Competencias Directivas: un Concepto en Desarrollo

having the potential to influence the performance of a high yielding dairy cow. Condensed tannins are found naturally within forage peas and occur at varying levels depending on the flower colour, peas which have coloured flowers tend to have higher levels than peas with white flowers (Wang et al., 1998). Previously, it has been shown that forage peas have high CP contents of between 170 to 220 g/kg DM (Mustafa et al., 2000; Hart et al., 2011) with the current study reporting values of 161 and 195 g/kg DM in experiment 1, whilst in experiment 2, CP levels of 208 and 195 g/kg DM were reported for red pea and white pea silage,

respectively. However, in forage legumes this protein tends to be degraded rapidly within in the rumen resulting in low quantities of RUP available in the small

intestine (Sinclair et al., 2009). Tannins have the ability to form a tannin-protein complex at pH 4.5 – 7.0 in either the silo or rumen, therefore improving the availability of RUP in the small intestine (McMahon et al., 2000). The microbial protein in a high yielding dairy cow is not sufficient to supply the cow’s MP requirement, therefore the cow requires RUP in the diet to meet these requirements (McDonald et al., 2011). The addition of condensed tannins in experiment 1, had no effect on CP content of forage pea silages suggesting that there was no reduction in proteolysis in the silo therefore there was no increase in the amount of RUP available. Similar to experiment 1, Adesogan and Salawu,

155 (2002) observed no change in total N when 16 g/kg DM condensed tannins were added to a pea-wheat bi-crop at ensiling. Although, ammonia-nitrogen content was reduced, with a content of 130 g/kg total N for silage with no tannin and 94.7 g/kg total N for silage that had 16 g/kg DM condensed tannin added. Although in the current study, the addition of condensed tannins to forage peas (Chapter 4) had no change on ammonia-nitrogen content. Therefore, it can be concluded that the addition of condensed tannins to forage pea silages has very little influence on nutritive value or availability of RUP. Consequently, experiment 2 investigated the effects of the natural levels of condensed tannin in forage pea silages on dairy cow performance.

In experiment 2, the total replacement of grass silage with either red or white pea silage reduced DM intake and milk yield which may be attributed to the difference in CP content of the forages however had no effect on diet digestibility. In a previous study conducted by Sinclair et al. (2009), it was found that the inclusion of pea silage in the diet of dairy cows improved DM intakes compared to cows fed grass silage, although milk yields were unaffected. These differences may be attributable to the amount of forage pea silage included in the diet, in experiment 2 forage pea silages were included at 0.4 of the forage DM whereas in Sinclair et al. (2009) the forage pea silages were included at 0.5 of the forage DM. Sinclair et al. (2009) concluded that the lack of change in milk yield may be due to the dairy cow utilising the nutrients from the forage pea silages for her body stores rather than milk production although there was no BCS change observed. In the current study, there was no change in BCS which averaged 2.87 suggesting that the cow was not utilising the nutrients from the forage pea silages for her body stores.

Since completion of experiment 2, more recent research (Broderick et al., 2017) investigated the effects of increasing levels of naturally occurring condensed tannins from 5.1 to 16.6 g/kg DM in birdsfoot trefoil silage on performance of high yielding dairy cows. These levels of condensed tannins in the birdsfoot trefoil silages were much lower than in the forage pea silages of experiment 2 which averaged 62.9 g/kg DM. In two trials conducted by Broderick et al. (2017), DM intake and milk yield averaged the highest at 26.8 and 41.3 kg/d, respectively, in cows fed birdsfoot trefoil silage containing 10 g/kg DM condensed tannins. In contrast, Hymes-Fecht et al. (2013) found that milk yield was highest in cows fed birdsfoot trefoil silage containing the highest amount of condensed tannin at 15.7

156 g/kg DM although DM intake was unaffected. These contradicting results from previous studies and the current study make it unclear what the optimal level of naturally occurring condensed tannins in forage legumes would be for optimising DM intake and milk production in dairy cows. It has been suggested (Hymes-Fecht

et al., 2013 and Gerlach et al., 2018) that levels of higher than 30 g/kg DM of condensed tannins in the diet can have a detrimental effect on DM intake and milk yield, therefore in the current study, this may be the reason there was a reduced DM intake and milk yield compared to cows fed grass silage. However, these higher levels of condensed tannin were used in the current study due to a previous study by Sinclair et al. (2009) observing improved DM intakes with forage pea silage containing condensed tannin at levels higher than 30 g/kg DM.

Savings can be made by the reduction of the use of imported dietary protein with the use of forage legumes in the diet of high yielding dairy cows due to their high crude protein content. In experiment 2, there was a saving in soyabean meal of approximately 0.4 kg/cow/day when cows were fed forage pea silages, this would be a significant saving for a commercial dairy cow herd. For a 100 cow herd with a lactation period of 305 days this saving would equate to a 12.2 tonne reduction in SBM which would be an overall cost saving of £3891 at £319 per tonne.

Therefore, the overall conclusion from experiment 2 and the previous published studies would suggest that any commercial advantage from feeding forage legumes containing condensed tannins to high yielding dairy cows will be based on savings in N from fertiliser or dietary protein (Sinclair et al., 2009).

6.3 The effect of hydrolysable tannins on nutritive value and dairy cow