|Technical Guide for SMEs in the Dairy Industry (CDI, 1999, 74 p.)|
|PART 1 - DAIRY FARMING|
|CHAPTER II: REPRODUCTION OF CATTLE|
The study on the reproduction of cattle will be focused on the female.
The indigenous breeds reach puberty at 26 months on average, whereas it takes only 10 to 11 months for the exotic breeds. The variation factor is the feed, although a female is considered to have reached puberty when its weight is 2/3 of the weight of the adult.
The age at which cows first calve is 3 years 9 months for local breeds and 24 months for exotic breeds. The interval between calvings, an important factor in fertility and productivity, is 473 days for local breeds and 360 days for exotic breeds. This interval depends on several factors, namely:
- Return to heat: this takes an average of 45 days for milk cows, and 60 to 90 days for suckler cows;
- Average number of inseminations per impregnation: 1.25 for heifers and 2.24 for cows;
- Interval between calving and first insemination: 56 days ± 28.
Consequently, these are factors that depend on farming practice and herd maintenance, and therefore the post-calving period.
During her genitally active period, the female experiences a number of structural modifications which occur in the same order at periodic intervals: these constitute the sexual cycle or oestrous cycle. These activities are only interrupted by gestation or certain gynaecological problems.
The length of the cycle is 21 days for cows and 20 days for heifers.
The cycle is subdivided into 3 stages:
- pro-oestrus, which corresponds to the stage of follicle growth,
- and post-oestrus. The follicle, which has ovulated, turns into a corpus luteum with a stage of formation, functioning and dehiscence.
The most important element for the farmer is oestrus. This is the only visible element. It is marked by the arrival of the females heat and, above all, acceptance of coupling. It is therefore the favourable period for artificial insemination or covering, and subsequently impregnation.
It is essential for this strategic period to be identified.
There are several identification methods:
- Direct observation by the farmer, which can be continuous for 24h/24h or discontinuous for 20 minutes an hour.
- Indirect observation with the help of markers placed either in the male (ink system) or the female (paste placed on the sacrum, which is rubbed off during coupling).
Direct observation produces better results than indirect observation, which can be misleading due to unproductive couplings.
The duration of the heat is very short for native breeds: 10 to 13 hours, whilst it is 12 to 24 hours for exotic breeds.
Signs of a cow being on heat comprise:
- A major sign is acceptance by the cow in question of coupling with one of her fellow creatures,
- secondary signs such as:
· emission of cervical mucus
· congestion of the vulva
· tonus of the uterine horn
· extreme mobility on the part of the cow