Heifers are particularly sensitive to large changes in diet composition at or near breeding time. These effects are most noticeable with AI bred heifers.
Much of the work Dr. George Perry of South Dakota State University has presented is focused on drylot vs. pasture systems for developing heifers pre-breeding and studying the impact of dietary change on animal behavior, animal gain and AI pregnancy rates.
If heifers are developed in a drylot and then turned out to pasture, several behavioral and performance issues emerge.
One study indicated that heifers developed in a drylot took 70% more steps on the first day they were turned out to pasture than heifers developed on pasture (17,000 steps vs. 10,000 steps).
Additional research was shown that indicated drylot developed heifers can lose as much as 3 pounds per day the first week they are turned out to pasture.
This increase in energy expenditure and reduced intake can have impacts on AI pregnancy rates in heifers.
This response was seen in a study which looked at turning drylot developed heifers out to pasture immediately after AI breeding and providing them with either 5 pounds of dried distillers grains or no supplement.
Weight gain and AI pregnancy rate was checked 42 days after AI. The supplemented heifers had gained 32 pounds in 42 days and had a 76% pregnancy rate to AI. The unsupplemented heifers lost 5 pounds in 42 days and had a 61% pregnancy rate to AI.
Some of these impacts affect natural service breeding systems also. Perry cited a study reported in 1999, concluding that a decrease in feed intake from 120% of maintenance to 40% of maintenance resulted in a loss of over 50 pounds during a two-week period and 60% of heifers stopped ovulating within 13 to 15 days of diet change.
While this may sound extreme, the studies mentioned above show that dry matter intake and heifer growth rate can dramatically decrease if drylot or supplemented heifers are abruptly shifted to pasture only diets.
The main take home message is to avoid abrupt diet changes immediately after AI or before turning out bulls for natural service breeding. If heifers are shifted from drylot development to pasture, supplement additional energy to overcome some of these negative effects. Heifers transition from hay-based rations to grazing, ease that shift through continued supplementation to avoid drastic reductions in energy intake to improve AI or early breeding success.
For additional information on heifer development, contact schmitze@missouri.edu or call the Pettis County Extension Center at 660-827-0591.
