Nutrition-Reproduction Interactions in the Third Trimester Cow
by Rachel Endecott
MSU Extension Beef Cattle Specialist


New energy requirements increase 14 to 15 percent and protein requirements increase 26 to 27 percent from mid-gestation to late gestation. This increase in nutrient requirements during this stage of gestation is due to the rapidly increasing growth of the fetus.  The fetus accomplishes only 25 percent of its growth during the first two-thirds of gestation.  The remaining 75 percent of fetal growth is attained during the last 90 days of gestation.  In fact, the developing conceptus (fetus plus membranes and fluids) requires 100 to 150 pounds of weight gain during the last third of gestation!  If the cow is unable to meet the nutrient demands of the growing conceptus from her diet, maternal reserves of energy and protein will be used and the cow will lose body condition.  Because of the weight gain of the fetus, this loss in body condition may not result in a change in body weight on the scale.  

Both energy and protein nutrition during the prepartum period can impact postpartum reproductive performance.  Cows fed pre-calving diets inadequate in either protein or energy exhibit longer days from calving to first estrus and lower pregnancy rates compared to cows fed diets with adequate amounts of protein and energy.   

Both body weight and body condition scores (1-9 scale, 1 = emaciated, 9 = obese) have been used as simple indicators of nutrient status and possible rebreeding performance after calving.  Research indicates that a target body condition score greater than or equal to 5 before calving will help to ensure nutrient stores adequate for postpartum reproductive performance.  Of course, this is not to say that all cows in body condition score 5 or greater will be pregnant in the fall, nor is it to say that cows thinner than condition score 5 will not get bred.  Postpartum influences on and changes in body weight and body condition score also play important roles in rebreeding efficiency.
Rapid fetal growth during the third trimester of gestation dramatically increases cow nutrient requirements.  Pre-calving nutrition can have far-reaching impacts on future reproductive performance and subsequent year’s calf crop.  Monitoring nutrient status throughout late gestation and early lactation allows for implementation of management changes to achieve ranch goals for cow performance.

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