Potassium, Sodium and Magnesium in Grass and Animal Nutrition: A Question of Balance

Keywords: balances, nutrient budgets, K flows, animal nutrition, nutrient interactions, farming systems, manures and slurries, metabolic disorders.

Potassium (K) is a key nutrient with widespread impact and functions within the complexity of grassland based livestock production. It has received relatively little research attention in recent years, but much is known about its behaviour, content and specific roles in each of the component parts of a livestock farm. Requirements for optimised supplies to forage crops are well known, as is K distribution and availability in soils. The interactions of K with other elements which are of importance to the nutritional well-being of livestock, especially lactating ruminants, have also been well described, and whilst the effects can be confounded by interactions with other factors, good advice is available to minimise risk. Nevertheless, it is timely to consider K in a more systematic way within the whole production cycle so that any potential imbalances in supply per se, or in relation to other nutritional requirements for optimised plant and animal performance, can be identified at an early stage. Knowledge of soil, field and farm K balances is one means of providing guidance on actions that may need to be taken from both tactical and strategic perspectives. Examples are provided of effects at various stages of the production cycle and of opportunities to improve balances.

S C Jarvis, formerly at: The Institute of Grassland and Environmental Research, North Wyke Research Station, Okehampton, Devon EX20 2SB, UK.
G E J Fisher, Novus Europe S.A./N.V., Congleton, Cheshire ST7 3BS, UK.

17 pages, 8 figures, 5 tables, 9 references.


Potassium Requirements for Grass Cut for Silage - A Review

Keywords: grassland fertilizers, potassium, nitrogen-potassium interactions, manures.

The management of productive grass swards is primarily driven by the appropriate use of relatively large dressings of nitrogen, both from fertilizers and manures. The basis for current fertilizer recommendations was laid by a large number of experiments carried out in the UK between the 1940s and 1960s. These experiments are identified and their findings reported. Both costs and outputs must be assessed. Increasingly aspects relating to water quality and pollution hazards to the environment must be considered. High yields of conserved grass inevitably remove large quantities of potassium from the field when harvested. This potassium must be replaced if soil fertility is not to decline. The percentage of potassium in the herbage DM (especially late in the previous season) is a good indicator of the essential need for potassium fertilisation. Values below 2.0% potassium are indicative of serious depletion which should receive attention. UK experiments suggest that optimum yields can be obtained when herbage potassium concentration remains above 3.0% DM at the end of the previous season.

The critically important relationship between the requirements for nitrogen and potassium by high-yielding grass swards is discussed and the extensive experimental evidence is reviewed.

Emeritus Professor Gordon Hemingway, Glasgow University Veterinary School, Bearsden, Glasgow G61 1QH, UK.

16 pages, 12 tables, 22 references.