Chromium (Cr) is considered an essential nutrient for acting on the metabolism of carbohydrates, lipids, and protein (Ohh and Lee, 2005). It is a component of the glucose tolerance factor, which increases the insulin signalization and stimulates the uptake of glucose and amino acids by target cells (Amata, 2013). A nutritional Cr deficiency can cause glucose intolerance, increase body fat and blood levels of insulin, cholesterol, and triacylglycerols, and reduce body protein in pigs (Pechova and Pavlata, 2007).

There is evidence that Cr supplementation improves the immune system (Wang et al., 2007; Tian et al., 2014), protein and fat metabolism (Untea et al., 2017), weight gain and feed efficiency (Li et al., 2013; Peres et al., 2014), carcass characteristics (Jackson et al., 2009; Park et al., 2009; Wang et al., 2014), and meat quality in pigs (Li et al., 2013). However, other studies did not show the same effects with dietary Cr supplementation (Tian et al., 2014; Tian et al., 2015; Marcolla et al., 2017). The source and concentration of Cr, supplementation period, nutritional status, stress status, health status, age, and genetics can be involved in the variation of responses observed among the studies (Amata, 2013). The main nutritional requirement guides (NRC, 2012; De Blas et al., 2013; Rostagno et al., 2017) do not present a minimal level of dietary Cr for pigs.

There is evidence that Cr supplementation period may affect the responses of the pigs. In a 29-day trial before slaughter, Rodrigues et al. (2020) did not verify any effect of CrY on growth performance. However, Boleman et al. (1995) observed an increase in carcass muscle percentage and a reduction in carcass fat with a 50-day Cr supplementation; and Caramori Júnior et al. (2017) reported an increase in muscle depth with a Cr supplementation period of 66 days, and Matthews et al. (2001) showed an increase in carcass length with supplementation of 102 days.

These responses raise the hypothesis that increasing the dietary Cr supplementation period may improve the growth performance and carcass characteristics of pigs. Thus, this study aimed to evaluate different CrY supplementation periods on the performance and quantitative characteristics of the carcass of growing and finishing pigs.

The experiment was carried out in an experimental farm located in Terenos, MS, Brazil (20°26′32" S latitude and 54°51′37" W longitude). Research was approved by the Institutional Committee on Animal Use (protocol number 625/2014).

We used eighty barrows (Duroc/Pietran × Large White/Landrace) with an initial body weight of 24.5±2.4 kg. Environmental variables were assessed daily throughout the experimental period, using a dry bulb thermometer, a wet bulb thermometer, and black globe thermometer installed in the center of the room. The black globe temperature and relative humidity index (BGHI) was calculated according to Buffington et al. (1981).

The animals were distributed in a completely randomized block design in one of dietary treatments [control: CrY-free diet for 94 days (25 to 110 kg); Cr25-110 kg: 0.4 mg kg⁻¹ of CrY in the diet for 94 days (25 to 110 kg); Cr50-110 kg: 0.4 mg kg⁻¹ of CrY in the diet for 65 days (50 to 110 kg); and Cr70-110 kg: 0.4 mg kg⁻¹ of CrY in the diet for 43 days (70 to 110 kg)], with ten replicates (pens) of two pigs each. The initial weight was adopted as a criterion for block formation.

Diets (Table 1) were formulated to meet the nutritional requirements of pigs with high genetic potential and medium-superior growth performance according to the recommendations of Rostagno et al. (2011). The CrY was included in diets by adding a commercial product containing 1,600 mg kg⁻¹ of CrY replacing the inert ingredient (kaolin).

Pigs received feed and water ad libitum. The experimental period lasted 94 days. The animals were weighed at the beginning and at the end of the experimental phases (phase 1: 25 to 50 kg, phase 2: 50 to 70 kg, and phase 3: 70 to 110 kg) to calculate daily feed intake, daily weight gain, and feed conversion. Daily digestible lysine intake, daily crude protein intake, and daily metabolizable energy intake were estimated.

On day 95, all pigs were transported (approximately 1 h) to a commercial slaughterhouse and kept in pens with free access to water for approximately 6 h before slaughter. Hot carcass weight was taken immediately after evisceration. Backfat thickness, longissimus dorsi muscle depth, and predicted lean meat percentage were measured using a probe (Hennessy Granding System) inserted between the last thoracic and first lumbar vertebrae 5 cm from the middle line on the left side of the hot carcass). The predicted lean meat amount was calculated by multiplying the hot carcass weight by the predicted lean meat percentage. The bonification index, which is a factor correction that differentiates each hot carcass individually by the predicted lean meat percentage, was determined according to Guidoni (2000).

Data were analyzed as a completely randomized block design using PROC GLM of SAS (Statistical Analysis System, version 9.4) with dietary treatment as a fixed effect and weight block as a random effect. The experimental unit was the pen for all analyses. The following statistical model was used:

in which Y_ijk is the quantitative response variable, μ is the overall mean, T is the effect of the i-th treatment, B is the effect of the j-th block, and ε is the random error. Dunnett's test was applied to compare the results of the control diet with the inclusion of chromium at each phase according to Sampaio (1998): for the first phase, three control diets versus one chromium diet; for the second phase, two control diets versus two chromium diets; and for the third phase, a control diet versus three chromium diets. For the performance in the total period and for the carcass results, the degrees of freedom of chromium inclusion period were decomposed into orthogonal polynomials, to get the regression equations. Significance was set at P<0.05.

The mean air temperature, relative air humidity, black globe temperature, and BGHI recorded during the experimental period were 27.7±2.0 °C, 75.0±12.0%, 28.1±2.1 °C, and 72.2±1.9 in phase 1; 27.2±2.1 °C, 78.0±9.0%, 27.6±2.1 °C, and 77.5±2.4 in phase 2; and 27.0±2.9 °C, 73.3±10.6%, 27.4±2.9 °C, and 77.0±3.6 in phase 3, respectively (Table 2).

There were no effects (P>0.05) of dietary CrY supplementation on growth performance for any of the evaluated phases (Table 3). The CrY supplementation also did not influence (P>0.05) the quantitative carcass characteristics evaluated (Table 4).

In the present study, the recorded mean environmental temperatures (27.7±2.0, 27.2±2.1, and 27.0±2.9 °C for phases 1, 2 and 3, respectively) are considered above the ideal for growing-finishing pigs (Nienaber et al., 1987). One of the primary effects observed in pigs subjected to heat stress is the reduction in feed intake (Campos et al., 2017). However, in the present study, even under air temperatures higher than the ideal, feed intake and weight gain of the animals were not altered and stayed in accordance with the Brazilian table of poultry and swine (Rostagno et al., 2017). This effect may be related to the presence of the water gutter in the pen contributing to heat dissipation.

All experimental diets were formulated to have equal nutrient concentration, except for the Cr level. Once there were no differences in ADFI, nutrient intake was similar for all treatments. A previous study found inconsistent responses of the effects of Cr supplementation on pig's growth performance and carcass characteristics (Gebhardt et al., 2019a).

In pigs, Cr can promote the development of muscle tissue, due to the additional energy generated by the increase in glucose uptake by insulin-sensitive cells, which can later be used for protein synthesis, supporting the muscle growth and cell maintenance (Park et al., 2009). Thus, it was expected that the growth performance of the animals evaluated in the present study would be improved, considering the action of Cr as a digestibility enhancer and nutrient partitioner (Lindemann et al., 2008).

The results of the present study corroborate with Tian et al. (2014), who found no effect on growth performance for growing pigs (30 to 50 kg) fed diets supplemented with Cr methionine (0.8 mg kg⁻¹) during 35 days, and Matthews et al. (2005), who reported no improvement in growth performance for finishing pigs (73 to 115 kg) supplemented with 0.2 mg kg⁻¹ of Cr propionate for 54 days.

On the other hand, when investigating the supplementation of increasing dietary levels of Cr methionine (0.3, 0.6, and 0.9 mg kg⁻¹) for 28 days in barrows from 75 to 100 kg, Li et al. (2013) observed a linear increase in weight gain and feed intake, 20 and 26% greater than the control diet, respectively. The increase in daily feed intake was also observed by Gebhardt et al. (2019b), when evaluating the isolated effect of Cr picolinate (0.2 mg kg⁻¹) in pigs from 27 to 130 kg.

In turn, Peres et al. (2014) observed an improvement of approximately 5% in weight gain and 7% in feed conversion in pigs from 60 to 107 kg fed diets with 0.2 mg kg⁻¹ of Cr methionine. Xu et al. (2017) also found an improvement in feed conversion with Cr methionine supplementation in diets for pigs from 50 to 110 kg.

The results observed for carcass characteristic are accordance with the responses observed by Matthews et al. (2001, 2005), Zhang et al. (2011), Peres et al. (2014), Gebhardt et al. (2019a), Gebardht et al. (2019b), and Rodrigues et al. (2020), who also did not observe the effect of supplementing organic sources of Cr on the quantitative carcass characteristics in the growth and finishing phases.

In the present study, it was expected that the Cr supplementation would provide a positive effect on carcass traits due to its mechanisms of action in maintaining glucose homeostasis, potentiating insulin in metabolizing fat, and increasing the uptake of glucose and amino acids for protein synthesis (Amata, 2013).

It can be inferred that the supplementation period was not a critical factor for the lack of a significant effect on the carcass characteristics in the present study, but probably the level of Cr supplementation. Caramori Júnior et al. (2017) reported an increase in muscle depth when evaluating a higher level of CrY (0.8 mg kg⁻¹) in the diet for finishing pigs for a shorter period (66 days).

It is important to note that there is no nutritional recommendation for minimal Cr requirements for finishing pigs (Rostagno et al., 2017), and that the level of 0.4 mg kg⁻¹ of Cr supplemented over a long period was not sufficient to promote positive effects on carcass characteristics. This hypothesis is corroborated by Lindemann and Lu (2019), who pointed out that the variability of the effects of Cr supplementation on carcass characteristics can be associated with the supplementation level and period, as well as body weight of pigs. For this reason, further studies are suggested to elucidate not only the period, but also the level of supplementation to better understand the results of the present and other studies.

The supplementation of 0.4 mg kg⁻¹ of CrY in diets for growing and finishing pigs from 25 to 110 kg does not affect growth performance and quantitative carcass characteristics.