Targeted Gene Expression Profiling in Beef Cattle under Finishing Conditions in the South Eastern United States: Identification of regulatory factors involved in metabolic efficiency, energy partitioning, and intramuscular fat deposition

Abstract

Three studies were conducted in order to examine the molecular events associated with metabolic efficiency in specific skeletal muscle and adipose tissue depots in finishing beef cattle. In the first study a total of 15 male cattle (n = 7 Bulls and 8 Steers) were selected from the initial progeny of a Residual Feed Intake (RFI) selection program to determine the relationship between RFI and the transcriptomic signature of performance efficiency in finishing beef cattle. Calves were individually fed twice daily for 84 days and refusals were collected once daily. Feed intake data was collected for each animal and used to determine residual feed intake (RFI). At day 84, skeletal muscle and adipose biopsies were collected for analysis of regulatory gene expression related to protein turnover and lipid metabolism. Overall, the results from this study indicate that RFI appears to be related to genes involved in protein turnover and to a lesser extent, other metabolic genes; however genes involved in AT metabolism did not appear to be related. It is possible that the range of RFI values in bulls and steers was too narrow to ascertain a relationship between fattening and feed efficiency and future studies should utilize more divergent cattle populations with respect to RFI. In a second study was conducted to determine the effect of days on feed and beta-agonist administration on the expression of regulatory genes in skeletal muscle and adipose tissue of finishing heifers. Seventy-one crossbred heifers were stratified according to height and height and assigned to one of six pens (12 cattle per pen). Cattle had continuous access to automatic water troughs and each pen contained 12 Calan Gates® to allow for individual feed provision and intake determination. Treatment groups consisted of days on feed (DOF, n= 16 per group) with the following assignments: 79, 100, 121, and 142 DOF. For each DOF group, half (n=8) of the animals were treated with ractopamine hydrochloride (RAC) 300mg/hd/d for the final 35d vi prior to harvest while the other half served as controls (CON). At slaughter, skeletal muscle and adipose samples were collected to determine regulatory gene expression. These data did not identify coordinated regulation of metabolic pathways in response to RAC administration in any of the DOF groups however temporal patterns of gene expression were observed that are consistent with the order and priority of tissue development in finishing cattle. The final study looked at the effects of feed restriction and re-feeding on gene expression patterns in muscle and adipose tissue of forage fed beef. Temporal regulation of gene expression was observed between biopsy dates and plane of nutrition differentially regulated growth initially however poor forage growth led to a lack of robustness in the treatment groups as the experiment proceeded and weight gains began to equalize between groups. This led to a lack of coordinated regulation of the selected genes between treatment groups. As a whole, this body of work provides the basis for future studies and has established that transcriptomic/gene expression profiling in beef cattle determined by quantitative RT-PCR of skeletal muscle and adipose tissue samples can be an effective approach when used in conjunction with more elaborate approaches to identify and manipulate the molecular distinctions that are inherent to more efficient animals. This will eventually lead to innovations to improve production efficiency and carcass value by augmenting the growth of specific tissues independently of less valuable ones.