FIGURING ADJUSTED WEIGHTS OF LAMBS
by Martha McGrath
In raising Coopworths, we are encouraged to select breeding animals on performance in order to continue genetic improvement. One of the important performance areas is lamb growth. The growth of the lamb from birth to weaning is largely influenced by the mothering and milking ability of the dam, and weights taken at about 60 days are often called the pre-weaning or weaning weight (ww).
The period of growth after weaning, is largely influenced by the lambs own genetic ability to gain weight on the feed available. Weights taken at about 120 days are called post weaning weights (pww). Together, these two figures can be very useful in selecting replacement animals. If you only look at the raw data, though, and not at the number of lambs the ewe raised or the age of the dam, you may end up selecting for single births, or against longevity of the ewes.
One way to "level the playing field" is to weigh the lambs, and then "adjust" the weight. Adjusted weights allow fair comparisons within the management group, and accurate selections of superior animals. Another advantage to adjusting the weight is that the entire lamb crop can be weighed at one time, and adjusted for age of the lamb, rather than weighing each lamb when it is 60 days old.
Using mathematic formulas, weights are adjusted for age and sex of lamb, age of dam, and type of birth and rearing (single, twin, triplet, etc.). All actual weights of lambs are adjusted to a standard age, and are also adjusted to the equivalent of a ewe lamb raised as a single from a mature ewe, three to six years of age. Some of the computer programs for flock management will do these calculations for you, but they can also be done by hand.
This is explained on the web site Calculating Adjusted Preweaning and Postweaning Weights
Doing the Math
There are two steps when doing the calculations by hand. First, the actual weights of the lambs are adjusted to the same age for each lamb.
The formula for this is;
For example, a lamb is weighed at 52 days of age. His actual weight was 48 lbs, and his birth weight was 9.5 lbs. To adjust his weight to 60 days, we subtract the birth weight from the actual weight; 48 - 9.5 = 38.5, divided by 52 (age when weighed) equals .74 (which is also his Average Daily Gain, or ADG, to this point), then multiply by adjustment age of 60, equals 44.4 lbs., and add his birth weight back on (44.4 + 9.5) = 53.9 lbs. adjusted weight for 60 days.
Then the age adjusted weight is multiplied by the "adjustment factor" for sex of lamb, age of dam, and type of birth and rearing.
Multiplicative Adjustment Factors(For Adjusting Lamb Preweaning and Weaning Weights to a Common Age of Dam, Lamb Sex, and Lamb Type of Birth/Rearing)
After adjusting weight to same age for each lamb, multiply the "age adjusted weight" by the factor in the table to arrive at the "adjusted weight" for the lamb. Lambs born in litters greater than three should use the triplet (3/3) adjustment factor.
In the above example, if the lamb was a ram, twin raised as a twin, out of a 2 year old ewe, the multiplier would be 1.17 ( 53.9 lbs X 1.17 = 63.06 lbs adjusted 60 day weight).
I know that this seems like a lot of work, but it really does allow you to compare rate of gain in your lambs. It would be worthwhile in selecting replacement animals from a group of lambs that have already been culled for correct breed type.
All of the tables and mathematical formulas, including a table for tabulating the age in days for a lamb, are in the breeding chapter of the SID Sheep Production Handbook. This is a HUGE loose-leaf book, about 1400 pages, just updated, with chapters on Sheep Breeding, Forages, Handling, Health, Management, Marketing, Nutrition, Predator Control, Quality Assurance, Reproduction, Sheep Care, Wool, and Contact Lists for State Extension Personnel, State Extension Veterinarians and State Animal Health Officers.
Jim and Martha McGrath
178 Lough Rd.
Franklin, WV, 26807