Forage Hygiene

by Dr Derek Cuddeford, Royal (Dick) School of Veterinary Studies, University of Edinburgh

Forage may be defined as bulky food or fodder like grass or hay for horses and cattle and is the base feed for all herbivores. These bulky feeds are coarse and fibrous and contain roughage, which is fibrous, indigestible material that is found throughout a range of vegetable foodstuffs in varying proportions. Herbivores possess the ability to utilize roughage courtesy of a  symbiotic microflora that inhabits their gut. In contrast,  non-herbivorous monogastrics do not have the specific organisms necessary to degrade roughage effectively so that it only performs a largely physical function rather than acting as a source of nutrients. Under natural conditions, forage is the sole source of energy and nutrients for horses and ponies. Because it is bulky and fibrous it requires a lot of mouth processing before it can be swallowed and of course, this means lots of chewing over an extended period of time. The time required to harvest the forage (foraging!) depends on its availability. Extremes would be hay in a hay rack in the stable compared to limited grass growing over several hectares. A 500 kg horse might consume 12.5kg hay in the stable compared to 50kg of grass in a field!

Thus it should be clear that forage is the key ingredient in any horse diet and that all horses and ponies must be fed forage; the exact quantity depending on the animal’s activity and demand for energy. In all situations, the hygienic status of the forage is its most important characteristic because it can affect the animal’s respiratory health (and yours too!) so nutritional quality will always be of secondary importance. Most horses and ponies kept in the UK will be fed either fresh growing grass or conserved forage that is all grown in the UK. Thus, UK forage will be grown under UK climatic conditions and, as all of us know, rain features rather too often on a day to day basis. This means that the method of harvesting will significantly affect the hygienic status of the crop concerned.

Forage crops that are cut and rapidly removed from the field for artificial dehydration will, in most cases, have the highest hygiene status. It is possible to purchase artificially dried grass and Lucerne (alfalfa) in the UK. The only problem that is likely is that of “dust”. This “dust” can be produced as a result of leaf shatter that occurs through handling and it is most likely with legumes because when they are dried the leaves become very brittle and easily break. Dust particles represent a physical threat to the respiratory system as those of us who have been forced to work in a dusty atmosphere well know. Forage processors are well aware of this risk and thus treat the dried crop with molasses-based products or syrups. These effectively bind the dust particles together thereby removing the threat. However, artificial dehydration together with processing the dried crop mean that whilst these products have the highest hygienic status of all forages they are also the most expensive. In mitigation, they usually also have the highest nutrient values.

Conventional hay making relies on “curing” or drying the forage in situ in a field at the mercy of the elements. One can be lucky (dream on…) and get a spell of sunny, windy weather which results in rapid drying with minimum handling. A rapidly made hay crop is recognizable by its green colour which indicates limited exposure to sunlight and thus limited photo oxidation of the plant pigments. This sort of crop is a joy to handle and smells irresistible-horses love it too! Remember though that it can be ruined if poorly stored. Rapidly made hay will be dust- and mould-free provided the water content has been reduced to 14% or below. Hay made during unsettled weather can be of poor nutritive value but not necessarily of low hygiene status. Provided it is thoroughly dried prior to baling it will not support the growth of fungi and thus no fungal spores will be produced.

What to do with hay that is contaminated with fungal spores? Conventionally hay is soaked in water to prevent any fungal spores getting airborne and presenting a respiratory challenge to the horse. Length of soak varies from 30 minutes to 2-4hours to overnight. It has been established that soaking results in a loss of dry matter in the form of nutrients; the extent of loss being affected by the duration of soaking. If the purpose is solely to “bind” spores then long soaks are unnecessary. However, soaking hay can be a real drag and recently steamers have become available for use with conserved forage. Steaming time is usually between 50 and 90 minutes according to which type of machine you use and it has been shown to effectively reduce “dust” thereby improving hygienic quality. The only significant nutrient losses appear to be of water-soluble carbohydrate so in this respect the steaming process is more effective than soaking which leads to indiscriminate losses of nutrients.

The best alternative to hay making is that of silage making which requires a much shorter period of fine weather because the crop is conserved at a much higher water content and thus the “curing” process is curtailed. Farmers have traditionally made grass silage for their cows that contains between 26 and 30% dry matter. Horse owners have always been reluctant to use such material for their horses because it is acidic and not easily traded/moved/supplied. As a result big bale silage was produced with a dry matter of 35+% that could be delivered to stables. From this has evolved haylage which is a very high dry matter version containing between 55 and ~70% dry matter that is available in small or big bales. Clearly the higher dry matter versions must reside in the field longer to reduce water content thereby increasing vulnerability to indifferent weather. Low dry matter silage is of high hygienic quality and as dry matter content increases there is a greater risk of mould growth if the material is not kept absolutely airtight. Whilst haylages generally are of higher hygienic status than hays there is always the risk of secondary fermentation if the big bale is opened and remains so for a number of days. The use of steam treatment has been tested in this context and shown to significantly reduce contamination of haylage. However this seems to be a somewhat retrograde step as the production of a haylage should result in a better, safer product. In my opinion, the best way forward is to purchase either small bale haylage which is used up rapidly or, to buy big square bale haylage. The latter can be opened from one end and then resealed to prevent the development of moulds, etc. Provided the haylage remains sealed it will be stable and the fermentation will act as a method of preservation of the forage.

In conclusion, the highest hygiene status is achieved in “manufactured” forages whereas farm-saved forages can have a very variable status. High dry matter content is critical for hays and for silages/haylages it is essential that they remain anaerobic/airtight in order to maintain their hygiene status. Steaming or soaking remain the options for improving the “safety” of contaminated forages. Alternatively you could burn it (difficult to do with haylages!) or give it to cattle………….

Author: The Editor

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