Comparison of the herbage quality of Summer grasses and crops

By Dr Bill Fulkerson - Norco Milk Supply Research & Development Officer

A study conducted on the Northern Rivers area of NSW compared the herbage quality of 4 summer grasses (Kikuyu, Setaria, Rhodes Grass and Paspalum) and 2 summer crops (Siberian Millet and BMR choice Sorghum). The samples were collected from commercial dairy paddocks and managed in such a way as to maximize herbage quality (grasses  were grazed about every 2 weeks, Millet was grazed at about 40cm height and the Sorghum at 1m height).

Comparison of Digestibility and Protein

For the grasses, the nutrient values are an average of samples taken at the end November, mid-January and end of March, Millet was sampled at the end of November and the Sorghum in early December and mid February (see Table 1 below).

The Metabolisable Energy (ME) of Kikuyu, Setaria, Paspalum and Millet were similar, Rhodes Grass was significantly lower and BMR sorghum was significantly higher. Typical values for Ryegrass are also given.  In practice, the difference in milk production potential between Kikuyu at 9.5 MJ/kg DM and Rhodes Grass at 7.9 MJ/kg DM would be  about 4 L/cow/day milk and between Kikuyu and BMR Sorghum about 2.7 L milk/cow/day.

When BMR was cut 3 weeks after optimal, the ME fell to 9.4 MJ/kg DM. Even so the loss of quality over time is less with BMR than for other Sorghums.

Comparison of Mineral Content

The Total Calcium is much lower in both Kikuyu and Setaria than the other forages, hence the need to supplement cows with lime or other sources of calcium.

The magnesium is low in Setaria and Rhodes Grass. It should be above 0.22% and can also be deficient in Ryegrass in June/July. 

The sodium is very low in Kikuyu, Paspalum and Sorghum and hence the need to supplement cows with salt when grazing these pastures.

The sulphur can be low in Kikuyu, Setaria and Sorghum.

Availability of Calcium

Whilst the Total Calcium is already low in Kikuyu and Setaria, much of this is bound to oxalates, making the available Calcium much lower still, as shown in Table 3 below.

In Ryegrass, the total calcium content is high and very little is bound to oxalates. In contrast, there is 5 times less calcium available in Kikuyu than Ryegrass and in Setaria there is even less available and it is therefore obvious that rations formulated for cows grazing ryegrass are completely inadequate for cows grazing Kikuyu and Setaria in terms of mineral supplementation.

The need to adapt cows to summer grasses with high levels of oxalates

Whereas some of the oxalates bind calcium, making it unavailable to the animal, the remaining soluble oxalates are toxic to the unadapted animal; it binds to the calcium in the blood disrupting muscle and lungs function. This is seen as sick cows and can sometimes be fatal. It is not uncommon to lose a number of unadapted animals if they are suddenly moved from Ryegrass to Setaria.

It takes about 4 days for the rumen bugs that break down the oxalates to build up to sufficient levels to breakdown and harmlessly excrete the oxalates. During the adaption period just give the cows ¼ or half hour on the Setaria but they need to be readapted if they have been off Setaria for more than 2 weeks. If the Setaria has been oversown with ryegrass and continually grazed then the cows get enough to adapt to the high oxalates. Setaria has achieved a bad name because of the problem of high oxalates but if properly understood and managed it can be as productive as Kikuyu.

This adaption also applies to Kikuyu pasture but to a far lesser extent.