratio of 45:55%. Subsequently, until the start of the experimental period, the alfalfa hay was gradually replaced every two days with 15% of the cereal grain to be used, according to each treatment, until its complete removal. The feeding trial began after the acclimation period and was extended until the moment each lamb would reach the pre-established slaughter weight of 32 kg, which corresponds to 60% of the adult weight of their mothers, as recommended by Butterfield (1988).

Upon reaching the weight defined for slaughter, lambs were deprived of solid feed for 14 h. After this time, they were weighed to determine the slaughter weight (SW), and then slaughtered. After each slaughter operation, the carcass was weighed individually and then chilled for 24 h in a cold room at 2 ºC. The following carcass characteristics were measured: hot carcass weight (HCW), cold carcass weight (CCW), hot carcass yield (HCY, obtained as HCW/SW × 100), cold carcass weight yield (CCY, determined as CCW/SW × 100), and rate of cooler shrink loss (RCSL, calculated as 100 - (CCY/HCY) × 100). Conformation was assessed on the cold carcass of each animal on a scale of 1 (poor) to 5 (excellent). The degree of fatness (DF) was determined on a scale of 1 (emaciated) to 5 (obese), according to methodologies described by Osório et al. (1998).

Next, the loin-eye area was obtained by exposing the Longissimus dorsi muscle after a transverse section was made in the carcass between the 12th and 13th ribs; its outline was then traced onto tracing paper (MÜLLER, 1980). SITER 3.1 software model A2 was used to determine and record the area, as described by Giotto (2001). In the same region, following Osório et al. (1998), backfat thickness was measured with a caliper and texture was evaluated subjectively. In this step, we observed the size of fiber bundles oriented lengthwise in the muscle and assigned a score of 1 to 5 (1 = too thick, 2 = thick, 3 = medium, 4 = thin, and 5 = too thin). Marbling was determined according to the amount of intramuscular fat observed in the Longissimus dorsi muscle, also on a scale of 1 to 5 (1 = inexistent, 2 = little, 3 = good, 4 = too much, and 5 = excessive). Lastly, the color present on the cut was assigned a score of 1 to 5 as well (1 = light pink, 2 = pink, 3 = light red, 4 = red, and 5 = dark red). Subsequently, the right half of the carcass was weighed and separated by region into the following cuts: neck, shoulder, ribs, and leg (OSÓRIO et al., 1998). After this separation step, these primal cuts were weighed and their percentage was calculated relative to the cold carcass weight. The right shoulder of each lamb was dissected and separated into the fractions bone, muscle, fat (backfat, scapular fat, and intermuscular fat), and other structures (ganglia, fascia, tendons, and great nerves) to determine the tissue composition of the meat.

After data collection, results were subjected to analysis of variance and means were compared by Tukey's test, adopting the 5% significance level. Analyses were performed using SAS (2004) statistical package. The mathematical model below was used:

Y_ij = observation referring to animal j on treatment i;

µ = overall mean of the observations;

α_i = effect of treatment i; and

ε_ij = random error associated with each observation.

Results and Discussion

Slaughter weight did not differ significantly across the treatments, demonstrating uniformity among the experimental units. In the analysis of hot and cold carcass weights and yields, however, the lambs fed the corn grain-based diet showed significantly higher values than those receiving the other treatments (Table 2). This result can be explained by the fact that, irrespective of the treatment, the lambs had homogeneous characteristics at the onset of the experiment in terms of genotype, sex, and age, which suggests this finding is a direct consequence of the feed provided to them. Therefore, one can infer that the lambs on the corn grain treatment had better body development and fat content when compared with those receiving the other treatments, which can be confirmed by their degree of fatness and backfat thickness, which were also higher in that group.

The carcass yield is known to be greatly influenced by the effect of the gastrointestinal content weight at the time of slaughter (LAWRENCE; FOWLER, 2002). In the present experiment, although the lambs were deprived of solid feed for 14 h pre-slaughter, when the chemical composition of the experimental diets was evaluated, the corn grain-based diet showed to have a lower proportion of slowly digestible or completely indigestible fraction of feedstuffs, as represented by NDF and ADF; a larger proportion of the fraction of greater digestibility; and a higher rate of passage through the gastrointestinal tract, as represented by non-structural carbohydrates (Table 1). This aspect might have influenced the results of carcass yield across the treatments, with the lambs fed black oat, white oat, and rice in hulls displaying larger proportions of gastrointestinal content (23.53%, 22.84%, and 22.54%, respectively) at slaughter in comparison with those fed corn grain (11.07%). These findings corroborate Pires et al. (2006), who evaluated carcass characteristics of feedlot-finished Ile de France × Texel crossbred lambs receiving diets with different fiber contents and observed that a reduction in the dietary NDF content led to improved carcass yields. Those authors attributed this result to a decrease in gastrointestinal content at the time of slaughter.

Findings similar to those observed in the current study for the hot and cold carcass yields of lambs receiving the corn grain-based treatment were obtained by Borges et al. (2011), who examined the effects of different levels of substitution of corn grain for black oat on the performance of feedlot Texel lambs. The authors observed, for control treatment (no substitution; diet containing 84.75% whole grain corn) mean hot and cold carcass yields of 52.33% and 50.59%, respectively.

The rate of cooler shrink loss indicates the percentage of weight that is lost during the carcass chilling process. Lower losses occurred in the meat of lambs fed the corn grain diet, which may be considered a direct effect of the backfat thickness and degree of fatness of animals. As stated by Osório and Osório (2003), the backfat works positively by protecting the carcass from dehydration during chilling, preventing darkening of the outer part of muscles, in addition to not compromising the meat quality. The indicated value of 2.73% regardless of the treatment can be considered normal for shrink loss, considering that such losses should be between 3.0 and 4.0%, according to Carvalho et al. (2017).

Loin-eye area was larger in the lambs receiving the corn grain treatment (Table 2), and this can be considered a consequence of the higher growth rate (BERNARDES et al., 2015) and cold carcass weight of animals receiving that treatment. This may be viewed as a positive aspect regarding carcass quality, since, as stated by Cartaxo and Sousa (2008), an increase in loin-eye area will lead to larger amounts of muscle in the carcass. A smaller area of 11.22 cm² was observed by Pires et al. (2006); however, that value is near those obtained by the lambs receiving the other treatments, in the present experiment.

The corn treatment resulted in meat with higher color values than the white oat diet, but did not differ from the rice or black oat treatments. According to Sousa et al. (2008), the color plays an important role in the sensory quality of meat and stands out as the main factor of appreciation at the time of purchase.

Despite the distinct periods of confinement of the lambs on different treatments as a function of the observed differences in weight gain (BERNARDES et al., 2015), the animals were slaughtered at similar live weights. Therefore, the weights of neck, shoulder, ribs, and leg were higher (P<0.05) in the group fed the high-corn grain diet (Table 3), as they showed the best cold carcass yield and weight of all treatments, which mean heavier cuts from their carcass.

Lambs on the treatment with high grain corn had the lowest percentage of leg in the carcass, which may be viewed a negative aspect according to Araújo Filho et al. (2007), who declared that the leg is considered the most noble of sheep carcasses, as it contains the largest accumulation of muscle mass. Moreover, it is normally the cut of highest commercial value, which depreciates the animal carcass from the economic standpoint.

By contrast, the proportion of ribs in the lambs fed the high-corn grain diet was larger than that observed in the lambs consuming white oat and rice in hulls, but did not differ from the black oat treatment. Considering the assertion of Kempster et al. (1987), this result can be explained by the fact that fat deposition in the carcass is faster via the loin and ribs. This finding might thus be associated with greater deposition of fat in that region in the animals fed the higher-energy diets.

Similar results were reported by Carvalho et al. (2005), who evaluated feedlot-finished lambs of different genotypes and obtained Texel lambs slaughtered at an average live weight of 37.73 kg with mean values of 7.43% neck, 20.52% shoulder, 39.29% ribs, and 32.74% leg.

The muscle portion in lambs fed the high corn-grain diet was heavier than that observed in all other treatment groups, which is positive for that diet from the consumer's standpoint. Similarly, the weights of subcutaneous, internal, and total fats were higher in the corn-grain treatment (Table 4). The higher weight of tissues in the lambs on that treatment is a consequence of their heavier shoulder cut in relation to the lambs receiving the high-grain diets based on white oat, black oat, and rice in hulls.

When the tissue components were evaluated in percentage terms, the proportion of muscle did not differ significantly across the treatments, averaging 53.91%. The percentage of bone, in turn, was lower, whereas the percentages of subcutaneous, intermuscular, and total fats rose in the lambs fed corn grain in comparison with the other treatment groups. This increase in fat content may be explained by the higher energy content of the diet based on corn grain provided to the animals. This result may be considered negative from the consumer-market perspective, since excess fat repels consumers, which are increasingly concerned with healthy eating habits.

Results in line with ours were obtained by Klein Júnior et al. (2008), who evaluated the shoulder tissue composition in Ideal crossbred lambs and found mean bone and muscle contents of 19.7% and 53.4%, respectively, in castrated animals. However, the fat content detected by those authors in the shoulder was 22.3%, which is slightly higher than that found in this study. Rosa et al. (2002) observed similar values (19.8% bone, 59.6% muscle, and 20.2% fat) in Texel lambs slaughtered at 33 kg live weight.

Conclusions

The use of high-corn grain diets, as compared with high-grain diets based on black oat, white oat, or rice in the hulls, can be recommended for the finishing of feedlot lambs when they are slaughtered at similar weights, as they provide improved quantitative and qualitative carcass traits, which are important from the production standpoint and for the consumer market.

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Author notes

*Author for correspondence