The objectives of this study were, to evaluate potential differences in feedlot performance, blood variables of lambs classified according to RFI, and to estimate correlation between feed efficiency represented by RFI, FCR, GF, and some metabolites blood parameters. The main findings of this study were; firstly, the low RFI animals had a greater GFE, and lower DMI and FCR values, with approximately a similar ADG and final body weight. Secondly; the RFI trait had a positive correlation with total protein, globulin, urea, HDL, phosphorus, and negative correlation with creatinine. FCR had a positive correlation with total protein and AST.
Consistent with our results, many previous studies have found variation in feed efficiency using the RFI approach between lambs, with the most efficient animals having lower dry matter intake [7, 17].
The mean values of RFI of the most efficient and least efficient group was -0.28 Kg DM/day and 0.34 Kg DM/day, respectively, making a total difference of 0.62 Kg DM/day between the two groups. This difference was larger than the results found by Edson et al. [7] in their evaluation of lle de France lambs, in which RFI values between efficiency groups ranged from -0.42 to 0.14 Kg DM/day. The results obtained for DMI, indicated that low RFI animals had 10.71% lower daily DMI than the med RFI animals and 14.28% than high RFI animals. This difference was slightly higher than the values of Low-RFI lle de France lambs, that had a 12.92% lower DMI than high-RFI lambs.
In the current study, the overall mean for ADG was 0.21 and 0.19 kg/day in low and high RFI groups, respectively. Basarab et al. [19] reported that the relationship between RFI and ADG was zero suggesting that RFI could be used as an indicator of the animal’s maintenance requirements rather than growth, size and/or appetite. Therefore, RFI could become an important tool to increase profitability in the meat production industry. François et al. [20] and Snowder and VanVleck [21] reported a moderate heritability for RFI in sheep of 0.30 and 0.26, respectively, which indicates the possibility of response to individual selection. However, it is essential to know the effects of using RFI as a genetic selection criterion in sheep.
A higher level of total protein, globulin, aspartate amino transferase and urea (the markers of liver function for higher protein catabolism) provide evidence for greater protein turnover in high RFI lambs than low RFI. The same results were demonstrated in previous studies in cattle, the high RFI animals had greatest total plasma protein levels [14]. The increasing level of blood protein may reflect the increasing rate of protein turnover in less efficient animals [22].
The higher globulin concentration in inefficient lambs may be associated with a greater stress response in these animals include an increase in metabolic rate and energy consumption coupled with increase in catabolic processes such as increased lipolysis and protein degradation [16]. Ricon-Delgado et al. [17], suggesting that less efficient sheep is on average more excitable or easily stressed than more efficient sheep.
The mean values of blood urea nitrogen (BUN) differed significantly (P=0.02) between high and low RFI groups. A significant correlation (rFCR = 0.40) was observed between RFI value and BUN concentration. Previous reports in cattle by Richardson et al. [14] have found greater blood concentrations of urea in less efficient genotypes. This may be due to a greater protein intake in high RFI animals, a greater rate of body protein degradation, or deviation in the supply of amino acids due in part to variation in the efficiency of microbial protein production in the rumen [23, 24]. Blood urea concentration has been reported as negatively correlated with high lean growth and positively correlated with backfat depth in sheep [25].
Creatinine is a metabolite product of creatine phosphate, an energy storage compound in the muscle, that has a positive association with muscle mass in sheep [25], cattle [26] and negatively with fat depth in sheep [25]. A negative correlation (r= -0.38) between RFI and creatinine was shown in this study (P= 0.02), a similar negative correlation (r = -0.45) between serum creatinine and RFI for beef steers was observed by Richardson et al. [14]. Moreover, Richardson et al. [14] reported that animal selected for RFI may result in higher concentrations of blood creatinine and lead to changes in composition of the body with lower fat content in the carcass and a higher proportion of muscle mass in the carcass.
The mean values of AST activity were found to be 124.07 and 162.90 (U/L) in low and high RFI groups, respectively. AST is a key enzyme in amino acid metabolism and it acts as a marker of liver function and indicates higher levels of protein catabolism in the liver of less efficient steers [14]. Richardson and Herd [27] reported that the high level of AST was found in the cattle with high RFI. Therefore, metabolites may give indicator of differences in animal metabolism which may reflect genetics differences in feed efficiency.
The positive correlation between RFI and HDL (RFIr=0.49), was in the same line with result obtained by Rauw et al. [28] who found that RFI tended to correlate positively with HDL.
Ions transport contributes 20% of the variation in basal energy expenditure between animals [29]. No strong correlation was observed between RFI and mineral concentrations except for phosphorus (RFIr=0.32), which is essential for cellular biology and energy metabolism with implications for protein synthesis [30]. Higher concentrations of phosphorus in young cattle have been directly related to growth hormone activity, which promotes intestinal phosphate absorption and renal phosphate re-absorption [31]. Phosphorus also contributes to the production of the muscle storage molecules, kreatine phosphate and ATP [32].