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dairy cows

Nutritional solutions help tackle heat stress this summer

The negative impact of heat stress on dairy cows is often underestimated, yet the cumulative effects of reduced yield, poorer reproductive performance and compromised animal health can significantly cut herd profitability. In the United States, for example, it’s claimed that heat stress costs the dairy industry around US$900 million/year.

“The temperature above which you’d normally begin to see a reduction in performance is around 25°C, but when the relative humidity (amount of moisture in the air) is high, this can be as low as 20°C in strong radiant sunshine,” states Trident technical manager Dr Michael Marsden. “And it’s an issue that is very relevant to UK conditions as our ‘muggy’ climate makes it much harder for cows to lose heat.

“The huge amounts of metabolic heat generated by milk production mean that most high yielding dairy cows are likely to suffer from heat stress at some point during the summer. The result is that as intakes fall, the risk of acidosis increases as cows selectively reduce their intake of forage and milk yields drop rapidly (table 1).”

Table 1 – Dairy cow performance change as temperatures rise (Source: adapted from NRC, 1981)

Temperature

(°C)

Maintenance req.

(% of req at 20°C)

DMI

(kg/day)

Milk yield

(kg/cow/day)

Water intake

(litres/cow/day)

20

100

18.2

27

82

25

104

17.7

25

89

30

111

16.9

23

95

35

120

16.7

18

144

40

132

10.2

12

127

One of the most effective ways to minimise the negative impacts of heat stress is through manipulation of the ration. And according to Dr Marsden, the first step is to cut structural forage fibre content to the minimum to reduce both ration volume and fibre levels.

“But it’s important to compensate by feeding more digestible fibre, using sugar beet feed or soya hulls to replace some of the starchy concentrates in the ration,” he explains. “This helps maintain rumen function, milk fat production and reduces the risk of acidosis, whilst also creating a compact ration with a very high nutrient density.

“Fat supplements and rumen-bypass proteins should then be introduced to further support yields,” he adds.

Fat supplements like Golden Flake, for example, contain approximately three times as much energy as cereals, but have a much lower heat of digestion and zero risk of acidosis. Used at up to 3% of dry matter intake – but no more than 0.75kg/day – they can lift energy supply even if rumen activity is falling.

Protein supply must also be adjusted to minimise excess rumen degradable protein (RDP), the disposal of which increases energy requirements and produces more heat. Rebalance overall protein supply by increasing the level of digestible rumen undegraded protein (DUP) up to 40% of the total protein in the diet using rumen-protected protein feeds like SoyPass and ProtoTec.

“The inclusion of moist feeds and molasses-based liquid feeds will improve ration palatability and reduce sorting, helping keep intakes as high and as consistent as possible,” Dr Marsden states. “In addition, the provision of clean drinking water is obviously vital, and the cooler the better – cows will be reluctant to walk further than 250m to drink if troughs are sited poorly.

“And intakes can be further improved by increasing the amount of feed available during the cooler part of the day. Offering 60-70% of the daily feed intake between 8pm and 8am has been shown to successfully increase milk production during hot weather, even if it means allowing cows access to a ration indoors during the daytime, then grazing at night.”

The ration will also need to supply extra minerals. Sodium levels in urine rise by 80% during heat stress, whilst potassium is excreted in sweat and phosphorous uptake can be compromised.

Dietary guidelines are given in table 2, and extra trace minerals like cobalt, zinc, copper and manganese will assist with any additional lameness, mastitis and fertility problems. Finally, yeasts and rumen buffers like Vistacell and AcidBuf should be considered to further reduce the risk of acidosis and maintain rumen function and efficiency.

Table 2 – Macro mineral supplementation during heat stress

% of ration dry matter

Normal

Heat stress

Phosphorus

0.41

0.51

Magnesium

0.25

0.35 - 0.40

Potassium

1.00

1.3 – 1.6

Sodium

0.18

0.4 – 0.6

Chloride

0.25

0.25

“The key is to have a suitable heat stress ration formulated ready to use, based on existing feeding to allow for an easy transition,” concludes Dr Marsden. “Table 3 shows an example of the types of changes needed, with feeding switched over as soon as the cows begin to show signs of heat stress.

“The cost of the ration might have risen by 63p/cow/day in this example, but that’s a 20% saving compared to the 3 litre/cow drop in daily milk yields typical for this level of heat stress, plus the knock on effects on health and fertility. Just remember that if cows are outside grazing, it’s the actual temperature in direct sunshine that matters, not the shade temperature measured by the weather stations,” he adds. 

Table 3 – Ration adaptation to help alleviate heat stress in dairy cows (based on 40 litres/cow/day, 5% reduction in dry matter intake)

Ration (kg FW/head/day)

Traditional

grazing-plus-buffer

Adapted for heat stress

Grazed grass

45.0

35.0

Parlour concentrate

2.0

2.0

Buffer ration:

Grass silage

8.0

7.0

Maize silage

8.0

7.0

Straw

0.5

0.5

British beet molasses

1.0

2.0

Confectionery blend

2.5

3.0

Soya hulls

1.0

2.0

Rapemeal

1.5

-

Prototec (heat treated rapeseed meal)

1.0

Rumen-bypass protein (e.g. SoyPass)

0.25

1.0

Protected fat (e.g. Golden Flake)

0.25

0.50

Vitamin / mineral premix

0.175

0.175

Yeast (e.g. Vistacell %)

-

0.05

Energy density (MJ ME/kg DM)

12.1

12.6

Crude protein (% DM)

18

17.7

Ration cost (£/head/day) 1

3.25

3.88

1 Prices quoted correct at time of going to press, 29t bulk loads delivered on-farm within 50 miles of source, prices will vary with load sizes and distance from source.

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