Over-conditioned cows, those with excess body condition (BCS > 6.0), present unique challenges, especially as they approach calving. Our pasture-based systems combined with a relatively good summer mean we’re encountering a lot of these cows on farm. While adequate nutrition is crucial for dairy cows, excessive body condition can lead to significant health and production issues. Identifying and managing over-conditioned cows through appropriate mineral supplementation (as just one of several important strategies) can mitigate risks and improve outcomes during the calving period.
Identifying over-conditioned cows
Identifying over-conditioned cows involves regular body condition scoring (BCS) at critical points in the production cycle, including pre-calving, post-calving, mating, and drying off. Over-conditioned cows typically display noticeable fat deposits around the tailhead, over the ribs, and along the spine. Their appearance is smooth, with a lack of visible bone structure, making anatomical points such as the short ribs, hips, and pin bones difficult to examine by feel. Pronounced fat pads in the brisket and around the pelvis further indicate over-conditioning. Regular and accurate BCS assessments are key to managing body condition, particularly in our seasonal calving systems where pasture quality can fluctuate.
Challenges of over-conditioned cows at calving
These cows face several challenges during calving. One major issue is dystocia, or difficult calving, due to excessive body fat reducing pelvic space and increasing calf size, which can complicate deliveries. Moreover, high body fat predisposes these cows to metabolic disorders such as ketosis and fatty liver syndrome because of inefficient mobilisation of stored fat, which is often exacerbated by grazing on lush, high-energy pastures. These cows are also more prone to reduced fertility, experiencing delayed returns to oestrus and lower conception rates, which can disrupt the timing of subsequent calving and affect overall herd fertility. Addressing these challenges requires a comprehensive approach, including proper mineral supplementation.
Management via mineral supplementation
Managing over-conditioned cows effectively involves addressing nutritional imbalances through mineral supplementation, which plays a central role in supporting metabolic health and mitigating risks. Calcium and phosphorus are key for skeletal health and metabolic functions. Calcium is essential for muscle function, including uterine contractions during calving, while phosphorus supports energy metabolism and bone health. Over-conditioned cows, often susceptible to milk fever due to disrupted calcium homeostasis, require adequate dietary calcium to prevent hypocalcaemia. This issue is particularly relevant in New Zealand where high potassium content in pastures can affect calcium balance. Phosphorus supplements, such as dicalcium phosphate, help to balance high potassium levels in the diet, maintaining a recommended Ca:P ratio of approximately 2:1.
Magnesium is another critical mineral, essential for nerve function and calcium regulation. Magnesium deficiency can lead to grass tetany and also exacerbate milk fever, especially in pasture-based systems where high potassium levels inhibit magnesium absorption. It is important to supplement magnesium at maintenance levels in dry and springer cow diets. Levels need to be substantially increased during early lactation to offset the higher metabolic demand. The amount of magnesium should be adjusted based on pasture analysis to ensure cows receive adequate levels.
Selenium and vitamin E play a significant role in antioxidant defence and immune function. Selenium, which is crucial due to its low levels in New Zealand soils, supports reproductive health and reduces the risk of retained placentas and post-calving infections. Vitamin E complements selenium in enhancing immune function and protecting cells from oxidative stress. Selenium, in the form of seleno-methionine, selenium yeast, or a rumen protected form will be preferable as these forms will be stored and utilised more efficiently. Alternatively, if a water-soluble form is the only option, sodium selenate will be the best option. Ensuring adequate Vitamin E through pasture management or supplementation, is essential, especially during dry periods when cows might have limited access to fresh forage.
Important trace minerals
Trace minerals such as zinc, copper, and manganese are involved in enzyme function and reproductive health. Zinc supports skin integrity and hoof health, essential for maintaining mobility and reducing the risk of lameness. Our grass-based diets often lack sufficient trace minerals, impacting metabolic and immune functions. Supplementing with organic trace minerals improves absorption and bioavailability, addressing deficiencies and avoiding imbalances that can impair metabolic functions.
Potassium and sodium are also essential for fluid balance, nerve function, and acid-base equilibrium. However, New Zealand’s pastures typically have high potassium levels, which can interfere with magnesium and calcium absorption, heightening the risk of metabolic disorders. Maintaining an appropriate dietary cation-anion difference (DCAD) is crucial for preventing milk fever. Monitoring potassium levels in pasture and adjusting mineral supplementation accordingly, including the use of sodium bicarbonate or other buffering agents, helps to support metabolic stability and prevent acidosis.
Understanding the unique challenges of our pasture-based systems and addressing them with strategic mineral supplementation, pasture management, and regular BCS assessments will go a long way to reducing the risk of metabolic disorders and improving calving outcomes.
First published in Dairy News, July 2024
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