The relationship between feed intake and liveweight in domestic animals.

Feed intake changes as animals age and grow. A constraint of most functional forms used to describe this relationship is that intake is maximum only once an animal reaches its mature weight. Often such is not the case and maximum intake is achieved earlier. Our aim was to describe a form unburdened by such a constraint and to determine its utility to describe the relationship between feed intake and liveweight across multiple species. Twelve data sets representing seven domestic animal species (cattle, chicken, dog, pig, rat, sheep, and turkey) with a wide range of mature weights were used. Average daily ad libitum feed intakes and liveweights were available on either a weekly or fortnightly basis. Rates of intake were scaled to mature intake. Within each set, the quadratic regression of scaled intake on the degree of maturity in weight was fitted. This form provided a very good description of the relationship between these variables (R2 > 0.86) and, for all but one case, a realistic prediction of mature intake. With one exception, intake reached its maximum value at a liveweight below its mature value. Furthermore, by appropriately scaling the relationship between intake and liveweight, the data could be described by a function with a single parameter with general relevance across species. By expressing the rate of intake as a function of its value at maturity, a quadratic form provides a robust and general description of the relationship between feed intake scaled to mature intake and degree of maturity in weight.

Partitioning of limiting protein and energy in the growing pig: description of the problem, possible rules and their qualitative evaluation.

A core part of any animal growth model is how it predicts the partitioning of dietary protein and energy to protein and lipid retention for different genotypes at different degrees of maturity. Rules of partitioning need to be combined with protein and energy systems to make predictions. The animal needs describing in relation to its genotype, live weight and, possibly, body composition. Some existing partitioning rules will apply over rather narrow ranges of food composition, animal and environment. Ideally, a rule would apply over the whole of the possible experimental space (scope). The live weight range over which it will apply should at least extend beyond the 'slaughter weight range', and ideally would include the period from the start of feeding through to maturity. Solutions proposed in the literature to the partitioning problem are described in detail and criticised in relation to their scope, generality and economy of parameters. They all raise the issue, at least implicitly, of the factors that affect the net marginal efficiency of using absorbed dietary protein for protein retention. This is identified as the crucial problem to solve. A problem identified as important is whether the effects of animal and food composition variables are independent of each other or not. Of the rules in the literature, several could be rejected on qualitative grounds. Those rules that survived were taken forward for further critical and quantitative analysis in the companion paper.

Partitioning of limiting protein and energy in the growing pig: testing quantitative rules against experimental data.

Literature solutions to the problem of protein and energy partitioning in the growing pig are quantitatively examined. Possible effects of live weight, genotype and food composition on the marginal response in protein retention to protein and energy intakes, on protein and energy-limiting foods are quantified. No evidence was found that the marginal response in protein retention to ideal protein supply, when protein intake is limiting, is affected by live weight, genotype or environmental temperature. There was good evidence that live weight does not affect the marginal response in protein retention to energy intake when protein intake is not limiting. Limited data for different genotypes suggested no effects on this response. A general quantitative partitioning rule is proposed that has two key parameters; e(p)* (the maximum marginal efficiency for retaining the first limiting amino acid) and R* (the maximum value of R, the energy to protein ratio of the food, MJ metabolisable energy (ME)/kg digestible crude protein (DCP), when e(p)* is just achieved). When R

The problem of predicting food intake during the period of adaptation to a new food: a model.

A model is described which aims to predict intake immediately following a change from one food to another that is higher in bulk content; it deals with the transition from one 'equilibrium' intake to another. The system considered is an immature pig fed ad libitum on a single homogeneous food, which is balanced for nutrients and contains no toxins so that the first limiting resource is always energy. It is assumed that an animal has a desired rate of food intake (DFI) which is that needed to meet the energy requirements for protein and lipid deposition and for maintenance. DFI may not be achieved if a bulk constraint to intake exists. Where a bulk constraint operates intake is calculated as constrained food intake (CFI) where (where WHC is the water-holding capacity of the food (kg water/kg dry food) and Cwhc is the animal's capacity for WHC (units/kg live weight per d)). Where intake is not constrained it is assumed that genetic potential will be achieved. Potential growth rate is described by the Gompertz growth function. Where intake is constrained, growth will be less than the potential. Constrained growth rate is predicted as where W is pig weight (kg), EI is energy intake (MJ/d), Em is the energy required for maintenance (MJ/d) and eg is the energy required for unit gain (MJ/kg). The value of eg depends on weight and the fattening characteristics of the pig. Actual growth is predicted to be the lesser of potential and constrained growth. To deal with adaptation it is assumed that the time taken to reach equilibrium depends on the difference in WHC values between the previous and current food and that the capacity to consume food bulk is related to the WHC of the current food. It is proposed that the capacity for WHC on the first day on a new food will be equal to the current capacity for WHC on the last day of the previous food. Thus where FI is food intake (kg/d). Thereafter Cwhc will gradually increase over time to a maximum of 0.27 g/kg. The rate of change in Cwhc is made to be the same for all pigs and all foods. The increase in capacity over time is assumed to be linear at the rate of 0.01 units/d. The model was tested using published data. Qualitatively the predictions of the model were in close agreement with the relevant observed data in at least some cases. It is concluded that the underlying theoretical assumptions of the model are reasonable. However, the model fails to predict initial intake when changed to foods high in wheat-bran content and fails to predict the intake of a non-limiting food where compensatory increases in intake and gain occur. The model could be adapted to overcome the first failure by taking into account the time course of digestive efficiency following a change in food. To deal with the second would require a sufficient understanding of the time course of compensatory growth.

The short-term feeding behavior of growing pigs fed foods differing in bulk content.

We investigated the effects of foods of different bulk on the short-term feeding behavior (STFB) of 16 individually housed pigs. The three foods used had different bulk contents [low-control (C), medium-70% wheat bran (WB), and high-70% sugar beet pulp (SBP)]. We expected the different intakes of the foods to be reflected in differences in STFB. Three hypotheses were developed based on ideas about the way in which a physical constraint to intake could arise. H(1): there would be less diurnal variation in feeding on high-bulk foods that limit food intake. H(2): feeding patterns on the bulky foods would be less flexible than those on C. H(3): a change in food type would result in food intake and STFB being rapidly altered to become appropriate to the new food. There were significant differences in food intake and STFB between the foods as intended. Pigs fed WB and SBP spent longer eating and had a slower feeding rate (FR) than pigs fed C. H(1) was rejected, as there was no difference in diurnal variation in intake between the foods. Feeding was not extended into the night on WB and SBP and the proportion of feeding that occurred during the night was the same for all three foods. H(2) was supported, as pigs fed WB and SBP were unable to maintain food intake and performance when time of access to the feeder was reduced. There was no adaptive change in STFB. H(3) was supported as a change from WB or SBP to C, or vice versa, caused a rapid change in STFB so that it became appropriate to the new food. It is concluded that physical constraints to food intake, caused by food bulk, may bring about changes in STFB and that they are important for the regulation of intake of such foods.