Improving the utilisation of home grown forage legumes by high yielding dairy cows

In recent years, the interest in the use of home grown protein sources for the dairy industry has increased due to fluctuations in cost and availability of soyabean meal (SBM). Forage legumes have been of particular interest due to their high crude protein content, ranging between 170 to 220 g/kg DM. Protein in forage legumes is degraded rapidly within the rumen, however a high yielding dairy cow requires high levels of by-pass protein to maintain milk yield. Tannins are phenolic compounds that can bind to forage protein in the silo or rumen, providing protection from microbial breakdown, before dissociating in the small intestine, increasing the amount of by-pass protein available from the forage legume. Studies were therefore conducted to evaluate the effects of tannins, either naturally occurring or added to forage legumes at ensiling, on the nutritive value, nitrogen utilisation and performance of high yielding dairy cows. In the first study, the effect of the addition of condensed or hydrolysable tannins at inclusion rates of 0, 25, 50 or 75 g/kg DM at ensiling on the nutritional value of forage legumes was investigated. Forage source influenced all nutritional values including DM, CP, ash, NDF and ammonia nitrogen content, whilst forage DM content increased from 329 to 364 g/kg as inclusion of tannin increased from 0 to 75 g/kg DM. Addition of tannins influenced forage pH with forages supplemented with condensed tannins having a higher pH than those supplemented with hydrolysable tannins. Forage pea silages have varying levels of naturally occurring condensed tannins and also a high crude protein content and the second study investigated the effect of the inclusions of forage pea silages differing in tannin content on dairy cow performance. Dairy cows fed pea silage containing low levels of condensed tannin had the lowest dry matter intake whilst milk yield was highest in cows fed grass silage. Similarly, milk protein was highest in cows fed grass silage, but there was no effect of treatment on milk fat content. Milk fat content of C18:2 n-6 was highest in cows when fed grass silage (P < 0.001), whilst C18:3 n-3 was highest in cows fed pea silage containing high levels of tannin. In comparison to forage pea silages, lucerne and red clover silages have limited amounts of naturally occurring tannins but also have a high content of crude protein. The addition of hydrolysable tannins at ensiling can bind to the forage protein in the silo improving the availability of by-pass protein from the forage, therefore the third study investigated the addition of hydrolysable tannins at 25 g/kg DM to lucerne and red vi clover silage on the performance of lactating dairy cows. The inclusion of tannin had no effect on dairy cow performance, however forage source influenced dry matter intake with cows fed lucerne silage having the highest intake (P < 0.05), similarly, OM and N intake was highest in cows fed lucerne silage. In both dairy cow studies, DM, N and OM digestibility were not significantly affected by forage type or the presence of tannins. In conclusion, any commercial advantage from feeding forage legumes to high yielding dairy cows will be based on savings in N from fertiliser or dietary protein rather than animal performance, whilst tannins will have little effect on milk performance.