Introduction

The production of beef with better tenderness is an ongoing challenge in beef farming because it is one of the most relevant factors affecting consumer acceptance and reflects the decisions along the production chain by the producers and industry. Although the consumer perception of tenderness is quite subjective, there are indications that shear force of up to 4.6 kgf/cm³ is close to the cutoff for classifying meat as tender⁽¹⁾.

There are several factors that influence meat tenderness, including genetic aspects⁽²⁾, characteristics of production systems^(3-6), and post-mortem factors⁽⁷⁾. Therefore, it is essential to model the main variables that explain variations in the tenderness of meat from beef cattle to better substantiate decisions within the scope of animal husbandry.

In Brazil, a reasonable number of scientific publications are related to the tenderness of beef from animals who mostly finished in confinement. The scientific basis of these studies is derived from the Newtonian and Cartesian philosophy of research, which analyzes an event from its decomposition into factors, which may still be decomposed into more factors up to the lowest level⁽⁸⁾. This way of thinking makes complex biological variables with great economic impact, such as meat tenderness, difficult to understand and manipulate owing to the influence of a large number of factors.

In this context, meta-analytical procedures using multivariate statistics can help understand these responses and can help model variables according to the results obtained from other characteristics that are easy to identify and control. In this study, we aimed to evaluate the main factors that influence meat tenderness and the probability of obtaining tender meat from confined calves.

Materials and methods

We evaluated the previously published studies on carcasses and beef from steers that were published between January 1999 and April 2019 and could be accessed through SciELO and Google Academic search platforms. The phrase "carcass and beef from steers" was used to select relevant publications in Portuguese and English, and a list 76 articles was obtained.

Studies meeting the following requirements were included in the analyses: (i) the calves should be finished in confinement; (ii) reported data should include the genetic group and/or breed of the calves, the weight at the beginning of termination, percentage of diet concentrate, termination period, and sensory characteristics of meat; (iii) shear strength should be evaluated; (iv) the methodology for determining the sensory characteristics of the meat should follow that described by Metz et al.⁽⁹⁾; and (v) should include all variables used in this study for logistic regression analysis.

After filtering using these criteria, 35 articles remained. Individual treatments in each article was considered a sample unit (one line) in the database (Annex 1). For each sample unit, relevant information was extracted from the material and methods and results sections, and the treatment was discussed.

The factors and variables considered for the research were as follows: the region of study (South - 1, Southeast - 2, Midwest - 3, North – 4, and Northeast - 5), genetic predominance (Zebu – 1, Continental Taurine – 2, British Taurine – 3, Synthetic – 4, and Cross - 5), percentage of Zebu background in the genome, body weight at the beginning of termination, percentage of diet concentrate, period of termination, age at slaughter, weight at slaughter, weight of warm carcass, thickness of subcutaneous fat, loin eye area, conformation, shear strength, marbling, juiciness, texture, and coloring of meat.

Pearson's linear correlation was performed to analyze the relation between the shear force and each variable in the database. A diagnosis of multicollinearity among the predictor variables was performed through the analysis of Pearson's correlation matrix and the measures of variance inflation factor, condition index, eigenvalues (λ), and proportions of variance associated with each λ⁽¹⁰⁾. The variables that correlated with a maximum of 15% significance by Pearson's correlation and that did not demonstrate collinearity were further analyzed using the statistics tool stepwise for the determination of the proportion of the synchronized effect of variables with the shear force using the partial and total determination coefficients (R²). Subsequently, the data were subjected to logistic regression test using the SAS LOGISTIC procedure. Meat with a shear force value lower than 4.6 kgf/cm³ was classified as soft; meat with an higher sheer force value was considered hard⁽¹⁾. Several multiple regression models with linear, quadratic effects, and interactions between the effects were tested using the stepwise software using 25% significance level to enter and 30% to remain in the model⁽¹¹⁾. The choice of the best model to be adopted was based on Hosmer and Lemeshow's test of fit quality⁽¹¹⁾. After model fitting (estimation of βi parameters), the quality of the fitted model and the individual significance values of the parameters were tested by the Likelihood Ratio Test at 5% significance.

The best calculated multiple regression mathematical model adjusted for the probability of the i-th calf providing soft meat and, accordingly, considered for data analysis is described as follows.

where P_ijklmno is the probability of obtaining tender meat from a calf with variables live weight i(χ_1i), percentage of concentrate in the diet j(χ_2j), period of termination k(χ_3k), age at slaughter l(χ_4l), degree of meat marbling m(χ_5m), interaction between the termination period and concentrate percentage in the diet n(χ_6n), and interaction between the weight at the beginning of the term and percentage of concentrate in the diet (χ_7o). The multipliers β₀ to β₇ are the regression coefficients associated with the regression variables χ₀, and ε_ijklmno is the random error associated with each observation.

The odds ratio estimated by OR = exp (bk), which is the ratio of the proportions for two possible results, i.e., the ratio between success (πj) and failure (1 - πj) to obtain soft or not soft meat, respectively, was used for the interpretation of the coefficients. The odds ratio was based on the mean denominator of the data set for each model. The units of changes of regression variables were determined according to the researchers' experience for the best interpretation of results as 1 month for the slaughter age, 10 kg for the initial weight, 5% for the percentage of concentrate in the diet, one day for the finishing period, and 0.5 points for the degree of marbling.

Results

The percentage of zebu blood in the genotypes of the calves evaluated in this study was 42.42% (Table 1). The average body weight (BW) of calves was 293.8 kg at the beginning of termination, and they received approximately 42.6% of concentrate in the diet (based on dry matter) with an average termination period of 114 days. The average age and weight at slaughter were 20.7 months and 430.3 kg of BW, respectively. On average, subcutaneous fat thickness in the carcass was 4.62 mm, the marbling score of the meat was 5.05 points, and the shear force was 5.34 kgf/cm³.

The region of study contributed 2% to the variation in shear strength (Table 2). The zebu background in the genome contributed 11.8% of the variation in softness, while 31.4% of the variation was related to the percentage of concentrate used in diets. The weight at the beginning of termination (4.44%), termination period (4.49%), slaughter age (2.39%), and degree of marbling of the meat (2.89%) explain 14.21% of the variation in shear strength. There was an interaction between the termination period and the percentage of concentrate in the diet (2.97%). Together, these factors explain 62.45% of the shear strength of confined calf meat included in studies in Brazil in the last 20 years.

The weight at the beginning of termination, percentage of concentrate in the diet, termination period, age at slaughter, and the degree of marbling of the meat were the factors in the logistic regression equation to explain the chance of tenderness (Table 3). By Hosmer and Lemeshow's test (p = 0.8176), this model predicts satisfactorily the variation in softness. By the odds ratio test, it was possible to identify that for every 10 kg increase in the initial weight of the termination of 430.3 kg of BW, a reduction in the probability of softness of 61.6% is expected. For each 5% increase in the concentrate in diet, on top of besides the 42.6% on average received, there will be a 2.7-fold increase in the probability of obtaining tender meat. The longer the confinement time is, the greater the chance of obtaining tender meat. Every day beyond the 114-day period that the calves remain in confinement, a 2-fold increase in the chance of obtaining tender meat is expected. Age, which shows an inverse effect, reduces the chance of obtaining tender meat; for each month added to the slaughter age of 20.7 months, there was a 31.9% reduction in the chance of obtaining tender meat. Marbling had a positive effect, where an increase of 0.5 points from 5.05 points results in 74.5% increase in the probability of obtaining tender meat.

Discussion

The characteristics of the production systems analyzed in most studies on confined cattle in Brazil reflect the main technical recommendations that are applicable to the rural areas in the country: a volume to concentrate ratio of 60:40; finishing period of up to 120 days; fat cover median of 3 to 6 mm; and slaughter weight between 400 and 450 kg of BW. An exception was the warm carcass weight (230 kg, considering a yield of approximately 54%), which could incur a penalty because of the low weight required in certain more demanding carcass-quality programs. The other production variables such as age at slaughter, period of confinement, and subcutaneous fat thickness, had values that would fit in most carcass quality programs in the country and that payed above the usual market values⁽¹²⁾.

The average shear strength was above the values considered between intermediate and soft, up to 4.6 kgf/cm³⁽¹⁾, indicating that most surveys produced meats that were not considered soft. The high proportion of zebu blood in animal genotypes was one of the main causes for higher meat shear strength. Currently, it is estimated that 80% of the Brazilian herd is composed of animals of zebu origin owing to the tolerance their tropical climate and better resistance to ectoparasites⁽¹³⁾. Pacheco et al.⁽⁵⁾ used meta-analysis to evaluate the characteristics of meat from slaughter cows in Brazil by contrast analysis and found that animals with defined zebu genotypes had increased shear strength by 1.05 kgf/cm³ compared with animals of continental and/or mixed breeds. Of all reproduction biotechnologies, artificial insemination can be used for breeding bovines, making it possible to increase the production of meat with better tenderness.

The significant effect of BW at the beginning of termination on the shear force reflected negatively on meat softness, which is related to the fact that heavier calves receiving more energetic diets at the beginning of termination live for a short period. Another hypothesis that can support this result is related to differences in the feeding efficiency of calves; more efficient cattle show lower rates of protein turnover and increased muscle growth rate owing to higher activity of the enzyme calpastatin⁽¹⁴⁾. Although this enzyme contributes to the improvement of performance, allowing greater weight at the end of termination, its effects post-mortem do not favor meat tenderness. This is because, the enzyme decreases the activity of calpain in the muscle, negatively affecting meat tenderness by decreasing protein degradation⁽¹⁵⁾.

The percentage of concentrate had the greatest impact on the shear force and had the greatest interaction with the variables tested. Increasing the proportion of concentrate is the main strategy adopted by producers to increase the energy level of cattle diets. Although corn is largely the main energy ingredient in the diet, Brazil has great productive and diversified potential with regard to agriculture, which enables other sources such as wheat bran, rice bran, sorghum or millet grains, and soybean husk to be used for feeding ruminants. These ingredients can be used as alternatives to corn without reducing the energy level of the diet and with the possibility of reducing production costs when the price of corn grain is high. Although the percentage of concentrate is related to the termination period and diet energy level, it is also directly related to diet fiber content. Concentrate is one of the factors responsible for ensuring higher activity of calpastatin in the muscle, consequently making the meat less tender⁽⁶⁾.

Termination period is recognized by its quadratic effect on meat tenderness, depending on the age. Few studies have reported that tenderness increases with increasing finishing time of up to 120 and 130 days^{(16, 17)}. After this period, the efficiency of weight gain decreases along with increasing age, to an extent where the meat quality decreases⁽¹⁸⁾. Age, in turn, shows an inverse correlation with tenderness, and in the present study, it had the same representation in the variability of shear force as the finishing time. This is because, as age progresses, intra- and intermolecular "cross-links" (intermolecular covalent bonds) are formed in collagen, which improve collagen stability, decreasing collagen thermal solubility and making the flesh more resistant⁽¹⁹⁾.

The interactions observed between initial weight, termination period, and concentrate percentage are of great relevance because they affect shear force variability and meat tenderness in a joint and systematic way. However, this evaluation should be analyzed very sparingly, considering the economic efficiency of the system. In a meta-analysis of scientific articles published in Brazil, Cattelam et al.⁽²⁰⁾ correlated the effects of slaughter age, termination period, and diet energy level using a decision tree. They found that more than 145 days of confinement are required to terminate super precocious calves (up to 18 months of age). To reduce this period, concentrate levels of >69% and energy levels of ≥71% TDN would be required. These results show that producing soft meats in Brazil may represent a high-risk strategy from an economic point of view and that such differentiated meats require a growing market.

Meat marbling is highly valued in the industry owing to its relationship with palatability and softness, especially when the latter is evaluated by a panel of evaluators. Meat marbling produces a lubricant effect and decreases tension between the layers of meat connective tissue during chewing. However, its contribution to shear strength is less evident, with a maximum contribution of 5% to softness variability ⁽⁷⁾. Marbling fat is closely related to fat thickness in the carcass⁽²¹⁾. When the amount of fat is scarce, the carcass cooling process is more accelerated as muscles are protected from cooling to a lesser extent, which negatively effects several meat quality characteristics⁽²²⁾.

Conclusions

A higher proportion of variability in the tenderness of meat from confined calves can be explained by factors related to the genetic background and the characteristics of production systems. When the initial weight at termination is higher, it is common to reduce the percentage of concentrate in the diet and shorten the period until slaughter. Under these conditions, the probability of obtaining tender meat is lower. In contrast, reducing the age at which calves are slaughtered and increasing the degree of meat marbling can significantly improve the chance of obtaining tender meat.

References