Metabolic and physiological mechanisms responsible for variation in feed efficiency

Authors: Foote, Andrew

Submitted to: Meeting Proceedings
Publication Type: Proceedings
Publication Acceptance Date: 4/26/2018
Publication Date: 5/17/2018
Citation: Foote, A.P. 2018. Metabolic and physiological mechanisms responsible for variation in feed efficiency. Proceedings of the XI SIMCORTE –XI Simpósio de Produção de Gado de Corte (International Symposium on Beef Cattle Production), May 31-June 2, 2018, Viçosa, Brazil. p. 115-129.

Interpretive Summary:

Technical Abstract: There has been an increase in the number of experiments in the past few years that explore the underlying mechanisms involved in feed efficiency of beef cattle. This is a byproduct of the need to improve feed efficiency to increase the sustainability of beef production and improve the economic situation of cattle production. In the United States beef cattle industry, feed costs account of approximately 55 to 75% of total production costs (Trenkle and Willham, 1977; Moore et al., 2009; Ahola and Hill, 2012). Improvements in the conversion of consumed nutrient to usable product (e.g. muscle, milk) along with continued improvements in growth rates is key to improving the environmental and economic impact of beef cattle production. Feed efficiency can be defined in many ways. A simple and basic definition is the ratio of inputs and outputs such as Feed:Gain or Gain:Feed (G:F). The ratio of G:F is the preferable ratio so that it can be defined in situations where body weight is maintained or lost. G:F is highly correlated to average daily body weight gain (ADG) (Foote et al., 2014), so it is assumed that if a favorable G:F is selected for, a concomitant increase in ADG would also be observed. An increase in ADG would likely be associated with an increased mature body size, which is less than ideal. Koch et al. (1963) presented the concept of two measures of feed efficiency, which include residual gain and residual feed intake (RFI). The authors identified residual gain as being the ideal measure of feed efficiency, yet RFI has become the more popular measure of feed efficiency in beef cattle. RFI is essentially the difference in expected feed intake for and animal (based on body weight and growth rate) and the observed feed intake. A negative (or low) RFI is considered ideal and represents an efficient animal while positive (or high) RFI is inefficient. RFI is uncorrelated to ADG and is mainly driven by feed intake. Residual gain is the difference is expected ADG (based on body size and feed intake) and the actual ADG and is highly correlated to ADG but not DMI. It is likely that there is no single best measure of feed efficiency because different production setting may call for different levels of either DMI or ADG. For instance, in production settings where the available forage is of poor quality, it may be necessary to select animals that have a greater potential for DMI, so that they can consume enough forage to meet their nutrient requirements. This may not be the best situation to select for cattle with low (negative) RFI. The more ideal approach is to understand the underlying biology of the regulation of growth and feed intake. These component traits could be used in an index that includes other factors such as economic factors to select for an ideal feed efficiency phenotype. The need to understand the underlying biological mechanisms that contribute to variation in feed efficiency is multifaceted. First, measuring feed efficiency on individual animals is difficult and expensive. In order to find genetic components that contribute to feed efficiency, data on extremely large numbers of animals is necessary. Labor and equipment costs limit the ability to conduct this kind of experiment. Understanding the biology could lead to easier to measure traits that could expedite the genetic evaluation of feed efficiency. Secondly, it is important to have a thorough understanding of the traits that are being selected for. This has been previously observed in extreme selection of fast growing cattle that leads to increased mature body size, therefore greatly increasing the maintenance costs of the cow herd. If producers make selection decisions using an index or calculated value, it may not be clear what effect this has on the physiology of the animals. Therefore, it is important to understand the underlying physiological mechanisms