MANAGEMENT AND ECONOMICS OF EXTENDED CALVING INTERVALS WITH USE OF BOVINE SOMATOTROPIN

The objective of this study was to investigate a reproductive manageme nt strategy that optimized profitability per cow per day of productive life when bovine somatotropin (bST) was used. Nine herds were involve d with a portion of animals randomly assigned to one of four treatment s: breeding at first oestrus after 60 days in milk (DIM, 13.2-month ca lving 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 assigne d to the study served as controls. All treatment animals received bST commencing at 63 days after parturition. Effects of bST supplementatio n with delayed pregnancy and without pregnancy were determined using t he 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 m anaged for a 13.2-month calving interval. Milk yield response to bST i ncreased as lactation progressed, leading to a significant increase in persistency. When days of productive life were standardized to 4.35 y rs 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 ca lving interval compared with a 13.2-month calving interval. Profitabil ity was increased by approximately US$0.75 per day per animal for anim als on a 16.5-month calving interval as compared to a 13.2-month calvi ng interval. On a herd basis, through two years of study, extended cal ving interval resulted to fewer calvings, lower incidence of postpartu m metabolic diseases, lower veterinary costs, less culling with fewer replacements needed, and an overall improvement in herd life, animal w ell-being and dairy farm profitability. (C) 1997 Elsevier Science B.V.