Guidelines for reducing the spread of calving
Commercial beef producers who are striving for maximum efficiency should be aiming for 95% of cows to calve in a 9-week period. This procedure explores the date of removal of bulls or the duration of mating to reduce the spread of calving and to sustain the annual calving date(s).
The ideal calving distribution should consist of 65% of calves dropped in the first three weeks, followed by 20% and 10% in the two subsequent three-week periods respectively. With this level of reproductive efficiency you will produce an even line of calves of roughly the same age making them easier to manage and market. To achieve this, guidelines for the length of mating period are:
• Maximum – bulls run with cows for 60 days;
• Minimum – bulls run with cows for 45 days. When the bulls are not run with the cows for long enough, calving percentage is decreased.
These recommendations allow all females to have a minimum of two oestrus cycles during mating. The limits can also be applied strictly for heifers.
When the bulls are run with the cows for too long, the period of calving is also too long and results in:
• A reduced ability to maintain a 365-day calving interval;
• An unnecessary increase in the use of high quality pasture by the breeding herd and a reduced amount available for growing stock;
• Difficulty in obtaining the required mating weight at the nominated mating date for heifers that conceive later in the mating period, an increase in heifer culling and difficulty in maintaining the desired herd age structure;
• An increase in the cost of supervision of calving from heifers (and cows with twins);
• A spread in calf weights that will delay weaning date until the lightest calves reach target weight; and
• A large spread in weaning weights that can lead to problems such as calf marking and associated husbandry procedures when seasonal conditions are less favourable.
To achieve the recommended mating periods, apply these rules:
• Manage the mating so that 80% of cows conceive by the end of the second oestrus (heat) cycle.
• Remove bulls when the maximum period is reached, providing that the assessment of calving patterns indicates that a 60-day joining is sufficient for a satisfactory pregnancy rate.
• Adopt a strategy to realign the herd’s calving pattern when more than 20% of the cows are conceiving on the third cycle.
• Do not use the same bulls in succeeding seasons if the mating histogram shows more than 40% of cows are not conceiving at the first mating.
As a guide, if a calving date histogram of the herd in the previous year shows that more than 20% of cows conceived in the third cycle, the reproductive capacity and age structure of the herd will need to be realigned over several years to avoid economic penalties for the enterprise.
The recommended minimum and maximum number of mating days is based on the assumption that more than 80% of cows conceived by the second oestrus cycle in the previous year’s mating. If less than 80% of cows conceived by the end of the second oestrus cycle in the previous year, initiate a program to realign the herd’s calving pattern. In this case the mating period may need to be extended beyond the recommended maximum limit to ensure satisfactory throughput in the short-term.
Changing calving time
The calving pattern of cows is very repeatable and it is difficult to bring the date of calving for late-calving cows forward by more than a three-week cycle per year without risking an unacceptably high rate of empty cows.
Where it is necessary to move the calving date forward by more than six weeks (over two years) it is best to leave the current cow herd in its established calving pattern and join the replacement heifers to calve at the desired calving time and period. As the number of young breeders (calving at the desired time) builds up, the older, out-of-sequence cows can be culled. After about five years, the herd will be calving at the required time over two heat cycles with minimal risk of low conception rates.
What to measure and when
• Record the date on which each cow calved to develop the ‘calving histogram’.
• Record the number of days that bulls are with cows.
Ensure that the investment of labour to record information on calving patterns and difficulties benefits the breeding operation.
