RELATIONSHIP OF POSTPARTUM INTERVAL IN MATURE BEEF-COWS WITH NUTRITIONAL ENVIRONMENT, BIOLOGICAL TYPE AND SERUM IGF-1 CONCENTRATIONS

Mature cows of nine breeds were fed (four cows per level within breed) at either 0.55, 0.71, 0.88 or 1.05 MJ metabolizable energy (ME) per k g initial body weight0.75 per day during non-lactational periods for a n average of 4 years. During lactation, energy allowances were increas ed by 0.17 MJ ME per kg body weight0.75 per day. Breeds represented fo ur biological types: moderate genetic potential for milk yield and gro wth (Angus, Hereford, Red Poll), moderate milk and high growth (Charol ais, Limousin), high milk and moderate growth (Braunvieh, Pinzgauer) a nd high milk and growth (Gelbvieh, Simmental). The objective was to te st the effects of biological type and nutritional environment on lengt h of post-partum interval as determined from serum progesterone levels . The potential of serum insulin-like growth factor-1 (IGF-1) level at 3 weeks post partum for predicting the length of the post-partum part um interval was also evaluated. Non-parametric statistical analyses ap propriate for right-censored survival data were used to test effects o n post-partum interval because collection of serum was terminated prio r to ovulation in some cows. Energy availability X biological type eff ects on post-partum interval were significant. Increased energy availa bility tended to decrease post-partum interval in all biological types with high potential growth types exhibiting the greatest positive res ponse to increased food. Post-partum interval for types with high grow th potential was shorter within food level when associated with high r ather than moderate potential for milk yield. Serum IGF-1 was related to post-partum interval at lower energy availability but appeared to h ave limited usefulness for identifying cows with extended intervals. I nherent direct and maternal production potential interact with availab le food resources to influence the post-partum interval of mature beef cows.