Could Summer Heat Stress Still Be Affecting Your Cows?

For cows affected during early and mid-lactation, heat stress can have implications beyond the period of hot weather itself.

Intestinal damage associated with leaky gut can create a significant immune challenge, while liver function may be impaired and fat accumulation increased. Foot health also needs consideration, as cows may spend longer standing during hot weather in an attempt to cool themselves, increasing the risk of lameness and hoof fat pad loss.

Heat stress can also affect mammary cell physiology. Elevated stress hormones, reduced dry matter intake and reduced blood flow can make effective tissue repair more difficult.

Nutritionally, good levels of dietary zinc can support intestinal health, while choline and methionine can support liver function. Protected zinc can be used to support foot health, while methionine and adequate metabolisable protein can help support mammary tissue repair.

Where fertility has already been affected by heat stress, there is little that can be done to reverse that initial impact. The priority now is to maximise fertility in the cows that still need to get in calf.

Delayed fertility can have consequences beyond conception itself. Cows getting in calf later than planned may also reach peak milk later, affecting the timing of future production.

Omega-3 can play a role in supporting fertility, including egg development and the progression of good dominant follicles.

For cows exposed to heat stress during late gestation or the dry period, the effects can continue beyond calving.

Reduced DMI, impaired udder redevelopment and repair, reduced foetal growth, intestinal challenges, reduced liver function and increased risks around displaced abomasum, post-calving mastitis and ketosis are all important considerations. Heat stress during this period can also affect milk production in the subsequent lactation.

Research adapted from Fabris et al. (2019) shows a clear difference in milk production between cows cooled throughout the dry period and those exposed to heat stress, with around a 10 to 12 kg difference in peak milk.

The research also shows that heat stress during late gestation can reduce mammary cell proliferation before calving, affecting mammary secretory capacity in the following lactation.

Image description: Effect of heat stress during early,late, and the entire dry period on dairy cattle. (Adapted from Fabris et al., 2019. JDS:102:5647- Univ of Florida)

Premier Nutrition’s TMS data indicated that as THI increases from a winter average of around 50 to summer scores in excess of 75, dry cows without cooling can experience a fall in rumen fill of more than 0.77.

This reduction in rumen fill can have a significant impact on subsequent performance, potentially resulting in a fall in peak milk production of at least 1.5kg/day, and more than 4kg/day where rumen fill falls to 2 or below.

For dry cows approaching calving, the priority should therefore be to maximise DMI and rumen fill. Good-quality forage with no spoilage should form the basis of this approach, helping cows enter the transition period in the best possible position.

Image description: A chart comparing average rumen fill scores in dry and fresh cows during January, February, July and August 2020, alongside average temperature humidity index. It shows rumen fill declining as summer heat and humidity increase.

Where facilities allow, cooling dry cows in the dry cow pens remains the ideal approach. Where that is not possible, nutrition should focus on supporting the areas most likely to have been challenged.

Alongside maximising DMI and rumen fill, protected zinc can support intestinal integrity, while choline and methionine can support liver function. UFAC’s Omega 3 Supplement (Ruminant) can support fertility, including egg development and the progression of dominant follicles post-calving, while adequate metabolisable protein can support maternal protein reserves and mammary development.

Where additional metabolisable protein is required, Enviropro can be used within the ration.

The important point is not to assume that the impact of heat stress ends with the immediate period of hot weather. Reviewing DMI, rumen fill, fertility and the nutritional support provided to affected cows can help prepare them more effectively for the dry period, transition and next lactation.