The element sodium (Na) is essential for all animals. It functions as a major extracellular cation and is essential for maintaining osmotic pressure in the body as well as water regulation. It is essential for transporting nutrients around the body and removing waste from cells. It is also involved in nerve impulse transmission and muscle function.
Sodium is lost in milk, urine, faeces and sweat – this loss increases in high temperatures and during exercise. For ruminant animals, sodium has an added importance in saliva, which is produced in vast amounts daily to help to buffer the acid produced from fermentation in the rumen. Production of the saliva is stimulated by the chewing of feed, particularly the process of rumination.
A dairy cow may produce up to 150 litres of saliva per day, depending on the diet it receives. This is a lot when compared to a monogastric animal of a similar size. Roughage/long fibre has the effect of increasing rumination activity, which in turn increases the amount of saliva that’s produced.
Saliva contains sodium bicarbonate, which has a buffering effect and counteracts the acidic by-products of carbohydrate (fibre, starch and sugar) digestion. It also helps to keep the rumen pH at a level that is optimal for the microorganisms living there to function. For this reason ruminant animals have a higher requirement for sodium, and a lack of it can depress rumen functionality.
Sodium is also linked to total feed intake, as ruminants tend to prefer salty foods over more bland foods. Water intake can also be affected by sodium, and adding sodium to the diet can help to stimulate that.
Sodium in forage
Sodium is found in forage because it is taken up by plants as they grow. However, sodium can leach from soils and the amount of sodium in the soil can vary depending on farm location. A coastal farm will have more sodium in the soil due to higher levels in rain, whereas an inland farm is more likely to be sodium deficient. Areas with high rainfall may be more prone to sodium leaching from soils. Some particular forage species are well known to not take up sodium from the soil even if present in adequate amounts. These include maize, lucerne and kikuyu.
DairyNZ has put the recommended sodium level for lactating dairy cows at 0.2 percent of total dry matter. It also suggests that the sodium level of pasture ranges from 0.03 percent to 0.6 percent (Facts and Figures 2017). This means that at times pasture is deficient in sodium compared to animal requirements.
Of the last 43 pasture samples submitted by NRM for which sodium level was requested for analysis, the average sodium content was 0.25 percent; however, about 50 percent of the samples were below the recommended level of 0.2 percent for milking cows. Some concentrate feeds such as palm kernel and grain are also low in sodium, which makes supplementation even more important when these feeds are included in the diet.
Sodium deficiency in ruminants can cause reduced appetite, reduced water intake, weight loss, reduced milk production and pica (an appetite for substances that are largely non-nutritive). Milk is 87 percent water, so a cow that is not drinking enough water will have a significant drop in milk production. With severe deficiency, animals may become dehydrated, uncoordinated when moving, shiver, and even die from arrhythmia of the heart.
Sodium is not stored in the body in significant amounts, which means that it’s best to supplement it little and often. The most common form of supplementation for sodium is sodium chloride, more commonly known as salt. Sodium chloride contains 40 percent sodium. Salt can be applied to forage as a fertiliser, dusted on pasture or included in water, although these methods can often have high wastage (particularly dusting), be time consuming and unreliable.
Offering salt to animals in blocks or free-access rock salt in troughs can work really well and it allows the animals to regulate how much sodium they need. One downfall of salt blocks is that animals may not be able to consume enough to meet requirements due to time constraints.
Salt can also be included in compound feed and blends, which is a good way of ensuring each animal receives adequate sodium every day. The salt can help to increase the intake of feeds by increasing palatability. Ruminant diets that contain forages low in sodium (such as maize silage) will need to be balanced with higher levels of supplemented sodium each day and several routes of supplementation may be best.
While salt can help to increase the palatability of feeds, too much of a good thing is not always best and there is a point where too much salt can cause water/feed to become unpalatable, so caution is required.
Article supplied by Stacey Cosnett, Nutritionist.
SODIUM LEVELS IN COMMON FEEDS
|Feed||Sodium level (%DM)|
|Soya bean hulls||0.01|
|Palm kernel expeller||0.02|
|Dried distillers grain||0.30|
The table shows that many supplementary feeds are low in sodium compared to requirements, which for lactating dairy cows is 0.2 percent of total dry matter.