The ALG supplement used in this study was high in DHA, and as the inclusion level of ALG increased the supply of DHA increased to provide approximately 0, 8, 16 and 24 g/cow per d. These dietary inclusion levels were selected as higher amounts have been associated with a decrease in animal performance and milk fat content (Franklin et al., 1999; Boeckaert et al., 2008). For example, supplementation with marine lipids at high
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rates has often been reported to decrease DMI in both dairy cows (Franklin et al., 1999; Moate et al., 2013) and sheep (Toral et al., 2010). In the current study there was no effect of treatment on DMI, which averaged 23.3 kg/d, a finding in accordance with both Stamey et al., (2012) and Vahmani et al., (2013) who reported no effect of feeding 200 g/d of ALG or FO to Holstein cows. Similarly, Bichi et al., (2013) also reported no effect of feeding ALG on DMI in lactating ewes when supplemented at 8 g/kg DM. In the current study the highest inclusion of ALG provided a similar DHA supply to that used in study of Moate et al. (2013), who also observed no effect on DMI. However, at a higher inclusion level of 50 g DHA/cow per d resulted in a 6 % decrease in DMI, with an 11 % decrease at an
inclusion level of 75 g/cow per day (Moate et al., 2013), and it would therefore appear that supplying DHA from ALG at up to 25 g/d can be achieved without a negative impact on intake.
It has been reported that supplementation with ALG at the rate of 43 g/kg DMI decreased milk yield by 45 % when administered directly through a rumen fistula,
(Boeckaert et al., 2008), mainly as a consequence of reduced DMI. In contrast there was no effect of ALG supplementation on milk yield in the current study, a finding similar to several others (AbuGhazaleh et al., 2009; Stamey et al., 2012; Vahmani et al., 2013). In contrast, Hostens et al., (2011) and Sinedino et al., (2017) reported an increase in milk yield when 224 g of ALG containing 44 g DHA and 100 g ALG containing 10 g DHA was fed daily to dairy cows for 46, and 120 d postpartum respectively. This difference may be explained by the longer term feeding of ALG in both studies, whereas in the current study the level of ALG inclusion was changed every 4 weeks.
Milk fat depression induced by ALG supplementation has been reported in both dairy cows (Sinedino et al., 2017; Moate et al., 2013; Vahmani et al., 2013) and sheep (Bichi et al., 2013). The exact mechanism behind milk fat depression following
supplementation with marine oils such as ALG or FO is however, unclear (Bichi et al., 2013). Bauman and Griinari (2003) described how unique FA intermediates that are produced through the biohydrogenation of PUFA can cause an inhibitory effect on milk fat synthesis, with one intermediate identified as a potent inhibitor of milk fat synthesis being trans-10 cis-12 CLA (Hussein et al., 2013; Peterson et al., 2003; Sinclair et al., 2007). However, other intermediates such as C18:1 trans-10 are also involved, and are often elevated in milk fat when milk fat depression is observed (Chilliard et al., 2001).
Supplementation of oil mixtures rich in PUFA or intermediaries of biohydrogenation in the rumen can strongly inhibit de novo synthesis and uptake of circulating FA by the
mammary gland (Hussein et al., 2013), and may therefore explain the results reported in the current study. For example, it has been reported in cell culture and rodent models that sterol regulatory element binding protein (SREBP) signaling is inhibited by PUFA
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expression of SREBP in the mammary tissue of cows fed FO or ALG compared to a control diet. Other authors have also observed that CLA causes a down-regulation in SREBP mRNA abundance and enzymatic activity in mammary tissue of dairy cows and sheep, which affects genes involved in the uptake, de novo synthesis, desaturation and esterification of FAs (Peterson et al., 2003; Hussein et al., 2013). In the current study there was a linear increase in both trans-10 cis-12 CLA, and C18:1 trans-10 as daily milk fat content and yield decreased with the addition of ALG in the diet, supporting the
findings that trans-10 cis-12 CLA is involved in milk fat depression and that C18:1 trans-10 may also be a contribute in dairy cows fed sources of marine oil.
Milk protein content and yield, as well as lactose yield were unaffected by ALG supplementation in the current study, a finding consistent with previous observations in cows fed ALG or FO (AbuGhazaleh et al., 2003; Stamey et al., 2012; Vahmani et al., 2013), and in sheep fed ALG (Bichi et al., 2013). Milk lactose content decreased linearly with the addition of ALG, a finding that contrasts with previous observations that reported that milk lactose content was unaffected by ALG supplementation (AbuGhazaleh et al., 2009; Vhamani et al., 2013), although the reason for this difference is unclear. There was no effect of dietary treatment on BCS or live weight, a finding in agreement with Glover et al. (2012), but there was a linear decrease in ECM as the level of ALG increased in the diet, which in combination with the similar DM intake between treatments, indicates that less energy may have been digested.
Few studies have evaluated the effect of ALG on whole tract digestibility, and making comparisons between studies is problematic as different sources of ALG have a diverse nutrient profile (Stokes et al., 2015). In the current study there was no difference in DM, OM or NDF intake between treatments, indicating that palatability and feed
preference were of minor concern, a finding in accordance with Stokes et al., (2015) when feeding ALG meal to sheep. However, similar to that of Stokes et al., (2015), there was a linear decrease in DM, OM and NDF digestibility with increasing rate of dietary inclusion of ALG. Diets high in PUFA have been shown to suppress the protozoal community in the rumen of cows and can also alter the Butyrivibrio related bacterial community, leading to the loss of some strains which are actively involved in biohydrogenation (Lourenco et al., 2010). In contrast Moate et al., (2013) reported an increase in the number of protozoa with the addition of ALG high in DHA in the diet of dairy cows, and concluded that when DHA is fed at a level that does not affect DMI, it does not alter rumen volatile fatty acid proportions, or enteric CH4 emissions, a finding supported by Klop et al., (2016). In
contrast Maia et al., (2007) reported that the activity of cellulolytic bacteria may be reduced by long chain PUFA, as these bacteria are inhibited by an accumulation of H2in
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