The current unavailability of monensin bolus, a valuable tool for managing the risk of ketosis, may pose challenges for some farmers.
Propionic acid production in the rumen
Monensin achieves its effect by increasing propionic acid production in the rumen. This was previously believed to be a consequence of a greater effect of monensin on Gram-positive bacteria than on Gram-negative bacteria. It has recently been shown that this is not the case, but rather that feed-carbohydrate-degrading bacteria, including Gram-negatives, have a greater adaptability to monensin than others (Guo et al, 2022). Propionic acid is converted in the liver of ruminants to glucose. No other volatile fatty acids are converted to glucose. Increasing the production of glucose and its concentration in the blood reduces excess fat mobilisation, thus reducing the incidence of ketosis. These effects explain why monensin reduces the risk of ketosis, displaced abomasum, and mastitis. In one study, it has also been found to increase milk yield (Mammi et al, 2021).
Managing body condition score
For cows in the latter part of their lactation transitioning to winter rations without monensin bolus, particular care should be taken to ensure they reach drying off in the right body condition. For thin cows, an extra 1.5 kg of cereals or concentrates daily for three months will improve body condition score by half a point (Kennedy, 2007). For fat cows, concentrates need to be reduced, and it may be necessary to limit their access to the best forage. During the dry period, limiting energy intake is important for cows in good body condition score (BCS). Douglas et al (2007) fed a ration containing 6.28 MJ net energy for lactation/kg from drying off ad libitum, or restricted to 80% of the net-energy-for-lactation requirement, to cows with a mean BCS of 2.81–3.11 over the dry period. They found that the cows fed the ration ad libitum showed a marked decrease in dry matter intake (DMI) before calving, and had higher plasma free-fatty-acid and beta-hydroxybutyrate concentrations, and lower postpartum DMI, than cows fed the restricted ration.
More recently, Richards et al (2020) compared dry cows fed a low-energy (5.6 MJ net energy for lactation/kg) diet, a high-energy (6.7 MJ/kg) diet, or the low-energy diet for the first half of the dry period and the high-energy diet for the second half of it, all ad libitum. They found less ketogenesis in the cows fed the low-energy diet than in cows fed the high-energy diet or those fed the low-energy diet than the high-energy diet.
Assessing risk in individual cows
If a cow reaches the latter dry period in the wrong body condition score, she should be managed individually. Both fat cows and thin cows are candidates for individual treatment, although body condition score is not a strong indicator of risk. A portable testing system for free or non-esterified fatty acids (NEFA) is now available, enabling better identification of cows that will benefit from intervention. In cows between 14 and two days from calving, an NEFA concentration of ≥0.3 mEq/l indicates an elevated risk of displaced abomasum, clinical ketosis, retained placenta, and metritis (Nydam et al, 2009).
Even at calving, it is not too late to use NEFA concentration to identify the cows most at risk. Melendez et al (2009) found that for a cow with an NEFA concentration greater than 1.2 mEq/l, the risk of mastitis was increased by a factor of 2.25. For a cow with a concentration greater than 1.6 mEq/l, it was increased by a factor of 3.03.
Anti-inflammatory drugs
Inflammation plays an important role in the development of ketosis by increasing insulin resistance. Thus, anti-inflammatory drugs could be useful in reducing the risk. Koracevic et al (2019) investigated the use of ketoprofen, given at 3 mg/kg liveweight daily for three days from calving. They found that TNF-α, IL-1α and IFN-γ were all lower in ketoprofen-treated cows than in controls; these cows also had lower NEFA and BHB concentrations in blood.
Strategic use of glucose precursors
Routinely pump-drenching cows at risk of various postparturient disorders, particularly hypocalcaemia, has become commonplace. The author investigated the strategic use of glucose precursors by pump-drenching a proprietary product containing them, Selekt Glycerol Plus, a couple of years ago. The intervention was made on the day of calving and again between 72 and 96 hours after calving. The effect of this intervention is similar to that of a monensin bolus, insofar as both reduce fat mobilisation by raising blood-glucose concentration. There were non-significant improvements in days to first insemination, and body condition score, in the cows given the glucose precursors. But the most dramatic effect was a highly significant increase in milk yield over the first 80 days of the lactation.
Not having monensin bolus will undoubtedly create some difficulties for dairy farmers. But there are ways for vets to help them limit or even avoid the problems that may result from its unavailability.