Concentrate Supplementation For The Pasture Farmer

A recent Meadow meeting held in the Eastern Cape concerning possible services that we could provide to pasture farmers and ways of improving our rations to meet their requirements, brought to my attention the fact that the role of supplements is greatly under estimated. The role of the dairy cow has changed on many pasture farms. Instead of being a high producing individual, the cow has become a means of merely converting grass into milk with the average of the herd being used as a bench mark for limited supplementation. The merits of this system include ease of cow management and attention to pasture management. Many dairy farmers have become better managers of their cheaper source of feed, pastures. This is invaluable in trying to improve dairy cow diets, as concentrate supplementation cannot make up for poor quality roughage. Key aspects to pasture management have been utilisation (calculated as consumption divided by growth and not as consumption divided by allowance) and measurement. Measuring pasture height allows farmers to estimate available dry matter and dairy cow intakes. Measuring leaf growth stage allows shortfalls or surpluses in pasture dry matter to be predicted before they become evident from measuring pasture dry matter. Leaf growth stage and pasture dry matter measurement need to be used together.

Supplements are important in improving the quality of the roughage base. Supplements lend flexibility to management systems, allowing for optimal utilisation through provision of security to increase stocking rates. Supplements also enable grazing rotations based on pasture leaf stage to be adhered by slowing the rotation down so that cattle do not graze pastures before they have reached optimum quality at the 3 or 4.5 leaves per tiller stage for ryegrass and kikuyu respectively. Once the pasture farmer has improved his pasture management and reached maximum carrying capacity of his farm, supplements become even more important in his ability to expand or intensify his operation. The efficiency of pasture farming is first achieved through sound pasture management. The efficiency of nutrient utilisation from quality pasture is achieved through concentrate supplementation.

Although feeding concentrates to increase production is often criticised because of the substitution effect, where a kilogram of expensive concentrate replaces a kilogram of cheaper grass, it is important to keep in mind that the energy content of South African pastures is lower than that of New Zealand and Australia. Quality of temperate pastures is adequate for lower production levels of up to 20 litres of milk, if the pasture is well managed with excess spring pasture being conserved and fed back in late summer, early autumn when there is a feed shortage. Supplementation is required for higher levels of production or when production per cow or per hectare needs to be increased to improve efficiency by bringing down unit costs. The success of supplementation depends on how well supplements can be incorporated into the pasture system.

The nutrients in the supplement must match that deficient in the pasture. This requires knowledge of how the nutrients in pasture change with season, species and growth stage. Nitrogen in the form of crude protein in feeds or fertiliser is expensive and is typically poorly utilised in dairy cows. Over feeding of protein, particularly rumen degradable protein is partly to blame, along with improper balance of protein fractions, poor quality protein sources, limiting amino acids and an imbalance of protein to energy. Protein synthesis is dependent on there being the right amino acids in the correct quantities present at the same time. The dietary protein content of rations can be reduced to improve nitrogen utilisation, without compromising production. Pasture protein should not limit milk production as a 600kg Friesland cow producing 30l of milk requires 15.8% crude protein, and this can be met even by kikuyu pasture, if it is fertilised properly. The conversion of ammonia to urea is metabolically expensive in terms of energy and milk yields decline. It is estimated in New Zealand that the energetic costs of converting ammonia to urea and the low carbohydrate availability associated with high CP pasture could decrease milk yields by 11l per day when ryegrass protein content increased from 20 to 35% of herbage DM. A negative energy balance reduces fertility and embryo development. It is possible that excessive CP intake in early lactation may affect reproduction and reduce fertility by altering the uterine environment and impairing the survival of sperm, ova and embryos. The effective monitoring of the protein to energy ratio of dairy cow diets is crucial in improving yields and reproductive performance.

According to Kolver, not all of the highly degradable protein content of ryegrass is degraded in the rumen because the rate of passage of the pasture is so fast. Although the pasture is 80% degradable only 60% is actually degraded and the cow requires more protein than theory suggests. According to Kolver, the pasture, which passes through the rumen undegraded, is said to be of similar structure to microbial protein, which would explain why there is seldom a response to bypass protein supplementation. However, pasture is low in the amino acids lysine and methionine.

Complete rations recommend that 34 to 38% of the diet should be readily fermentable carbohydrates and starch should account for 30% of the diet, which will supply enough energy for microbes to convert energy into protein. High quality pastures contain between 5 and 25% readily fermentable carbohydrates. Although pasture is deficient in energy in the form of soluble carbohydrates, energy is supplied in the form of highly degradable fibre. Starch and readily fermentable carbohydrates should improve fermentation in diets of fast fermenting grasses, but it is crucial that energy is available to the microbes when they need it in the construction of microbial protein. The appropriate energy source must be used to match roughage degradation. Feeding a combination of fast and slowly fermentable energy sources, and not just maize, increases the chances for energy availability at the right time in the rumen, and should overcome problems of excess ammonia.

Energy is important in fertility of dairy cows. Ensuring cows are in good condition at mating can do wonders for conception rates. This often cannot be achieved on pastures alone with the energy costs associated with nitrogen excretion. Reproductive targets and economic increases in milk yields cannot be achieved efficiently by treating cows as a herd. Treating cows as individuals on a pasture based system allows money, in the form of concentrates, to be spent on cows that will produce and those that need to gain condition not on those that will not.

(Dugmore, 2002) (Hutjens & Barmore, 1995) (Kirkman 2003)

Date published: 2003-12-01

Dugmore, Hutjens
& Barmore and Kirkman