Managing cows for high fertility
The key indicators of reproductive performance are body condition score for cows and liveweight for heifers. Birth weight can also have a direct bearing on the number of live calves born. To increase conception rates it is important to join females on a rising plane of nutrition. Condition score (or fat score) is a useful way to observe a change in the nutrition of cows, however the absolute score numbers and the relationships with fertility can differ between cattle breeds.
Guidelines for the minimum and maximum mating values for British breed heifers and cows are:
• Heifers – joining liveweight of 280kg, condition score 3 at 15 months of age
• Mature cows – condition score 2.5
• First calf cows – condition score 3.0
• Condition score 3.5
If the breeding herd is outside these recommended guidelines:
• Increase or decrease pasture available and/or pasture quality before mating to ensure condition score of cows or liveweight of weaner heifers remains within the recommended limits.
• Wean calves before cow condition score falls to 2.5.
• Supplementary feed a high quality diet to cows when condition score is below 2.0.
• Supplementary feed heifers (perhaps including the use of a rumen non-degradable protein source if pasture quality is low) to ensure they reach target weight.
• Assess animal health status, particularly for internal parasites (worms and fluke), and treat if there is a problem.
• Cull weaner heifers that fail to reach target liveweight or get in calf in two mating cycles.
EBVs also have a role in breeding for herd fertility:
• Selecting sires with a high EBV for scrotal circumference results in earlier onset of puberty.
• Days to calving EBV will also help in decreasing the interval between calving and conception.
• Calving difficulties are reduced by selecting sires with a low score EBV for gestation length, and birth weight.
• If the phenotype of the herd, especially heifers, is high yielding (ie Euro type) they tend to cycle later.
Artificial insemination as a mating option
If artificial insemination (AI) is used, the correct procedures are required to ensure high conception and calving rates. Results from an AI program are optimised by managing:
• Cow/heifer selection. All females in an AI program must be on a rising plane of nutrition. As a guide to best results, maiden heifers are under less stress than mature cows, but mature cows are easier to get in calf than first time calvers.
• Heat detection. The accurate detection of standing heat and the resulting timing of insemination are critical to the success of an AI program. Clear identification of individual animals, record keeping, visual observation for signs of heat and where necessary the use of heat detection aids are all critical factors in an AI program.
• Synchronisation of oestrus. This saves time and labour but it will only work on cattle that are actively cycling. There are two basic types of synchronisation:
– Prostaglandins – a hormone (administered by injection) that shortens the reproductive cycle;
– Progesterone implants – placed under the skin behind the ear or in the vagina. The implants are usually left in place for 11 days. This postpones the onset of oestrus until two days after removal.
• Care when handling semen. Semen is a live biological product that must be handled correctly. It is susceptible to temperature shock and exposure to sunlight, water, blood and poor hygiene.
• Insemination technique. The retro/vaginal technique of insemination gives the best results.
If an AI program is being considered, carefully assess the benefits and costs of the options. Calculate the costs of the various options in terms of $ per calf born to enable a comparison of mating systems. Attending a special AI training course is also recommended to gain the knowledge and skills to obtain the best results.
Cows with low fertility require more than two matings to conceive, making a 365-day calving interval difficult to maintain. This decreases the number of calves produced by the herd each year. When the number of calves per breeding female is below the herd genetic potential, the throughput of weaners and animals meeting market specifications is also reduced and enterprise profitability is likely to be below potential. The impacts will be greater when nutrition is marginal, and for females calving for the first time.
Focus on improving fertility by feeding to ensure target liveweight for heifers and condition score for cows or risk maintaining high culling rates of infertile cows.
High culling rates increase the number of replacement heifers required to maintain the herd breeding structure. This practice results in a higher proportion of quality pasture being used for maintenance of the breeding herd because of the amount of pasture needed to grow females up to first mating.
Purchase of in-calf heifers increases the risk of introducing infectious diseases and if these heifers are from an unknown genetic background, they may not be compatible with strategic business and breeding goals. Purchased heifers are also unlikely to match the planned calving period and could have higher rates of dystocia than mature cows.
What to measure and when
• Conception rates from natural mating or when an AI program is implemented;
• Condition score of cows at regular intervals according to the seasonal conditions – monitor after weaning of the last calves, from six weeks before calving, and then from calving to mating;
• Liveweight of weaner heifers every six weeks until mating.
The stock manager should also observe the breeding herd for evidence of female activity (cycling) prior to the commencement of mating.