28 <strong>AFMA</strong> MATRIX ● APRIL <strong>2013</strong>
Factors affecting the voluntary feed intake of livestock By Foch-Henry de Witt and Ockert Einkamerer, University of the Free State, Bloemfontein We often assume that because there is 15% protein or 60% carbohydrate in a ration, that the animal has to use all of it. This assumption is not true. In fact, we must consider that even the ingested nutrients are still “outside” the body until it has been absorbed. When any animal is presented with food, for some reasons that may not be known, the animal may consume 95 units of this material and refuse the balance. Increasing intake of high-producing animals is essential, but care must be taken to prevent metabolic disorders and excessive fat accumulation closer to the end of the feeding period. Feeding is a complex activity which includes such actions as the search for food, recognition and movement towards it, sensory appraisal, the initiation of eating and ingestion. It is necessary to consider why, in most mature animals, body weight is maintained more or less constant over long periods of time, even if feed is available ad libitum. Hence, the concepts of short- and long-term control of food intake must be considered. The former concerns initiation and cessation of individual meals, and the latter the maintenance of a long-term energy balance. Although thought to be similar, there are important differences between species. This depends mainly on the structure and function of their digestive tracts. Non-ruminants The most problematic factor in diet formulation for monogastric animals is probably the correct prediction of voluntary feed intake (VFI). Although diet formulators are normally skilled in the use of software programmes for “least-cost” formulation, they should continuously strive to improve their biological knowledge regarding the factors influencing VFI. The followings aspects will be briefly discussed to indicate their importance in VFI of pigs and chickens. Feed Due to the limited nutrient contribution of microbial fermentation in monogastric animals, pigs and chickens normally consume feed in accordance with the diet’s first limiting nutrient – thus any shortage of amino acids, minerals, vitamins and energy would provoke an increase in VFI. Additionally, the amino acid profile as well as the lysine against apparent metabolisable energy (AME) ratio, must concur with the requirements of the specific requirements of the animal. Generally, it is assumed that a decrease in dietary energy would result in an increase in VFI to compensate for the energy loss, until GUT capacity becomes the limiting factor. It is indicated that the threshold energy value for poultry is between 10,1 and 10,8 MJ AME/kg, depending on the environmental temperature, thereby suggesting that an energy intake below these values would negatively influence lean protein growth in favour of adipose tissue deposition. The physical form of the diet (wet/dry mash, crumbs and pellets), as well as particle size and the distribution of particles (% particles ≥1,0 mm & ≤3,5 mm), would not only influence VFI but also total available nutrient intake, rate of passage and digestibility coefficients of a given diet. Nutrient density or “bulkiness” of diets, which are mostly linked to the AME and crude fibre (CF) concentration of diets, plays an important role in VFI, especially in young animals with limited GUT capacity and a high rate of passage due to the rate of nutrient metabolism. In addition to that, the water-holding capacity (WHC) and non-starch polysaccharide (NSP) component of certain feed sources (especially if no synthetic enzymes are included and the diet was not exposed to heat/steam treatment) will impede VFI due to its interference with the rate of passage and digesta viscosity. “It is highly unlikely that feed with such a high PV will be fed to pigs and chickens under commercial conditions. It is also known in poultry that the relationship between water consumption and feed intake is linear” Feed acceptability due to rancidity “off odours” or “sourness” from wet fermentation of diets represent a classical interaction between “feed qualities versus animal tolerance” which will influence VFI – mostly in a negative manner. Although some literature indicates that a dietary peroxide value (PV) of 75 to 150 milli-equivalent peroxide per kilogram fat had no negative effect on feed intake and production performances of birds, it remains open for debate. It is highly unlikely that feed with such a high PV will be fed to pigs and chickens under commercial conditions. It is also known in poultry that the relationship between water consumption and feed intake is linear. Any factor that influences water intake (WI) would subsequently influence VFI. Simultaneously, water quality in terms of chemical and microbial contaminants will influence WI and eventually VFI of the animals due to Processing <strong>AFMA</strong> MATRIX ● APRIL <strong>2013</strong> 29