Blood BHB and Cow Performance
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Hyperketonaemia arises when cows, especially in early lactation, experience a severe negative energy balance, causing their liver to produce ketones, like BHB. This condition can occur if ketone production is too high and the cow's body can't use all the ketone bodies for energy. Having elevated BHB in the blood is termed hyperketonaemia. Notably, cows feeding mainly on pasture might have naturally higher blood BHB levels. This doesn't always mean they suffer from subclinical ketosis. In pasture-based systems, unlike housed systems abroad, moderately increased BHB levels aren't a standalone indicator of cow health. Always consider other factors, including blood glucose levels, body condition score, feeding levels, and clinical symptoms. Furthermore, treating cows with moderate hyperketonaemia with MPG drench isn't universally effective in pasture-based systems and might sometimes produce negative results.
A DairyNZ study under 'Pillars of a new dairy system' investigated subclinical ketosis in grazing cows and determined that elevated blood beta-hydroxybutyrate (BHB) concentrations were not always associated with poor reproductive performance under a pasture-based diet.
What is hyperketonaemia and how does it differ from ketosis?
All cows have some degree of negative energy balance during early lactation, where they lose body condition to provide energy for milk production. However, during severe negative energy balance, excessive amounts of body fat are mobilised, which the cow’s liver cannot convert to energy through the usual pathways.
Instead, it produces ketones, such as BHB, which the cow can use in small amounts as an alternative energy source. If ketone production becomes too high, the cow cannot use all the ketone bodies for energy and BHB concentrations increase in the blood. When this occurs, the cow may suffer from the metabolic disease, ketosis.
Figure 1. Blood beta-hydroxybutyrate concentrations in dairy cows fed the same energy as pasture (blue), or pasture plus grain (green). Milk energy output and BCS change were not different between groups (adapted from Roche et al., 2010).
An elevated concentration of BHB in blood is called hyperketonaemia. Severe hyperketonaemia (blood BHB greater than 3.0 mmol/L) is associated with clinical ketosis and requires treatment. It affects about 2 to 15% of cows in the herd; however, early lactating cows are more likely to have moderate hyperketonaemia (blood BHB between 1.2 and 3.0 mmol/L). Moderate hyperketonaemia is increasingly used to diagnose subclinical ketosis, which has been associated with increased uterine infections and reduced milk production and reproductive performance in housed cows.
However, blood BHB is also affected by carbohydrate source in the diet. Blood BHB concentrations are naturally higher in cows fed predominantly pasture because they produce more butyrate in their rumens than those eating a diet high in starch such as grains or total mixed rations (Figure 1). Therefore, in cows grazing pasture, moderate hyperketonaemia is not necessarily associated with subclinical ketosis and reduced cow performance.
Hyperketonaemia is common in pasture-based cows
In a 2010 study, 565 cows from 15 herds in the Waikato and Canterbury regions were tested weekly for blood BHB concentrations (Compton et al., 2015).
Figure 2. The daily % of cows that tested positive for moderate hyperketonaemia (blood BHB between 1.2 and 3.0 mmol/L) in 3 herds (Scott Farm, Waikato; Taranaki Agricultural Research Station (TARS), Taranaki; Lincoln University Dairy Farm (LUDF), Canterbury).
On average, 68% of cows had hyperketonaemia (blood BHB greater than 1.2 mmol/L) at least once during the first 5 weeks post-calving; however, the degree and timing of hyperketonaemia varied significantly between herds (min 12%; max 100%).
A more recent DairyNZ study in 2016 involved testing 1000 cows in 3 herds for blood BHB three times per week for the first 5 weeks post-calving (Phyn et al., 2017). The intensive sampling regime provided a very accurate pattern of moderate hyperketonaemia, which although highly variable across the 3 herds studied was still most prevalent during the first 7-10 days post-calving (Figure 2). On average, 75% of cows in these herds (min 65%; max 82%) had moderate hyperketonaemia at least once during the first 5 weeks post-calving, whereas 12% (min 6%; max 18%) had severe hyperketonaemia.
Hyperketonaemia is not always associated with poor cow performance in pasture-based cows
Many overseas studies conducted in housed cows indicate that moderate hyperketonaemia is associated with reduced reproduction and adverse health outcomes, and is, therefore, indicative of subclinical ketosis.
Figure 3. The 6-week In-Calf rates of cows that tested positive (Pos) for moderate hyperketonaemia (blood BHB between 1.2 and 3.0 mmol/L) at least once during the first 5 weeks of lactation, compared with cows that only ever tested negative (Neg; blood BHB below 1.2 mmol/L) in 3 herds (Scott Farm, Waikato; Taranaki Agricultural Research Station (TARS), Taranaki; Lincoln University Dairy Farm (LUDF), Canterbury).
In pasture-based cows, however, the associations are less clear, with inconsistencies between studies.
In the 2010 study, Compton et al. (2015) reported that cows with blood BHB greater than 1.2 mmol/L within 5 days of calving were 2.5 times more likely to have a uterine infection at week 5 post-calving. Furthermore, cows with a BHB test over 1.2 mmol/L at any stage during the first 5 weeks post-calving had a 7% lower 6-week In-Calf Rate.
In contrast, the 2016 DairyNZ study did not determine any consistent associations between moderate hyperketonaemia during the first 5 weeks post-calving and subsequent reproductive performance. The relationships were strongly herd-specific, because in one herd (LUDF) moderate hyperketonaemia was associated with a lower 6-week In-Calf rate but in the other two herds (Scott and WTARS) the opposite relationship occurred (Figure 3).
This means that moderately elevated BHB concentrations, especially between 1.0 and 2.0 mmol/L, are not suitable as a sole indicator of cow metabolic health status. A combination of variables should be considered including blood NEFA and glucose concentrations, BCS and feeding levels, clinical symptoms, and days post-calving.
Treating cows with moderate hyperketonaemia gives inconsistent results
Monopropylene glycol (MPG) is the active ingredient in energy drenches used to treat clinical ketosis. However, recent US studies indicate that daily MPG drenching of housed cows with moderate hyperketonaemia until they test below 1.2 mmol/L can improve cow metabolic status, survival rates and reproductive performance.
Figure 4. The effect of using monopropylene glycol (MPG) drench to treat cows diagnosed with moderate hyperketonaemia (blood BHB between 1.2 and 3.0 mmol/L) on 6-wk In-Calf rate in 3 herds (Scott Farm, Waikato; Taranaki Agricultural Research Station (TARS), Taranaki; Lincoln University Dairy Farm (LUDF), Canterbury).
In the 2016 DairyNZ study, half of the cows with moderate hyperketonaemia were drenched with MPG to determine if similar positive outcomes can be achieved in a NZ pasture-based system. Although treating hyperketonaemic cows with MPG temporarily reduced blood BHB concentrations, the effect on reproductive performance varied between herds (Figure 4). In one herd (TARS), hyperketonaemic cows treated with MPG had lower 6-wk In-Calf rates than untreated animals, but there were no differences in the other two herds (LUDF and Scott Farm).
These results indicate that, unlike in overseas housed cows, using a MPG drench to treat cows with moderate hyperketonaemia (without other symptoms of metabolic illness) is not a wide-spread strategy to improve performance in pasture-based systems and may, in some cases, have adverse effects.
Further information
Compton et al. 2015. Subclinical ketosis in post-partum dairy cows fed a predominantly pasture-based diet; defining cut-points for diagnosis using concentrations of beta-hydroxybutyrate in blood and determining prevalence. New Zealand Veterinary Journal, 63:241-248.
Phyn et al. 2017. Contract session: An overview of postpartum hyperketonaemia and its association with cow health and performance in pasture-based dairy systems. Proceedings of the New Zealand Society of Animal Production, 77:104-106.
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