Adjusting milk yield

The total milk production of a ewe is very much dependent on her lactation number. Milk yield increases from first lactation to second and to third. Milk yield generally peaks at the 3^rd or the 4^th lactation corresponding to full maturity of the ewe. Without adjusting the level of production for this non-genetic effect, it would be impossible to compare ewes of different age within a flock. Selection of replacement ewes would suffer and be slower than it should be because only lambs from 3^rd or 4^th lactation ewes would be kept as replacement.

Two methods of selection are offered to producers:

1. Calculation of EPD’s (Expected Progeny Differences). An EPD is an estimate of genetic value of an animal calculated from performance information from all relatives of that individual and is the most accurate estimate of genetic value possible. An EPD calculation for a prospective replacement (or ram) ewe would use the milk yields (or any other trait) of the individual’s dam, maternal grand-dam, paternal grand-dam, full-sisters, half-sisters, and any other female relatives with milk production records. Replacement animals are chosen among the ones having the highest EPDs. EPD calculations require relatively sophisticated statistical techniques and fairly large computing resources. When several flocks are genetically linked (generally by the use of common rams such as in a Sire Referencing Scheme) the comparison and ranking of animals between flocks become possible. EPDs permit the optimization of a selection program.
2. Adjustment of milk yield. Not every producer, however, belongs to a selection program but all need to select replacement ewes or rams from their best producing dams. Genetically, the best producing dam could be a 1st lactation ewe but her full potential would not be expressed because of a lack of maturity. Multiplicative coefficients should be used to adjust the milk yield. Thomas (1966) in the proceedings of the 2^nd Great Lakes Dairy Sheep Symposium (all Proceedings can be found at the following URL http://www.uwex.edu/ces/animalscience/sheep/) published multiplicative factors to adjust the milk yield to a mature ewe 4 to 7 years of age.

The adjustment factors presented by Thomas are, however, old and calculated with data collected from Spanish breeds that might not be representative of our dairy sheep of predominantly East Friesian breeding, with their larger frame and earlier maturity. It is, however, better to use them than using nothing at all.

In the Spooner Ag. Research Station flock, composed of 350 milking ewes of East Friesians and Lacaune breeding, it appears that there is a larger difference of production between 1st lactation (1 year of age) and 3^rd lactation (3 year of age) and a slightly lower difference between 2^nd and 3^rd. Moreover the ewes seem to reach full maturity and a maximum production at 3 years of age. The following adjustment factors might be more correct to use:

Total milk yield and curve of lactation of the Spooner flock are shown in the following graphs. Ewes were milked twice a day, starting 3 days after lambing. It is worth noting that 1st lactation ewes, although producing at a much lower level and for approximately 30 days less than older ewes, have a better persistency, meaning that the level of production decreases less rapidly on a monthly basis than older ewes.