3.1: Preweaning Phase

3.1.1: Measurement of birth weight. Calf birth weight is a useful indicator of calving difficulty. Therefore, selection of breeding animals for smaller birth weight appears to be an effective criterion for improving direct calving ease. Both sex of calf and age of the cow influence birth weight of the calf.

For recordkeeping, weight of females can be adjusted to a male basis by multiplying their birth weight times 1.07.

Factors for adjusting birth weights for differences in age of dam are in Appendix Table 12.2.

Adjustments that differ from the BIF standard in lines 1 and 2 of the table were developed by individual breed associations using breed data because the most appropriate adjustments for age of dam may differ from one breed to another. Therefore, adjustments derived separately for each breed can be substituted for the HIP standard. Breed associations are encouraged to develop their own adjustment factors for sex and age of dam. Caution should be taken to consider genetic trends and, to the extent possible, remove the effects of these trends from the estimated affects of age of dam on birth weight.

Season of birth also has an important effect on birth weight. Research indicates that fall calves have lighter birth weights than spring calves.

Birth weight adjusted for age of dam and sex should be expressed as a ratio to the average of the birth weights of contemporary calves born in the same season. Birth weight ratios can be used to make comparisons between calves from the same herd born in different seasons. To facilitate selection for lighter birth weights to improve calving ease, it is recommended that birth weight ratio be computed as follows:

3.1.2: Measurement of weaning weight (205 days). Weaning weights are obtained to evaluate differences in mothering ability of cows and to measure differences in growth potential of calves. For best estimates of genetic worth for weaning weight, it is necessary to adjust individual calf records to a standard basis. It is recommended that the weaning weight be standardized to 205 days and a mature dam equivalent, and that weights be recorded as dose to 205 days as possible. The recommended range is 160 to 250 days.

Calves weaned outside this range should be accounted for by a special management code and handled as a separate management group. Records of these calves should not be adjusted for age of dam, since appropriate correction factors are not available,

The 205-day weight is computed on the basis of average daily gain from birth to weaning, as follows:

• Subtract the actual birth weight from actual weight at weaning. (If actual birth weight is not available, substitute the appropriate standard birth weight as designated by the respective breed association for the sire breed of calf.) (See Appendix Table 12.2. The table replaces the former recommendation of using standard birth weight of 70 pounds for all breeds and crossbreds.)
• Divide the remainder by age in days at weaning to obtain average daily gain.
• Multiply the average daily gain by 205.
• Add the birth weight that was subtracted initially.

This provides an estimated 205-day weight, unadjusted for age of dam or sex of calf This procedure is summarized by the following formula:

To establish a uniform procedure for computing age of dam, the following classifications are recommended:

To adjust for age of dam, it is recommended that the adjustment factors in Appendix Table 12.2 be used to compute 205-day weights for the respective age of dam for each calf.

Research shows that the additive factors for age of dam adjust weaning weights more correctly than do multiplicative factors. Unlike multiplicative factors, additive factors neither favor heavy birth weights nor ‘balloon’ the effects on weaning weight of extra weight gain due to creep feeding. Therefore, BIF recommends the use of additive rather than multiplicative age-ofdarn adjustment factors.

3.1.3: Weaning weight ratio. Records on 205- day weight and 205-day weight ratio, adjusted for age of dam for individual animals, should be reported and published on the basis of each sex category (within-sex basis without a sex adjustment). Weaning weight ratios within sex groups are calculated by dividing each call’s 205-day weaning weight (adjusted for age of dam) by the average of its sex group and expressing it as a percentage of its sex group average. Thus, weaning weight ratios provide a record of each animal’s deviation in percentage terms from the average of its contemporaries and are useful in ranking individuals of each sex for making selections. For weight ratios to be of value, contemporaries should be herd mates, similar in age, and exposed to the same environmental influences.

3.4.4: Produce of dam summary. A record of lifetime productivity (produce-of-dam summary) is recommended. It provides valuable information for within-herd comparisons. It can be most helpful for identifying both the lowest producing cows to be culled and the cows that are consistently high producing.

It is recommended that the produce-of-dam summary include the following information:

Measures relating to reproductive efficiency:

• Age at first calving (months);
• Current age;
• Number of calves born (lifetime, see "Guidelines for Reproduction" section 2);
• Number of calves weaned (lifetime, see "Guidelines for Reproduction" section 2);
• Average age of calves when weaned;
• Average calving interval.

Measures relating to productivity

• Average birth weight ratio;
• Average weaning weight ratio of all calves weaned;
• Average adjusted 365-day weight ratio;
• Most Probable Producing Ability (MPPA).

3.1.5: Most Probable Producing Ability (MPPA). It is recommended that MPPA be included on produce-of-dam summaries and that ranking of dams be based on MPPA for 205-day weaning weight ratio. This is necessary to compare dams with different numbers of calf records in their averages. For example, suppose six cows have the following records of production:

In the example, cow A has the lowest lifetime average weaning weight ratio. However, this average weaning weight ratio for cow A is based on only a single calf for which environmental conditions or the calf’s genetic potential for growth might have been below the average of what the cow would normally produce. One or more calves from cows B or C could also have had a record of 85 or less. All three cows are probably low producers, but use of MPPA enables more accurate culling and, in this example, indicates that cows B and C are slightly lower producing cows than A.

MPPA for weaning weight ratio is computed by the following formula:

MPPA = H +	NR	(C - H)
	1 + (N - 1)R

where H =	the herd average weaning weight ratio,
N =	the number of calves included in the cow’s average,
R =	0.4, the repeatability factor for weaning weight ratio,
and C =	average for weaning weight ratio for all calves the cow has produced.

MPPA of cow D in the example above is computed as follows:

3.2: Postweaning Phase -- On the Farm and Ranch

3.2.1: Measurement of yearling weight (365 days) or long yearling weight (452 or 550 days). When cattle have been compared on a constant weight or age basis, yearling weight at 365 days or long yearling weight at 452 or 550 days are particularly important because of their high heritability and high genetic association with efficiency of gain and pounds of retail trimmed boneless beef produced.

Yearling weight should be computed and reported separately for each sex. In on-the-farm or ranch tests, the postweaning period should start on the date weaning weights are obtained (for example, actual weaning weight is used as initial weight on test). Research results show that the age-of-dam effects on 365-day weight are of approximately the same magnitude as age- of-dam effects at weaning. For this reason, it is appropriate in on-farm tests to add post-weaning gains in a 160-day, postweaning period to 205-day weaning weight, adjusted for age of dam to arrive at adjusted 365-day weight. The following formula is recommended:

The period between weaning weight and final weight should be at least 160 days. Final weight should not be taken at less than 330 days of age for any individual animal, and the average age for each sex-management group should be at least 365 days. It is recommended that the number of days between weaning and final weight be the same for all animals of the same sex in a herd. With this procedure, one need obtain only weaning weight and yearling weight on each animal. All growth periods in the animal’s life are included in this procedure.

Adjusted long yearling weight (452 or 550 days) for eacb sex should be computed in the same manner as adjusted 365-day weight:

To compute 452-day weight, substitute 247 for 345 in the equation. For bulls grown on intermediate feed levels, adjusted 452-day weight gives a better evaluation of growth potential than 365-day weight. Taking them to 550 days on a standard ration might result in their becoming excessively fat. Final weight should not be taken at less than 500 days of age when estimating 550-day weight or at less than 400 days when estimating 452-day weight.

3.2.2: Weight ratios. Weight ratios for either adjusted 365-day weight (yearling), adjusted 452-day weight, or adjusted 550-day weight (long yearlings) should be computed separately for each sex-management code group. Weight ratios should also be reported separately for each sex-management code group for ease of ranking individual animals for each sex in making selections.

Weight ratios for yearling weight can be biased downward if lighter calves are culled at weaning. Research has indicated that, with 25 percent, 50 percent, and 75 percent culling for low weaning weight, yearling weight ratios would be underestimated by 3 percent, 6 percent, and 9 percent for each calf, respectively, if the average yearling weights of selected calves are used to compute the ratio. This would occur because the average yearling weights of selected calves would exceed those for all calves weaned.

To adjust yearling weight ratio for selection on weaning weight (or culling of lighter calves at weaning), the following formula is recommended for computing yearling weight ratio:

When no calves are culled at weaning, the expression "W_u + P_s" in the above formula becomes the average of all animals in the sex-management code group. The remainder of the formula remains the same for expressing the ratio of an individual animal as a percentage of its sex-management code group. By keeping the averages corresponding to each calf on file, this type of indexing can be done even for noncontemporary weaning groups assembled for central tests.

Sire and group summaries for yearling weight ratio should be computed as:

3.3: Efficiency or Gain

Efficiency of gain in beef production is the ratio of nutrient input to beef output or the ratio of beef output to nutrient input. On a life cycle basis for either herds or individual cows, about 70 percent of the energy required for beef production is consumed by cows and bulls in the breeding herd and by their calves prior to weaning. About 30 percent of the energy required for beef production from weaning to slaughter is consumed by steers and heifers during the postweaning period. Depending on the cost of feed resources for the cowherd in relation to those for the feedlot, 45 to 55 percent of the total feed costs are incurred in the period from weaning to slaughter. Thus, efficiency of growth is usually measured by (1) cow efficiency and (2) postweaning efficiency.

3.3.1: Cow efficiency measures the ratio of beef output per unit of nutrient input. The beef output includes the beef from cull cows as well as progeny slaughtered. Beef output is relatively easy to assess. However, it has not been possible to measure nutrient input of reproducing animals under practical economical management systems.

One method of measurement that has been tried is the ratio of calf weight to cow weight as an estimate of cow efficiency. In this ratio, the numerator represented output, and the denominator, cow weight (or cow weight ^(0.75) ), represented input or feed required for maintenance of the breeding herd.

However, research involving individually fed calves and individually fed cows has shown that this ratio is not a good predictor of the ratio of weight of progeny to feed consumed by the cow and calf to weaning. One study showed that weaning weight of progeny alone is slightly more accurate than the ratio of calf weight to cow weight as an indicator of cow efficiency. Several other studies have shown that the ratio of calf weight to cow weight (or cow weight^0.75) is too biased (penalizing large cows and favoring small cows) to be of practical use as an indicator of cow efficiency.

3.3.2: Postweaning feed efficiency. Choice of interval of evaluation or end points has an important influence on ranking of animals and breeding groups for feed efficiency. The interval chosen depends on whether the objective is to maximize production efficiency of lean meat (weight or time end points) or meat of a constant fat-to-lean ratio (fatness end point).

For either weight-constant (for example, 1,000 pounds) or time-constant (for example, 365 days) end points, faster gaining cattle have greater feed efficiency. These cattle are characterized by: larger mature size; larger carcasses; higher percentage of retail product; a lower percentage of fat trim; and lower marbling levels. Variation in both--feed efficiency and the relationship between feed efficiency and growth rate--is maximized in weight-constant intervals (for example, 500 to 1,000 pounds). Cattle with more rapid growth rates require less feed per unit of gain as measured in weight-constant intervals primarily because they require fewer days and less feed for maintenance in a weight-constant interval.

The ranking of animals for feed efficiency in a time-constant interval is correlated positively with that for feed efficiency in a weight- constant interval. Faster gaining cattle tend to be more efficient in time-constant

evaluations (such as the 140-day postweaning test), but the variation in feed efficiency and the association with growth rate is reduced because larger faster gaining animals that maintain heavier weights throughout the time-constant test require more feed for maintenance. The BIF recommends that feed consumption per unit of gain evaluated in time-constant intervals (feed/gain) be adjusted for differences in maintenance requirement by a iado of test group average metabolib gight (W^07~) to individual metabolic weight (W~ - ), or pen averages (if tests are on pens of animals) as follows:

The midweights (W) used are estimated as &frtac12; (initial weight + final weight). This procedure adjusts feed/gain of heavier than average groups downward, and feed/gain of lighter than average groups upward

assuming that maintenance requirements are proportional to weight (in kilograms) raised to the 0.75 power. Results indicate that BIF-adjusted efficiency yields estimates that closely approximate feed efficiency estimates evaluated in a weight-constant interval. Calculation of BIF-adjusted efficiency based on the ratio of feed over gain in a time-constant interval (feed/gain) is usually easier to compute than feed/gain in weight-constant intervals.

Feed efficiency can also be evaluated to fatness end points (such as a small degree of marbling corresponding to USDA Choice, or to a fat thickness of 0.5). Animals with the greatest propensity to fatten are the most efficient when evaluated to a fatness end point because they require fewer days and less feed for maintenance to reach that point. It appears that feed efficiency to fatness end points is not strongly associated with growth rate or size.