WHEN silage quality is improved, milk yields increase, or alternatively, concentrate levels can be reduced. Therefore, improving silage quality should be a target for all dairy farmers in Northern Ireland.
However, there are concerns that when cows are offered silages with a very high digestibility, rumen health and digestive efficiency will be adversely affected due to insufficient fibre in the diet.
This situation may be made worse if high quality silages are supplemented with high levels of starch-based concentrates, which normally increase performance, but also potentially increase the risk of acidosis.
As a consequence, supplementing very high quality silages with fibre-based concentrates rather than starch-based concentrates is often advised.
In addition, the inclusion of small quantities of chopped straw in high quality diets is often advocated if diets are believed to contain insufficient ‘functional fibre’. In theory, straw inclusion will increase saliva production and rumination, and stabilise rumen pH but could also reduce intakes.
The objectives of the current study were to examine the effect of concentrate type (starch-based or fibre-based), and straw inclusion (straw or no straw) on dairy cow performance, when offered alongside a very high quality grass silage.
This short-term study involved 24 mid-lactation dairy cows. All cows were offered a high quality grass silage, which had a dry matter content of 42 per cent, a crude protein content of 17 per cent, and a metabolisable energy content of 12.1 MJ per kg of dry matter (DM).
Four treatments, comprising different supplementation
strategies, were examined, as follows:
n Starch-based concentrate, no straw
n Starch-based concentrate, plus chopped straw
n Fibre-based concentrate, no straw
n Fibre-based concentrate, plus chopped straw
All diets were prepared using a mixer wagon and offered as a total mixed ration. The mix provided approximately 12 kg concentrate per cow per day. Cows on the straw treatment were offered approximately 1.0 kg straw per day.
The starch-based concentrate contained a high proportion of maize meal and had a starch content of 33 per cent and a fibre content of 21 per cent (fresh basis). In contrast, the fibre-based concentrate contained 21 per cent starch and 31 per cent fibre (fresh basis), and contained a high proportion of soya hulls and beet pulp. Both concentrates had a crude protein content of 14.5 per cent (fresh basis).
The response to straw inclusion in the diet was the same with both types of concentrate (ie, there was no interaction), and this allows us to examine the effect of concentrate type (Table 1) and straw inclusion (Table 2) separately.
Effect of concentrate type
Silage dry matter intakes (DMI) of cows offered the fibre-based concentrate were 3.4 per cent lower than those of cows offered the starch-based concentrate, and this resulted in a reduction in total DM intake. While milk yield was unaffected by concentrate type, milk protein content was reduced by two per cent when the fibre-based concentrate was offered, perhaps due to the lower intake with this treatment.
While starch-based concentrates are often associated with a reduction in milk fat content, no such effect was observed in the current study.
This is likely due to the concentrates being formulated using ‘NutriOpt’, a rationing programme that takes into account the ‘structural fibre index’ and ‘acid load’ of the ration.
Therefore, although the fibre and starch content of the diets where different, the starch-based diet had enough effective dietary fibre to promote rumination and stimulate intakes.