Reproductive Momentum in Beef Cattle

Rancher profitability is driven by reproductive efficiency and market value of cattle sold. It is not unknown to producers that reproductive success is an important factor in their operation. However, in the beef cow-calf world it may not be routinely tracked as how to measure reproductive efficiency. The most common metric discussed is the percent of open cows with a low percent open being perceived as “good”. However, I would argue that the % of open cows is not really all that important without knowing how long the breeding season is and what the conception rate of the first and second cycles are. Before we can really measure or compare reproductive efficiency between herds we need to realize some of the biological limitations of breeding cattle:

  • Fertile bulls and fertile cows will achieve a conception rate or 60-70% on one breeding (~30% of matings will not lead to a fertilized oocyte or the embryo will die within 14 days and the female will resume cycling).
  • Lactating cows in good BCS will require a 50-80 day Post Partum Interval (PPI- time from calving to resuming normal fertile cycle).
  • First calf heifers in good BCS and ≥85% of mature body weight at calving will require an 80-100 day PPI.
  • If we expect a 365 day (12 month) calving cycle and gestation is 283 days that means the longest the PPI can be is 82 days for a cow.
  • First calf heifer PPI is longer than that of a cows by ~20 days so heifer breeding season should start 20 days prior to cow breeding season.

A few major factors can influence the PPI of cattle with the main effect being limitation of energy and protein in the diet prior to or after calving and cattle in poor BCS (<2.5/5) prior to and after calving. By having limited energy/protein in the ration or having animals in poor BCS at calving the PPI is delayed which then leads to fewer females that are cycling at the time of breeding. This makes for a scenario where the cows may be fertile but just not cycling yet and even with fertile bulls they can’t breed cows if they are not yet cycling.

An example of reproductive momentum can be seen when comparing two herds with similar percentage of open cows but one can have positive reproductive momentum, as with Herd A in Figure 1, and be much more profitable than another herd which has negative reproductive momentum, as Herd B. In the figure Herd A depicts a group of cows that have 90% of the cows cycling by the end of the first 21 day breeding period and a bull conception rate of 60-70%. Herd B depicts a herd with 50% of the cows cycling at the end of the first 21 day period and a bull conception rate of 60-70%. With fewer females cycling there is hence fewer animals that become pregnant in the first cycle and even second cycle as shown by lower conception rates. Although the overall % open is not dramatically different between the two herds there is a definite performance difference as herds with positive reproductive momentum will have more calves born during the first two cycles and hence have more calves and heavier calves as there are more older calves.

figure 1 percent pregnant by cycle

If left unchecked and no corrective action is put in place negative reproductive momentum created by few animals cycling in the first two cycles will slowly get worse with time as more animals get pushed into the second and third cycle and eventually end up with more open cows or more in a fourth cycle if cows exposed longer. It is difficult and takes time to be able to correct a scenario of negative reproductive momentum as it is difficult to dramatically change the PPI of the cow even with dramatic changes to energy/protein supplementation and better BCS in the herd. If the majority of the cows are pushed into the second and third cycles they are unlikely to be brought back into the first cycle by the large majority. As well if selecting heifer replacements out of the herd it may be difficult to get heifers that are old enough and mature prior to breeding to ensure positive reproductive momentum in the heifers as more of the heifers will be young due to the dams calving in later cycles and potentially not be mature enough and cycling themselves at breeding. However, creating momentum within a herd starts with the heifer and replacements as the producer can consider buying in animals that are older and more mature, they can ensure supplementation of heifers to ensure they are 55-65% of mature body weight at breeding, or implementation of an estrus synchronization program can also ensure creating positive reproductive momentum.

Your herds reproductive efficiency can be assessed with either calving data and grouping calves into 21 day periods or can be done at pregnancy diagnosis and staging of the pregnancies into 21 day periods. By staging pregnancies and assessing reproductive momentum at pregnancy diagnosis allows early detection as well as early implementation of any intervention strategies. As well it allows for an optimal time to assess BCS of cows, evaluation of forages and resource allotment, and discuss disease prevention strategies coming into calving season. Timing of pregnancy diagnosis needs to be able to facilitate the staging of pregnancies and for most veterinarians this is best done about 40 days after bulls are pulled on a roughly 65 day breeding season so that pregnancies are ~40-100 days. Not only can animals be grouped into 21 day periods but sorting the data by age can also be critical in identifying which age management group may not be performing well. This is most ideally done by sorting into 21 day periods for the heifers, second calvers, and mature cows.

figure 2 total calving distribution

figure 3 calving distribution by age group

In Figure 2 the overall calving distribution fits within our model of overall good reproductive efficiency. However, if we break he groups down as in the Figure 3 you can see that the second calf heifers are possibly being mis-managed as reproductive efficiency has slumped and likely a high percentage of heifers were not cycling in the first cycle which could pose a problem later on in the cow herd if the necessary management changes are not made as many females will likely leave early from being open or late calving if the same strategy persists.

Either working with your herd veterinarian on creating calving distribution data at pregnancy diagnosis or reviewing calving data can be a powerful tool for analyzing your herds reproductive efficiency.

Sources:

1. Reproductive Systems for North American Beef Cattle Herds

Larson, Robert L. et al.

Veterinary Clinics: Food Animal Practice , Volume 32 , Issue 2 , 249 – 266

2. Evaluating Information Obtained from Diagnosis of Pregnancy Status of Beef Herds

Larson, Robert L. et al.

Veterinary Clinics: Food Animal Practice , Volume 32 , Issue 2 , 319 – 334

 

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