Management and economics of extended calving intervals with use of bovine somatotropin

The objective of this study was to investigate a reproductive management strategy that optimized profitability per cow per day of productive life when bovine somatotropin (bST) was used. Nine herds were involved with a portion of animals randomly assigned to one of four treatments: breeding at first oestrus after 60 days in milk (DIM, 13.2-month calving interval (CI)); breeding at first oestrus after 150 days in milk (16.5-month calving interval); first lactation animals never re-bred; and second lactation animals never re-bred. All herdmates not assigned to the study served as controls. All treatment animals received bST commencing at 63 days after parturition. Effects of bST supplementation with delayed pregnancy and without pregnancy were determined using the Test Day Model(C) (TDM). Results indicated that with bST use a 16.5-month calving interval increased (P < 0.05) milk yield over animals managed for a 13.2-month calving interval. Milk yield response to bST increased as lactation progressed, leading to a significant increase in persistency. When days of productive life were standardized to 4.35 yrs as the typical life cycle and a calving interval of 16.5 months was used, milk was increased 4468 kg for the animals with the extended calving interval compared with a 13.2-month calving interval. Profitability was increased by approximately US$0.75 per day per animal for animals on a 16.5-month calving interval as compared to a 13.2-month calving interval. On a herd basis, through two years of study, extended calving interval resulted to fewer calvings, lower incidence of postpartum metabolic diseases, lower veterinary costs, less culling with fewer replacements needed, and an overall improvement in herd life, animal well-being and dairy farm profitability. (C) 1997 Elsevier Science B.V.