Introduction

In tropical regions, graze is the main nutritional resource for the production of beef cattle. Tropical grasses are the basis of the cattle grazing system and provide low-cost energy substrates, mainly fibrous carbohydrates¹. However, when used exclusively, without supplements, they rarely meet the nutritional requirements of the animals for adequate productivity, presenting some nutritional restrictions, mainly in protein and energy, which vary throughout the year¹, resulting in unsatisfactory animal performance.

Dry matter intake is undoubtedly one of the most important factors for animal performance and maintenance, since it is the starting point responsible for the entry of nutritional sources, mainly energy and protein, necessary to meet the requirements of maintenance and animal production. Therefore, animal supplementation aims to result in additional gains that depend on the objective of the producer, which can only be achieved with the intake of forage and mineral supplementation, allowing the animal to increase nutrient intake and improve their digestibility.

Thus, a program of continuous meat production that aims to be efficient and competitive, should seek to eliminate the negative phases of the production process, providing the animal conditions for adequate development throughout the year, in order to reach the conditions of early slaughter. An alternative for animals to be slaughtered younger is to keep the animals in graze during the rainy season, where there is high availability of forage with good nutritional value, aiming to reduce production costs and, during food restriction periods, an interesting strategy is to confine the animals. In addition to improving the final product, it accelerates capital turnover, reduces stocking in graze, increases the production scale of the property. The objective of this study was to evaluate the nutrient intake and digestibility, performance and economic viability of crossbred steers managed in Brachiaria brizantha cultivar Marandu graze and in feedlot during the rearing phase.

Material and methods

Field work was conducted on the Princesa do Mateiro Farm, located in the municipality of Ribeirão do Largo, state of Bahia, Brazil, with coordinates of 15° 26′ 46″ S and 40° 44′ 24″ W. In an area of 14 ha, divided into 12 paddocks with approximately 1.17 ha each, formed of ‘Brachiaria brizantha cultivar Marandu. The experiment started in February/2017 and ended on June/2017.

Fifty (50) crossbred Holstein x Zebu male steers with average initial weight of 275 ± 8.18 kg and 12 mo of age were used. The animals were distributed in a completely randomized design with 10 replicates per treatment: Mineral supplementation ad libitum (MS); nitrogen supplementation ad libitum (NS); concentrate supplementation in the order of 0.1% body weight (SC 1); concentrate supplementation in the order of 0.2% body weight (SC 2); total feedlot (C 3). The proportion of each ingredient in diets is described in Table 1.

Results

Discussion

The supply and the quality of the forage are determining factors for animal development and performance. The availability of potentially digestible dry matter found in this study was 2,000 kg ha^-1. The higher the content of potentially digestible dry matter, the better the biological performance and, consequently, the economic performance will be favored, since the cheapest basic nutritional resource available for livestock is grazing and, the better it is used, the greater the financial return¹⁵. When carrying out an extensive literature review¹⁶ recommend a minimum supply of potentially digestible dry matter of 6 % or (6 kg of MSpd per 100 kg of body weight). In this study, 8 kg was found, an MSpd value higher than those recommended by these authors.

The average forage supply observed in this study was 12.8 %. This is in agreement with that recommended by others¹⁶; they also recommend forage supply of 10 to 12 % for tropical grasses, evidencing that the value found is above the minimum recommended by the authors to assure forage availability with quality and quantity to the animals. The similarity for forage dry matter intake can be attributed to its expressive quality, which demonstrated crude protein levels of 9.5% MS of the forage, within the minimum limits of 7 to 11 % of DM in the diet.

The similarity in the total dry matter intake and in relation to the body weight for grazing animals was possibly due to the dry matter availability of good quality forage being the same for grazing animals and for being groups of homogeneous animals with the same age group. The similarity of these variables shows that grazed animals did not find quantitative and qualitative limitations during the experimental period regarding forage availability and possibly reached the maximum physical limit of intake.

According to Lazzarini et al¹⁷ the response to nitrogen supplementation in forage intake becomes less evident when the CP content of the basal diet is greater than 7 to 8 % in MS, as observed in this study: a crude protein of 9.5 %. This high protein content of the forage possibly contributed to the fact that there was no difference in dry matter intake among grazed animals.

Total dry matter intake and body weight gain were higher for animals in feedlot. This result can be justified by the fact that the animals were supplied with 100 % concentrate ad libitum; consequently, they had a greater weight gain due to the higher dry matter intake. Dry matter intake is undoubtedly one of the fundamental factors that influences animal performance, and it is the starting point for nutrient input, mainly protein and energy, necessary to meet the requirement of animal maintenance and production. This difference in intake may have occurred due to the higher dry matter intake of the group of confined animals, contributing to the increase in their body weight.

The values found for mean CP intake among rearing systems allowed to infer that the CP requirement of the animals in the BRCorte described by Valadares et al¹⁸ was satisfied by the diet available to the animals. The ingestion of protein by animals is of fundamental importance, since this nutrient is part of the synthesis of all body tissues, besides participating in the growth and microbial synthesis in the ruminal environment, a microbiota that has the function of degrading the compounds of the diet to release nutrients for absorption, in addition to producing microbial protein available for absorption in the small intestine.

The difference in crude protein intake for grazed animals receiving a mineral mixture may be justified by the higher non-protein nitrogen intake via supplementation provided to the other groups of animals. The similarity in the crude protein intake for grazing animals supplemented with nitrogen salt and protein / energy concentrate supplement evidences that this result may be associated to the dry matter intake, since it was similar, contributing to the fact that there was no difference in the intake of this nutrient. Crude protein intake was higher for animals in feedlot, compared to those supplemented with protein / energy concentrate. The crude protein content of the diet met the requirements of the animals and consequently contributed to a better performance, when compared to the other rearing systems.

It is presumed that part of the difference found for the intake of NDFap is due to the composition of the diet, in which its content was lower, respectively, in the diet of confined animals. These results demonstrate that the animals fed with one hundred percent of concentrate ingested fiber in less quantity, in relation to grazed animals.

The difference in the intake of EE, NFC, TDN, for grazed animals supplemented with mineral salt can be justified by the additional contribution of these nutrients from the supplementation, when compared to animals supplemented with concentrate. When the concentrate is offered to animals, it increases the concentration of the non-fibrous constituents in the diets, increasing the availability and nutrients in the gastrointestinal tract of the animals. Carbohydrates are a source of energy for ruminants, when converted to volatile fatty acids (acetic, butyric and propionic), are directed towards the deposition of muscle tissue.

The absence of differences in intake of EE, as well as NFC, TDN for animals supplemented with nitrogenous salt, compared to those supplemented with protein / energy concentrate, can be justified by the low level of concentrate supplementation (1 and 2 g/kg BW), not enough to influence the ingestion of these nutrients.

The intake of EE, NDFap and NFC, TDN, were higher for animals in feedlot. These results can be attributed to the fact that these animals receive a diet containing 100 % concentrate non-fibrous carbohydrate, contributing to an increase in the intake of these fractions due to the higher concentration of these nutrients in the diet, mainly due to the higher intake of NFC and other more digestible nutrients. Thus, the increase in the intake of these dietary components is unique and exclusively due to its greater contribution, provided by the diet. The reduction in fiber intake in neutral detergent corrected for ash and protein (FDNcp), was due to the lower participation in the diet. The dry matter intake is directly associated with performance, since it contributes to the determination of the amount of nutrients ingested, being sufficient to meet the energy and nutritional requirements of the animals and, consequently, the greater efficiency in animal production.

The difference in DM, EE, NDFap and NFC were different compared for grazed animals supplemented with mineral mixture, in relation to the other managements adopted. These results may be associated to the benefits generated in the digestibility of the fibrous fraction of the diet, when the concentrate supplement is added to the ruminant diet.

Thus, the digestibility of a given diet is a result of the interactive and associative effects of all the nutrients in the diet and not only of the isolated effect of a particular constituent of the food. Even with 100 % concentrate offered to the confined animals, digestibility was not impaired, probably due to the balance between the rumen degradable dietary protein and the energy content of the diet, since this association helps maintain fiber digestion, even in situations where starch-rich supplements are supplied to animals.

The difference in the DM, CP, NDFap, EE, and NFC for animals receiving nitrogen supplementation in grazing compared to grazed animals that received concentrate supplementation in the order of (1 and 2 g/kg) of body weight. The same behavior was observed in DM, CP, NDFap, for grazed animals supplemented with protein / energy concentrate, when compared to each other. This result demonstrates the benefits that the addition of the concentrate in the ruminant diet provides for the grazing animal production system. This difference in crude protein digestibility and in the other nutrients may be associated to the higher nitrogen supplementation in the ruminal environment, making it more favorable to the growth and development of the microorganisms present in the rumen, favoring the growth of the microbial population by balancing protein and energy in the diet.

There was a similarity in the total nutrient digestibility coefficient (TDN) for animals receiving nitrogen supplementation in grazing, compared to grazed animals receiving protein/energy concentrate supplementation in the order of 1 and 2 g/kg of body weight. The same performance for EE and NFC was observed for grazed animals that received protein/energy concentrate supplementation in the order of (1 and 2 g/kg) body weight, when compared. Possibly due to the nutritional requirements of the microbial population, it is possible that there was no limitation of these nutrients, leaving the ruminal environment under favorable conditions for microbial growth.

The best DM of CP, neutral detergent fiber corrected for ashes and NDFap, EE, and NFC for animals in feedlot in comparison to other management practices can be justified by the greater participation of these dietary nutrients in the diet. Since the association of structural and non-structural carbohydrates in the diet allows improvements in nutrient digestibility as a function of the synchrony of energy and protein availability, providing substrates to the microorganisms, resulting in improvements in the absorption efficiency of the ingested nutrients. It is worth mentioning that, when consuming diets with a lower proportion of fiber due to the increase in concentrate, ruminants can show a faster rate of passage and when the intake of diets containing high proportion of fiber occurs, the rate of passage occurs more slowly, therefore allowing greater nutrient digestibility. The difference in the digestibility coefficient of NDFap, EE, and NFC for the feedlot. system, in comparison to other rearing systems, probably occurred due to the greater contribution of nutrients from the concentrate supplement, improving the ruminal environment and increasing the digestibility of the fibrous fraction of the digestion.

The difference in the mean daily gain for grazing animals supplemented with mineral mixture, in comparison to the other managements adopted and for the animals in feedlot. when compared to grazing animals that received concentrate protein/energetic supplementation, may be associated with a higher intake of non-fibrous carbohydrate, contributing to a greater contribution of nutrients, leading to an improvement in animal performance. Non-fibrous carbohydrates are fast-degradation compounds consisting of starch, pectin and glucans of easy fermentation, providing a greater contribution of energy to the growth of the ruminal microorganisms, favoring nutrient digestion. The mean daily gain is an important index in beef cattle, and the profitability of the system depends on this gain.

The productive performance of grazed animals is directly related to the quality and quantity of forage available for grazing. These characteristics influence the intake of nutrients and nutritional attributes by grazing animals, and ingestion is the main determinant of animal performance.

The similarity in mean daily gain among grazed animals supplemented with nitrogen salt compared to animals supplemented with concentrate on the order of 1 and 2 g/kg of body weight is observed, since there was no difference in total dry matter intake, it contributes to the fact that there was no difference in animal performance. The mean daily gain, respectively, was a numerator to obtain an important variable of feed conversion (FC), in which it was not observed a difference in feed conversion between grazing managements. The similarity in the performance of the animals between grazing production systems demonstrates that, when there is a great quantity of forage and quality cattle raised in grazing can meet their requirements and achieve satisfactory gains in the period of high food availability, making the system economically viable.

Animals in feedlot perform better in comparison to those kept in grazing, since they spend less energy in search of food¹⁹, since the food is supplied directly in the trough and in a superior quality to that of grazing.

Rearing was economically viable for grazing and feedlot systems, a fact evidenced by the positive net income, characterized for the profit, since the gross income was able to cover the total cost of the rearing system.

The animals in feedlot obtained the highest gross margin in the activity, net margin and net profit observed, justified by its best performance. Therefore, grazed animals receiving concentrate supplementation in the order of 1 g/kg and in feedlot, allowed a greater return on invested capital (US$ invest / US$ ret) in the activity. The monthly rate of return was higher for animals in feedlot, which optimized animal performance. It is interesting to always seek a balance between productivity, which in this case is expressed by economic viability and performance.

Conclusions and implications

Considering the obtained results, it was possible to observe that the animals kept in graze with good availability of dry matter showed satisfactory performance. It is feasible to confine the animals in rearing, since it shortens the production cycle, generating favorable economic results.