Hidden dangers in pasture

Spring pasture may look ideal, but its high sugar and low fibre content can trigger ruminal acidosis, especially in high-producing cows or cows who have recently transitioned from a conserved or grain-based diet.

The rapid fermentation of the soluble carbohydrates in lush grass produces lactic acid in the rumen, lowering the rumen pH and disrupting the balance of bacteria in the gut.

This can lead to reduced appetite, milk drop, very low butterfat percentage, an inversion in the ratio between fat and protein, loose manure, lameness (via laminitis), and in severe cases, systemic illness.

It is essential to manage the risk of acidosis to ensure optimal herd health and welfare and to react quickly if you identify any of the symptoms described above.

Strategies to manage acidosis risk include the use of dietary buffers like sodium bicarbonate, magnesium oxide, calcium carbonate, various yeast products, bentonite clay, sodium sesquicarbonate and, in extreme risk cases, Virginiamycin.

Offering access to dry hay or straw to increase the amount of effective fibre in the diet can be beneficial. It can also prove economic if it maintains rumen health and component levels in the milk and any unconsumed pasture is harvested and conserved for summer feeding.

Introducing cows carefully onto high-risk pastures is an important strategy to maintain herd health in the spring.

Sub-acute rumen acidosis (SARA) can result in significant immune system suppression which can lead to an increased risk of significant diseases like salmonella and mastitis.

Keeping the rumen function stable during spring can help avoid production dips and reduce the risk of the secondary health issues like laminitis and mastitis.

There are several other very significant herd health risks that can be lurking in those rapidly growing spring pastures.

Milk fever (low blood calcium) typically affects older, high-producing cows in the first few days post-calving. The risk increases in spring due to lush pasture being low in calcium and high in potassium, which interferes with calcium absorption.

Grass tetany (low blood magnesium) also is a high risk on rapidly growing spring pastures, especially during cool wet spells.

Because magnesium is not stored in the body, a daily intake is required, and an acute shortage can lead to signs like twitching (even convulsing), staggering, aggression and even sudden death.

It is always a fair thing to assume that affected animals during a high-risk period have grass tetany until proven otherwise, and supplementing with magnesium and avoiding excitement and stress is a good management option.

Be aware that nervous ketosis can also present in a similar way but tends to be individual animals affected where grass tetany can affect whole groups of animals. It can be prevented by water or feed medication, and pasture dusting can also be of use.

Nitrate/nitrite poisoning is a very severe metabolic disease that can occur when cows are grazed on rapidly growing spring pasture, especially when it has been pushed along by the application of nitrogen fertiliser.

Pasture that has been fertilised with urea or ammonium based products should not be grazed during the highest risk period after fertiliser application (five to 14 days) especially if there are other factors that might increase the risk of toxicity like cool, cloudy or frosty weather that can affect nitrate breakdown.

If there is a risk of nitrate poisoning, do not allow hungry cows to access risky pasture.

Animals affected with nitrate/nitrite poisoning are often just found dead but clinically affected animals will be staggering, breathing hard and twitching, and the classic symptom is dark chocolate-brown blood.

Some other good strategies for managing risk include testing pasture samples for nitrate levels before feeding, not feeding high risk paddocks first thing in the morning and provide roughage so cows are not hungry before introduction to pasture.

If you suspect nitrate poisoning, remove the cattle immediately, and contact your veterinarian.

Spring is full of opportunity, but a single misstep in pasture or health management can create setbacks that echo into summer.

Dr Rob Bonanno is a ProDairy lead veterinarian based in Gippsland.