Substantial amounts of a herd manager’s time and energy are focused on improving reproductive performance. Despite an emphasis on breeding cows, it has been documented that the conception rate of the modern dairy cow is measurably lower than it was 40 to 50 years ago.
In 35-plus years of dairy practice, I learned long ago that you cannot always fix a herd reproduction problem from the back end of the cow. There are a tremendous number of factors happening at the front end of the cow that can impact the herd’s reproductive performance.
A large study conducted by the University of Guelph involving 1,341 cows in 18 commercial herds revealed that 19.5% of the cows were diagnosed as non-cycling or anestrus at 60 days in milk (DIM). The range of anestrous cows on a herd-specific basis was large: 5% to 44%. The study identified associations between specific animal risk factors and the incidence of anestrous cows (Table 1).
It explained the risk factors using a term called an odds ratio. For example, if a cow delivered twins, she was 2.3 times more likely to be anovulatory at 60 DIM, compared to a cow that had only one calf. Cows were more likely to be anovulatory if they had experienced dystocia, twin births, retained placenta or displaced abomasum. The study also established cows that had subclinical ketosis were at an increased risk of being anestrus.
A link was identified between being anestrus and the first 305-day milk projection. It was opposite to what most dairy farmers expect; as the cow’s first 305-day milk projection decreased below the herd average, the cow’s risk of being anestrus increased. Interestingly, if the cow’s first 305-day milk projection was at or above the herd average, there was no increase in the risk of anovulation compared to the herd average.
Reproductive performance of the anestrous cows was evaluated in the herds in the study (Table 2).
In herds using heat detection for first inseminations, the first artificial insemination (A.I.) occurred eight days later in anovulatory cows, compared to the cycling cows. The anovulatory cows had a lower first-service conception rate in both the heat detection herds and the herds using a timed A.I. program. The interval from calving to conception was 30 days longer if the cow was diagnosed as being anovulatory, meaning 156 days versus 126 days for the cycling cows.
There will always be individual high-producing cows that have difficulty conceiving but, on average, breeding performance is better in high-producing cows when compared to lower-performing cows. Many cows and heifers in the bottom half of the herd never make it to the breeding period due to metabolic and infectious diseases, death or culling.
To identify whether metabolic diseases and anovulatory cows are a factor in your herd, we can evaluate some basic indicators. The 21-day pregnancy risk (PR), sometimes referred to as the 21-day preg rate, is commonly used in many herds as a measure of reproduction. This performance indicator combines the effect of both the heat detection rate and the conception rate on overall herd reproduction. It allows us to measure the outcome of a huge number of factors that ultimately result in a pregnancy. The current industry PR average is around 15%. The goal is to have your herd PR at or above 20%, and some herds presently have PRs in the 28% to 30% range.
When the pregnancy rate is evaluated, opportunities for improvements in heat detection rates, conception rates or both can be identified. The conception rate, and to a lesser extent the heat detection rate, is related to nutrition and transition factors in cows. Heat detection success can be altered by management strategies to improve the number of heats seen and cows served. Altering conception rate is more of a challenge. Nutritional deficiencies, often caused by health issues, do have an impact on the reproductive performance of the cow.
So how does the front end of the cow affect reproduction? It’s all about dry matter intake (DMI).
Let’s review some nutrition basics. All animals use energy for four bodily functions:
- Maintenance: This is the energy required to stay alive – breathing, heart beating, staying warm, walking and numerous other bodily functions, such as tissue repair.
- Growth: This is the energy required to grow from a calf to a fully mature adult cow.
- Performance: This is the energy used to increase body reserves such as fat and muscle in a fattening steer or in a cow to produce milk.
- Reproduction: This is the energy used to become pregnant and then to grow the fetus.
The energy requirements for maintenance are always met first. After that, the cow uses the next available energy to either grow or perform. The energy demand for growth and performance is about equal. We all understand that a first-lactation animal does not milk as much as a mature cow because she is still growing.
Once energy requirements are met for maintenance, growth and performance, any excess energy can be used for reproduction, cycling and becoming pregnant. Becoming pregnant is a “luxury” item in terms of an animal’s nutrient usage. Once a pregnancy is established, the energy and nutrient requirement of the growing fetus will no longer be a luxury item and will be prioritized by the body ahead of the growth and performance requirements. For example, an underage heifer that becomes pregnant will have a reduced growth rate and a smaller mature bodyweight than her potential.
So why is this important to our 21-day preg rate?
If we evaluate the energy intake and allocation from a ration perspective, we can now predict whether the diet will, or will not, be able to support growth, milk production and reproduction. As seen in Figure 1, the Cornell NCPS model predicts that the metabolizable energy (ME) intake for the defined milk production will be slightly less than required and some body reserves will be used, a typical response at this stage of lactation.
Reproductive success is dependent on the health and nutrient balance of the cow, both pre- and post-calving. Even small decreases in DMI can result in an anovulatory cow. This can start in the dry period, with the risk continuing through the post-calving period, and may extend into the breeding period.
Decreased DMI has a tremendous effect on the energy balance of the cow. We can model the effects of changes in DMI using the NDS ration formulation tool to evaluate the same diet and estimate changes in the body condition score (BCS).
Five different scenarios are displayed in Table 3.
Diet A is the same diet described above. Diet B is the same diet fed to a cow that is 30 DIM. It shows adequate energy for the cow if she consumes that amount of dry matter and is going to decrease her body reserves, as modeled with a small decrease in BCS of -0.3 over the next 30 days.
Diets C, D and E is the same TMR diet but fed to a first-lactation heifer that is 22 DIM. Diet C estimates the dry matter that we expect her to eat while producing 34 litres of milk, with an estimated BCS loss of -0.5 over the next 30 days.
Diet D shows the energy available with a small, often undetectable 9% decrease in DMI. The decrease in DMI will only support 31.5 litres of milk with a BCS change of -0.5 over 30 days. To produce 34 litres of milk, she will have to use more of her body reserves, equaling a BCS loss of -0.59 over the next 30 days.
Diet E shows the energy available with a 17% decrease in DMI. This decrease in DMI will only support 28.2 litres of milk using the same body reserve expenditure, a BCS change of -0.5 BCS over 30 days. To produce 34 litres of milk, she will have to use a tremendous amount of her body reserves, a BCS loss of -0.84 over the next 30 days.
This is approximately 45 kilograms of bodyweight, an amount that is likely to result in anestrus. If this first-lactation cow doesn’t use any body condition to support her milk production, the diet can only support 21 litres of milk or 63% of the formulated diet.
The model predicts dramatic effects from even modest decreases in DMI and exposes why any compromise to a lactating cow’s appetite, whatever the cause, can affect her net energy balance. If the energy deficit is too large, the cow cannot recover sufficiently and will be at an increased risk of being anestrus at or beyond 60 DIM.
Dry and transition cow nutrition and management issues are correlated with herd reproduction. Solving front-end issues will improve your bottom line directly with fewer fresh cow issues and indirectly with improved herd reproductive performance. A successful reproductive program starts with optimal nutrition in the dry cow period and into lactation.
