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Dairy Corner: Rain can compromise good quality hay

Feeding and management options to consider

The other week I was driving in southern Manitoba and saw a group of dairy cattle grazing tall lush pasture. The grass was up to their bellies and I thought: “When was the last time that our pastures looked so good?” The next field had cut hay in wide swaths that looked rained-on and bleached. My heart sank.

While rain is great for pasture production, the rainy weather in the last couple of months has made harvesting good-quality dairy hay nearly impossible. While rained-on hay like I saw may not replace high-quality hay in a well-balanced dairy diet, we should acknowledge such damage. Effective actions can be taken to minimize its negative effects on dairy performance.

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It’s no secret rain damage on hay starts immediately after the first raindrops hit cut field grass and every minute afterward until swaths can be dried to under 15 per cent moisture and harvested into bales. For example, University of Wisconsin demonstrated that field plots of alfalfa hay lost 22 per cent of their dry matter content after a 2.5 cm (1 inch) rainfall fell on it after one day, while cured hay without any rain damage lost about six per cent dry matter. The researchers also found rained-on hay lost 44 per cent of its dry matter content after persistent rains lasted a few more days.

Impact on dairy production

In addition to dry matter field losses, here are three more fundamental problems connected with feeding rained-on hay to dairy cows:

1. Loss of dairy nutrients. Similar to field losses of rained-on hay, there is a leaching of water-soluble carbohydrates, which are a good source of energy for lactating cattle as well as a significant loss of soluble protein stored in its leaves. Not only is rained-on hay less nutritious, it tends to fill dairy cattle up quickly with indigestible fibre, which is fermented slowly by their rumen microbes. As a result, what energy and protein left over in damaged hay often cannot support even the lowest dietary nutrient requirements of lactating without some type of nutrient supplements.

2. Mould growth. If hay is harvested at moistures above 20-25 per cent, it becomes an environment for dangerous mould growth. I have seen many times in my travels, the leaves and stems of tough hay covered with white mould. Some practical field trials with white and other hay moulds have proven that cattle will eat most hay with about one to two per cent storage mould without any problem. However, cattle will likely reject feed hay with over 10 per cent mould contamination or at best it slow down consumption if they have no other forage choice.

Even in unpalatable mouldy bales not rejected by dairy cattle, there is a fair amount of essential nutrients (in addition to field nutrient losses) that are naturally consumed by mould organisms. It is estimated an established mould in damp hay reduces its energy content up to 15-20 per cent. That’s energy that could have been eaten otherwise in a mould-free state by lactating dairy cows and help contribute to milk production.

3. Caramelized hay. Mould growth produces a lot of heat, which not only denatures essential nutrients such as forage protein, but reduces overall nutrient digestibility of the hay. This process is known as the “Browning reaction.” It occurs when internal temperatures of poorly cured hay reaches 60 C (140 F), which binds heated forages’ carbohydrates and proteins together, and thus renders a significant amount of dietary protein unavailable to dairy cattle. In order to measure the extent of protein damage by caramelization; producers should take rained-on forage samples and request an ADIN (acid detergent insoluble nitrogen) analysis at a feed-testing laboratory.

Feeding options

So what can dairy producers do to minimize the adverse production effects of rained-on hay? Here are a few suggestions:

  • Consider using hay preservatives such as propionic acid sprayed on to cut-down forages. While propionic acid will not stop field losses, it can reduce “dry-down time” and hay can be harvested at a higher moisture.
  • Reduce the amount of poor-quality forages fed to dairy cows and complement with more silage or better quality hay. Avoid feeding extra grain to compensate for poor-quality forages.
  • Consider using “forage extenders” such as beet pulp to add dietary energy and digestible fibre.
  • Save it for the non-lactating cows. This hay is more fibrous and contains less energy and may have a place in dry cow diets later on.
  • Some producers have switched to round-baled hay silage, where cut forage is harvested at 50-60 per cent moisture compared to 18 per cent in cured dry hay. Studies on baled hay silage show significant reductions in comparative field and nutrient losses. Like cured hay, it works well with other dairy feeds such as corn silage to achieve high milk performance in lactating dairy cows.
  • Invest in an outside forage source, if available. It may be economic to buy better-quality dry hay to support milk production than to rely on nutrient-compromised hay.

About the author


Peter Vitti is an independent livestock nutritionist and consultant based in Winnipeg. To reach him call 204-254-7497 or by email at [email protected]



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