Livestock Health on Green Pastures

6/07/2007 12:00:00 AM

Dr Colin Trengove

High protein diets associated with winter and spring pastures can cause sudden death due to nitrate poisoning, red gut and bloat. They may also predispose animals to lameness, foot abscess, illthrift, scouring, tender wool, infertility, mastitis, abortions, stillbirths, weak neonates, retained foetal membranes and disease due to suppressed immunity.

Underlying high blood urea nitrogen (BUN) is expressed in these various forms.  How does this occur and what are the treatment options?

Requirements of a healthy rumen

The soil is the plant’s stomach similar to the role of the rumen in the ruminant. Plants can get carbon, nitrogen and oxygen from the atmosphere by respiration but most of the plants nutrients come from the soil. We cannot get nutrients from steak without it being digested in our stomach and plants cannot get nutrients from organic residues like stubble without them first being decomposed. Micro-organisms are vital to the uptake of minerals in all biological systems and a holistic approach to nutrition requires attention to the well-being of the microflora in the soil, plant and animal.

Nutritive value of green feed

Soil mineral balance is responsible for balanced uptake of nutrients by plants and this has a direct impact on:

• Percentage of wastage in a crop
• Flavour
• Shelf life
• Control of plant nitrates
• Degree of wind damage
• Quality of produce

Albrecht referred to the two main food groups as “grow foods” (nitrogen/protein) and “go foods” (carbon/carbohydrates and fats). Unfortunately, the conventional model for feeding livestock fails to look at the quality of protein, (is it amino acids or urea-rich, non-protein nitrogen?) or the quality of the energy (is it starch only, or does it also include cellulose, hemi-cellulose, organic acids, lipids, sugar pectins, and beta-glucans?). If we control plant nitrate levels then we control plant protein quality. Similarly, if we control plant nutrition and management, we also control waste, flavour and quality.

Photosynthesis requires a balanced supply of nutrients to ensure the production of quality protein, sugars, starch, pectins, cellulose, hemicellulose, fats and hormones by plants. Nutrient interactions, sunlight, moisture, soil structure, pests and diseases are some of the potential impediments to this process. Grazing management can also play a major role in photosynthesis and therefore plant protein and energy levels as well as total dry matter production. The change in feed value with stage of plant growth is readily influenced by the pasture management strategies employed by the grazier.

Specific animal health/ production issues on green feed

It is critical to realise that 60% of the true protein in forage and 100% of the non-protein nitrogen (NPN) degrades into rumen ammonia. Soluble protein as ammonia provides adequate “grow foods” for rumen microbes. A build-up of rumen ammonia without a corresponding balance of soluble energy causes the ammonia to spill through the rumen wall, now becoming Blood Urea Nitrogen (BUN). This is a toxic substance and at high enough levels can kill stock. This is observed when stock are grazing actively growing pasture, crops, lucerne following a dry spell; or during winter or early spring; or when pastures are being irrigated. During these periods, pasture protein can be as high as 43%, but more commonly 30 – 35%. A rapid rise in BUN can lead to sudden death eg nitrate poisoning, red gut. These conditions need to be differentiated from bloat, where sudden death is the result of asphyxiation through gas entrapment in the rumen. This occurs due to froth formation in the rumen associated with the excessive intake of highly soluble plant protein. In contrast, BUN at elevated levels over prolonged time, can cause lameness, foot abscess, illthrift, scouring, tender wool, high infertility rates, udder health issues (eg mastitis), abortions, unthrifty neonates, retained foetal membranes and various diseases due to suppressed immunity. In addition, this toxic “protein” requires that the ruminant remove it from the blood stream utilizing a series of enzymes synthesized in the liver and kidneys. The result is excreted as urea or uric acid in the urine. Unfortunately, the labour to remove waste is stolen energy that would otherwise be used to put on weight or produce milk - a “double whammy”!

Excess protein negates pasture energy (similar to the “Atkin’s diet”). For every 1 unit of surplus protein in the diet, there is a loss of 0.18 MJ of ME. An example is a winter pasture with 35% crude protein: the net energy of the pasture available to promote growth in lambs or milk production in ewes is reduced from 11MJ to 7MJ/kg DM as a result of rumen organisms using the energy to degrade the surplus protein. The provision of barley straw or oats in this diet would help to balance the excess protein and stimulate rumen fermentation.

Strategies to optimise animal health & production

All stock need a balance of protein and energy in their daily diet and this varies with their physiological state, age and growth rate. It is recommended that feed analyses be performed to establish the particular feed values for all feeds being considered for a ration, rather than rely on estimates. Knowing the protein and energy content of stored feeds and matching them to the protein and energy needs of livestock enables balance to be achieved in the protein and energy in their daily diet.

A number of prevention strategies are available to promote good energy:protein balance. These include leaving the grasses and weeds in the legume pasture rather than using herbicides to remove them. This is beneficial provided grass seeds are not a problem and the pasture is primarily for grazing. The purpose is to provide greater variety in the diet and reduce the overall fermentable nitrogen intake.

Another option is to allow grazing of more native or ‘rougher’ pastures at least every 3 days or providing low protein pasture hay or straw in the paddock ad lib to adult stock. In order to promote weight gain in lambs however, it is preferable to leave them in the legume pasture and feed ad lib oats to balance the excess nitrogen in the diet.

A one ml (or cc) dose of Vitamin A, D & E under the skin in sheep or 3 ml dose in cattle can act as a vital preventative treatment as non-protein nitrogen such as nitrates and nitrites in excess in lush green pasture commonly prevents the formation of vitamin A in the liver. This is an essential vitamin for immune strength in all animals. These injections need to be repeated every 6-8 weeks while grazing lush green feed or with prolonged dry feed intake.

Specially formulated foliar sprays can also be used to make winter pastures safe for grazing by modifying the energy:protein balance. These sprays also reduce the risk of bloat by converting soluble plant protein into rumen bypass protein and so minimising the risk of froth forming in the rumen or excessive ammonia levels causing a high BUN.

Ultimately, comprehensive soil and plant testing with experienced interpretation and recommendations is the most sustainable strategy. Acting on this information will promote balanced nutrition of legume dominant pastures and combined with good management will prevent production losses from occurring.

This is an excerpt from a more comprehensive paper available on request from APAL