Chromium in performance and metabolism of dairy cows

. The need for refined adjustments to nutritional requirements in dairy cow production systems is a demand for productive efficiency. Dairy cows face severe physiological and metabolic changes as the end of pregnancy and the beginning of lactation, requiring greater attention in nutritional aspects. However, chromium supplementation has been suggested to improve the metabolism of carbohydrates, lipids and proteins. Thus giving potential use for dairy cows. In this context, the objective was to conduct a literature review on the effects of chromium supplementation on the productive performance and metabolism of dairy cows. Therefore, chromium supplementation appears to improve milk production without affecting milk constituents. And apparently chromium supplementation reduces the blood concentration of NEFA. These results can be explained by a possible improvement in insulin metabolism, promoting an increase in insulin sensitivity in adipose tissue, consequently reducing lipolysis. What can change the energy partition in the mammary gland improving the processes of milk synthesis. However, many experimental results are contradictory in the literature, which can be explained by different stages of lactation, chromium source, stress conditions, supplementation period, type and content of carbohydrates in the diet. So in fact, there is a need to conduct a meta-analysis study with the available database to elucidate the real effect of chromium on the performance and metabolism of dairy cows.


Introduction
Chromium (Cr) was first described as an essential mineral for normal glucose metabolism in rats by Schwarz and Mertz (1959) and in humans by Jeejebhoy et al. (1977). Apparently, the positive responses of Cr supplementation seem to be related to the change in the energy partition Leiva et al., 2015) due to a potentiating effect of the action of insulin (Vincent, 2001).
Traditionally, conventional diets were supposed to meet the nutritional requirements of Cr in farm animals. However, studies suggest that Cr supplementation affects glucose and / or lipid metabolism (Gentry et al., 1999;Hayirli et al., 2001;Sumner et al., 2007). Fact that can be used to modulate animal metabolism and improve the production of ruminants.
The demand for Cr is typically increased during different forms of stress such as nutritional, metabolic and physical (Pechova and Pavlata, 2007). During the prepartum, delivery, lactogenesis and galactopoiesis periods, great metabolic stress is generated, causing immunosuppression and metabolic overload in dairy cows (Spears, 2000;NRC, 2001;Gulpete, 2018). Cr is involved in many metabolic functions (Mertz, 1993;Bryan et al., 2004;Vargas-Rodriguez et al., 2014) and is essential for the normal metabolism of carbohydrates, lipids and proteins (Vincent, 2004).
Therefore, Cr supplementation in dairy cows can have positive effects on metabolism and productive performance. Thus, the aim was to conduct a literature review to assess the effects of Cr on the metabolism and performance of dairy cows.
Therefore, Cr supplementation in dairy cows can have positive effects on metabolism and productive performance. Thus, the aim was to conduct a literature review to assess the effects of Cr on the metabolism and performance of dairy cows.

Contextualization and Analysis Chromium
Cr is a mineral that can be found in very low concentrations in natural ingredients commonly used in diets for farm animals (Bailey, 2014). It can be naturally detected in different oxidation states from -2 to +6, however with a greater predominance of hexavalent (Cr 6+ ) and trivalent (Cr 3+ ) Cr. Trivalent chromium is the most stable form found in living beings, considered a highly safe form of chromium (Lindeman, 1996). However, hexavalent chromium is mainly of industrial origin and is associated with chromium toxicity (Amata, 2013). Currently several forms of organic Cr are described in the literature as Cr-Propionate, Cr-methionine, Cr-picolinate, Crnicotinic acid complex and Cr-yeast.
According to Gäbel et al. (1987), the absorption of Cr in the rumen is negligible and the recovery of Cr in the rumen varies between 92 to 99%. Although most studies are conducted with rats, it has been suggested that Cr absorption occurs in the proximal part of the jejunum (Khan et al., 2014), duodenum and in the ileum (Chen et al., 1973).
Different factors can affect the absorption of Cr, however it is well accepted that the absorption and bioavailability of organic sources are better than inorganic sources (Lukaski, 1999;Zha et al., 2009). In the literature it is reported that the bioavailability of inorganic Cr is between 1 and 3%, while organic Cr is about 10 to 30 times more available (Forbes and Erdman, 1983). The low bioavailability of inorganic Cr is probably related to the formation of non-soluble Cr oxides (Chen et al., 1973;Pechova and Pavlata, 2007), due to interference in the ion forms of other minerals (Pechova and Pavlata, 2007), bonding Cr to natural forage compounds (Borel and Anderson, 1984;Pechova and Pavlata, 2007), or the slow conversion of inorganic Cr to bioactive form (Ranhotra and Gelroth, 1986).

Chromium role in metabolism
Currently, it is described that chromium acts on the activation of insulin via cromodulin, an oligopeptide consisting of glycine, cysteine, aspartate and glutamate (Yamamoto, et al., 1987). This oligopeptide is a cofactor for the action of insulin, mediated by stimulating the activity of protein tyrosine kinase, an insulin receptor (Ducros, 1992;Davis et al., 1996;Davis and Vincent, 1997). The stimulating effect of chromodulin seems to occur without affecting insulin concentration, indicating that this oligopeptide has an intrinsic role in insulin sensitivity (Khan et al., 2014), promoting a self-amplifying mechanism of insulin signaling (Vincent, 2000).
In adipose tissue, it is suggested that the action of insulin is mediated by the activation of phosphotyrosine phosphatase in the adipositary membrane (Ducros, 1992;Vincent, 2000;Davis et al., 1996) increasing glucose uptake. Additionally, the activation of this intracellular signaling pathway increases the rate of glucose uptake and positively regulates the levels of mRNA of the insulin receptor, type 4 glucose transporter (GLUT4), glycogen synthase and uncoupling protein-3 in skeletal muscle cells ( Davis et al., 1996), consequently improving insulin sensitivity, increasing the rate of glucose and amino acid uptake.

Productive performance
Diets for dairy cows can contain sufficient concentrations of Cr to meet the demands during a normal production period, but can become deficient in critical situations such as, late pregnancy, delivery, early lactation, weaning and transport (Sano et al., 1991;Mousaie et al., 2014;Vargas-Rodriguez et al., 2014;Yuan et al., 2014).
In the experiment by Kafilzadeh et al. (2012), supplementing chromium in multiparous dairy cows at the beginning of lactation, reported that MP was not affected. Soltan (2010) and Nikkhah et al. (2011) investigating the effect of Cr supplementation on the performance of multiparous dairy cows reared under heat stress at the beginning of lactation, reported that MP and DMI in cows supplemented with Cr were greater than the control group. Similarly in the experiment by Al-Saiady et al. (2004) with multiparous dairy cows in mid-lactation raised under thermal stress, it was reported that DMI and PM were higher for the group of cows supplemented with Cr. However, experiments using Cr supplementation for dairy cows demonstrate inconsistent results ( Table 1).

Composition of milk in lactating dairy cows
Several experiments evaluated the effects of Cr supplementation on the milk composition of dairy cows (Table 2). In general, it is reported that the fat, lactose, protein and non-fat solid content of milk was not affected by the addition of Cr in the diet (Yang et al., 1996;Soltan, 2010;Vargas-Rodriguez et al., 2014;Yasui et al., 2014). However, Kafilzadeh et al. (2012) reported that the lactose content in milk increased when the diet of multiparous dairy cows at the beginning of lactation was supplemented with Cr. Al-Saiady et al. (2004) reported that Cr levels in the diet did not affect the milk composition of multiparous dairy cows stressed by heat in the middle of lactation. In contrast Nikkhah et al. (2011) reported an increase in protein, fat and lactose content with Cr supplementation for cows under heat stress. However, other investigations have described that Cr supplementation did not influence the fat, protein and lactose content of milk in primiparous dairy cows (Yang et al., 1996;Bryan et al., 2004;Smith et al., 2005).

Blood parameters
There are conflicting reports regarding the effects of Cr supplementation on the serum parameters of dairy cows (Table 3). Kafilzadeh et al. (2012) investigated the effect of Cr supplementation on the serum parameters of multiparous dairy cows at the beginning of lactation and reported that Cr supplementation caused an increase in glucose and insulin and decreased concentrations of nonesterified fatty acids (NEFA), but not there was an effect on cortisol by Cr supplementation. Soltan (2010) reported that Cr supplementation decreased the level of cortisol and NEFA in multiparous dairy cows in the prepartum period, although glucose and insulin concentrations were not affected.   However, it was reported that the concentrations of glucose, insulin, glucagon, betahydroxybutyrate (BHBA) and NEFA were not affected by Cr supplementation in multiparous dairy cows during the prepartum period (Yasui et al., 1996;Smith et al. ., 2008). Similarly, Yuan et al. (2014) studied the effect of Cr supplementation on some serum parameters in primiparous and multiparous dairy cows in mid-lactation, reported that the concentrations of glucose, insulin, glucagon and NEFA were not affected by Cr supplementation.

Final considerations
The effects of Cr supplementation for dairy cows are inconsistent, making it difficult to determine whether Cr supplementation improves the productive performance of dairy cows.
In some studies, the increase in milk production may be due to the nutritional correction of Cr deficiency or change in the energy partition in the metabolism, increasing the energy availability for milk synthesis. Another mechanism that should be taken into account is that Cr can improve gluconeogenesis, thus increasing the use of glucose for the synthesis of lactose in the mammary gland, consecutively increasing milk production, due to the osmoregulatory effect of lactose.
In general, the composition of the milk does not appear to change. However, in relation to blood parameters, NEFA is the one with the greatest change, promoting a reduction in concentrations. These results suggest that a possible action of Cr on insulin sensitivity in adipose tissue may occur, providing an increase in glucose uptake, consequently increasing lipogenesis and reducing liquid lipolysis. This effect can help explain the energy partition in the metabolism, culminating in increased milk production.
However, the discrepancies between the results of the experiments can be explained by differences in the lactation stage, chromium source, stress conditions, supplementation period, type and content of carbohydrates in the diet. Thus, it is suggested the need to conduct a meta-analysis study with the available published database, to assess the real effect of Cr on the performance and metabolism of dairy cows.