Controlled Breeding Systems for Dairy Cows

Abstract

AbstractThe economic consequence of low heat detection rates is the main reason cattle reproduction research programmes continue to focus on developing practical controlled breeding systems for dairy cows. Three approaches can be taken to control the oestrous cycle in cattle: (i) Use of the luteolytic agent prostaglandin F2α alone or one of its potent analogues, (ii) Cycle regulation using short-term progestagen treatments and (Hi) Prior follicle wave synchrony followed by induced luteolysis. Administration of prostaglandin F2α (PGF2α) to cows after about Day 5 of the oestrous cycle causes immediate regression of the CL with progesterone concentrations declining rapidly to basal concentrations within 24 hours. Because PGF2α is only effective in animals between Days 5 and 17 of the oestrous cycle up to 40% of any group of randomly cyclic animals will not respond to a single administration. A number of PGF2α administration regimens have been developed for dairy cows. A two-injection regimen, with an interval of 9-13 days between successive administrations, elicits a higher response, as all animals possess a susceptible CL the time of the second injection. In general, cyclic heifers respond with good precision of heat onset to two PGF2α injections 11 days apart. Fixed-time AI at 72 and 96 hours after the second injection can result in acceptable pregnancy rates. In post partum dairy cows the slower turnover of follicle waves, contribute to the greater variation in oestrus onset, resulting in more variable and generally lower pregnancy rates to fixed-time AI at 72 and 96 hours. Consequently, intensive oestrous detection and AI on the basis of observed oestrus is more appropriate for lactating cows following PGF2α administration. It is generally accepted that fertility of the cyclic heifer after PGF2α treatment is not impaired or may even be higher when compared with untreated control animals inseminated at a natural oestrus. However, in lactating cows, conception rate following PGF2α treatment have been frequently lower (about l0%) than in cows bred following a natural heat. Recently, aprotocol to synchronise follicular development using GnRH and to induce luteal regression using PGF2α was developed In this protocol (Ovsynch), GnRH is administered to cows at random stages of their cycle 7 days before luteolysis is induced with PGF2α; a second GnRH injection at 36-48 hours later induces ovulation. All cows are inseminated once at 16-20 hours after the second GnRH injection. Ovsynch system is not recommended for use in heifers. The development of progestogen delivery devices, such as the PRID, CIDR and Crestar has facilitated the use ofexogenous progestogens for oestrous cycle synchronisation in cattle. Oestradiol is administered at the start of treatment to (i) shorten the life span of the corpus luteum (CL) and (ii) to induce the emergence a new follicle wave. Generally, a high proportion (up to 85%) of the cows that exhibit oestrus after removal of the progestogen do so between 36 and 60hours ofremoval. This allows for either one (54-56 hours) or two fixed-time AIs (48 & 72 hours) after progestogen withdrawal. Alternatively, cows can be observed and inseminated once at a detected oestrus. While fixed-time AI of all treated cows gives 100% submission rate conception rates tend to be lower than when AI is on the basis of observed heat. However, the overall number of cows becoming pregnant (submission x conception rate) is frequently the same, reflecting submission and conception rate differences for the two AI options. Choice of AI option will depend the herdsman 's ability to accurately detect heat, semen costs and labour availability.