THE COMPONENTS OF THE CALVING INTERVAL
The calving interval is defined as the period between one calving and the next and it is an overall measure of fertility status. There are several distinct stages however and these need to be identified before we can discuss the factors affecting fertility.
Take calving as the starting point. After calving, the cow must overcome any uterine infections. She must then begin her ovarian cycles, to come on heat and ovulate every 21 days, and she must cycle regularly without any abnormalities. In a herd using artificial insemination she has to be seen to be bulling so that she may be presented for AI, and this is known as heat detection. After service the egg must be fertilised and then the developing embryo must attach itself or implant onto the wall of the uterus. These two processes of fertilisation and implantation are together known as conception. Good conception rates, that is avoiding large numbers of repeat services, are very important in fertility management. Once the foetus has become established in the uterus, there is still the possibility of early foetal death or, at a later stage, abortion, defined as the premature expulsion of the calf. A successful calving should result in the production of a live calf, so the final hurdle is the elimination of stillbirths.
Components of a successful calving interval are:
• elimination of uterine infection
• commencement of ovarian activity and establishment of regular oestrous cycles
• visual observation of oestrus, i.e. heat detection
• fertilisation
• embryo recognition leading to implantation of the placenta onto the uterine wall
• minimising embryo deaths
• avoiding abortions
• production of a live calf
Each of these factors will be dealt with in detail later in the chapter, but to enable a better understanding of the processes involved, some of the physical and hormonal changes associated with the oestrous cycle are described.