Historically for seedstock cattle, body weight gain has been measured between weaning and yearling weights. However, depending on the intended use gain has had multiple trait definitions. In this article, we will assume that regardless of the trait definition the optimal application of animals' gain data is for the generation of EPDs. Because of the multiple sources of gain data, it is essential for organizations providing EPDs to record the characteristics of the source data. Additionally, care should be taken to appropriately include or exclude data for EPD production, because of the varying statistical characteristics of the types of gain measurements.
Average daily gain
In its simplest form average daily gain (ADG) has been the total gain between two weights taken a number of days apart divided by the number of days between the two weights. Of course, it is important to account for environmental differences (e.g., contemporary groups) and other non-genetic factors such as the sex of the animal, the age of the dam, and the animal's age at weighing. Generally, average daily gain is a measurement of the average daily body weight change of an animal over a specified period of time. Average daily gain is calculated as follows:
Prior to the widespread availability of EPDs and still, in some situations, it may also be expressed as a ratio to account for contemporary group effects as follows:
Because growth is non-linear and these equations are a linear approximation, care should be taken to ensure that the number of days between the two weights is not too long. However, the number of days between the two weights should be sufficiently long enough to obtain an accurate observation of the rate of gain. Also, it is important to compare animals' rate of gain when they are at the same phase of the growth curve.
Gain from genetic evaluations
Some organizations publish EPDs for average daily gain. In all cases in North America, this is the average daily gain prediction for the period from weaning to yearling. It is simply calculated as,
It is important to note that in many North American genetic evaluations, including those of private organizations, the yearling weight EPD is calculated from a multiple-trait analysis that includes models for weaning weight and post-weaning 160-day gain. While not published as post-weaning gain EPDs, the few organizations that publish ADG EPDs produce them using the post-weaning 160-day gain EPDs from this multiple-trait analysis. Fitting yearling weight instead of 160-day post-weaning gain would cause poorer performance of the iterative solvers used to solve for the EPD. Additionally, fitting yearling weight would require a second maternal effect be included in the model, making it a larger problem to solve without improving the quality of the resulting predictions. Yearling weight EPD are then obtained from the models that fit 160-day post-weaning gain by adding the post-weaning gain EPD to the weaning weight direct EPD.
In the past, when centralized bull testing was more common, gain during the test period was an essential component of those tests. Even today some breed organizations include these centralized test data in their genetic evaluations. Caution should be used to evaluate the impact of these types of data on genetic evaluations as the variance components may not be the same as for yearling and weaning weights. Ideally, gain/weight data from centralized tests would be used as a separate trait in a multi-trait evaluation of weaning weight and yearling weight (post-weaning gain). Alternatively, a heterogeneous variance model may be considered to account for the variance component differences. But this approach becomes more problematic if the gain test has repeated weights.
Feed intake and gain testing
Often feed intake tests will measure gain during the testing as part of an assessment of efficiency of feed utilization. However, while BIF does recommend a minimum of 42 days on a feed test, a longer feeding period is necessary for a gain test (Table 1). While shorter test periods result in less precise measures of average daily gain, this issue can be overcome by producing feed intake and gain EPDs from a sensible model that includes post-weaning gain and weaning weight as correlated traits, and includes complete pedigree information for all animals in the genetic evaluation. Because shorter test periods permit the collection of feed intake data on more animals, more selection candidates with a potentially higher accuracy of EPD will be available, thus, resulting in improved selection response. If additional precision in measuring test average daily gain were desired, then continuing to feed test animals outside of the feed intake collection facility after 42 days of measuring intake would accomplish this and still allow for measuring feed intake on additional animals.
Table 1. Average correlation and regression coefficients between standard (70-day) and varying shortened test period lengths for on-test average daily gain (ADG) and metabolic mid-weight (MMWT). (adapted from ) )
|Test Length||ADG Pearson Correlation||MMWT Pearson Correlation||ADG Regression||MMWT Regression|
Partial body weights
Benfield at al. described automated technology to collect partial body weights by having an RFID-tagged animal stand with its front feet on a platform scale. This permits collecting multiple weights per day when placed in front of a water trough or feed bunk and can be incorporated into a feeding test. The collection of many weight observations during the test period may permit the reduction of test period length required to collect sufficiently accurate gain observations to more closely match the minimum 42 days on test for feed intake.
MacNeil et al. showed that predicting body weight from partial body weight requires adjusting the partial body weight differently in different contemporary groups. Variation in the relative weights of fore- and hind-quarters of beef animals between breeds, and even within breeds, may explain some of the need for different adjustments. Finally, a multiplicative adjustment factor may not be effective for converting partial body weight to body weight. BIF recommends that full body weight be taken periodically throughout the test period to calibrate the conversion from partial body weight to body weight using linear regression. Predicting full body weight from partial body weight is likely to have acceptable accuracy in most applications if appropriate statistical procedures are used to make these adjustments.
Warm-up period recommendations
BIF recommends a warm-up or acclimation period of at least 21 days should be included in the gain test. The goal of the acclimation period is to reduce within contemporary group variation in feed intake due to non-genetic factors (e.g. pre-test environment) and to acclimate animals to both the test diet and the testing equipment. Animals should be transitioned from receiving to test diets gradually to minimize digestive system upset. Frequently these transitions will include moving animals from a primarily roughage-based receiving/backgrounding diet (either fed ad libitum or with restricted intake) to a higher energy, concentrate-based growing diet fed ad libitum. If calves entering a test have been previously transitioned to a diet that will be used in the formal feed intake test, then the acclimation period may be substantially reduced (by a week or more) to accommodate acclimation to the feed intake equipment only. However, users should be cautious that animals acclimated to a high concentrate diet are not restricted from that diet while training to prevent acidosis issues when they return to normal consumption levels.
Diets used in feeding tests will vary according to animal type, animal gender, environmental constraints, feed ingredient availability, cost, and management. Therefore, data collection should be implemented such that diets can be adjusted insofar as possible to a common nutritional base. All animals within one test should be fed the same test diet, and the diet should be formulated to provide essential nutrients and sufficient energy to ensure the expression of animal differences for intake. The ingredient composition of the diet should be recorded, and the ingredient composition of the diet maintained throughout the test period. It is desirable for samples of diet ingredients or of the complete diet to be sent to a commercial laboratory for complete chemical analysis.
Diets used in tests with growing bulls should contain at least 2.4 Mcal ME/(kg DM). Diets used in tests with finishing steers should contain at least 2.9 Mcal ME/(kg DM). There is a growing number of reports in the scientific literature in which data from intake tests are adjusted to common energy content, mainly to increase across-test comparability. That is, statistical adjustment to a constant energy density requires recording of enough chemical composition data on the diet(s) to derive metabolizable energy (ME) in megacalories (Mcal) on a dry matter basis. Average daily intake and functions of intake data should be reported on a dry matter basis. Expression of daily feed intake values on a dry matter basis removes variability in the moisture content across a diversity of diets, and increases the comparability across multiple tests and studies. As-fed measurement of daily feed intake can be recorded as well, but for further data analyses, sufficient information must be supplied to convert feed intake to a dry matter (DM) basis.
Contemporary grouping for genetic evaluation
The most efficient use of gain measures to achieve genetic progress is in genetic evaluation with EPD delivered to breeders for use in selection decisions. Contemporary groups for weight data from tests should consist of the weaning contemporary group in addition to any post weaning treatment differences including ‘on test’ date, test or acclimation duration, test location and/or test equipment, and minimum age on test. The combination of these factors into a single feed intake contemporary group as a fixed effect should result in a relatively equal pre-test environment resulting in equal ability to express genetic differences during the test. Alternatively, the contemporary group definition could be partitioned into two components, weaning and post-weaning, whereby weaning contemporary group is fitted as a random effect and nested within the fixed effect of post-weaning contemporary group. Including the grouping related to the trait being measured, in this case post-weaning performance, as a fixed effect avoids bias in the genetic predictions related to differences in management practices. Given testing stations may combine animals from multiple sources (breeders) and weaning contemporary groups in a single test pen, there may be a desire to make use of these data. In this case, including test group (pen, period/test, diet) as a separate effect in the model in addition to fitting post-weaning gain contemporary group as a separate effect could be done. This maximizes contemporary group size while still attempting to account for environmental differences. However, as a general rule mixed breeder contemporary groups are not advised. Appropriate contemporary grouping ensures that the within-group performance differences are minimally affected by differences in non-genetic factors thereby reducing bias in genetic predictors.
No organizations in North America produce feedlot gain EPDs for seedstock animals. In reality, all measures or predictions currently available for post-weaning gain are not economically relevant traits (ERT). It is reasonable to presume that feeding test gain, and, for that matter, feed consumption, are highly correlated to the feedlot performance of fed cattle offspring from the seedstock animals tested. However, there is insufficient knowledge of the precise relationship between North American feedlot gain and the other measures to make a statistically sound translation, or even know if it is necessary. Presumably at minimum seedstock test gain (and feed intake) would have at least different components of variance. Additionally, the rates of deposition of fat and muscle appear to be different between seedstock bulls and feedlot steers. Finally, feedlot gain tends to have a strong genotype by environment interaction. Different genotypes respond differently to factors such as feeding period length, ration composition, and weather.
Even when valuing animals as fat cattle or grade-and-yield, feedlot gain is not an ERT. It could be a useful indicator trait of feedlot feed consumption/cost if actual feedlot feed consumption EPD are not available. But since neither of these traits are used in routine genetic evaluation the point is moot. Perhaps in the future, this will change. In reality, all predictions of gain are not ERT. Gain is often used as a key performance indicator (KPI) or in combination with feed intake as an efficiency KPI. Gain itself should not be included in an economically optimal selection index. Sensible economic selection indices should instead include EPDs for weights when commercial animals are valued (e.g., fat cattle weight, or carcass weight) or indicator trait EPDs of those when pay-weight EPDs are not available.
The contents of this article are a result of an ad hoc BIF committee with additions, from various authors including MacNeil, Spangler, and Golden. This article is derivative of the original Guidlines wiki page Intake and Feed Efficiency which has been deleted.
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