Table 2. Estimated salivary excretion and forage:concentrate ratio (Jiang et al., 2017) Forage:conc ratio


40:60 50:50 60:40 70:30


Eating L/d 54 62 68 73


Resting L/d 99 88 85 83


across the rumen epithelium all having an influence (Aschenbach et al., 2011). As a consequence, the firmly held relationship between many dietary factors and rumen pH is often poor (White et al., 2017). In contrast to acetate, propionate and butyrate, the pKa of lactic


acid is considerably lower at 3.86. The consequence of this is that approximately 50% of lactic acid is dissociated at a pH of 3.8, making the effect on ruminal pH far greater (Aschenbach et al., 2011). Additionally, lactic acid is more slowly removed from the rumen, and concentrations of only 5 mmol/L can predispose the cow to acute acidosis. Despite the importance of lactate in SARA, uptake mechanisms for this VFA are more poorly understood, although negligible transport activity of lactate has been reported across the apical membrane of the rumen epithelium. More recent studies indicate small amounts of transfer during prolonged periods, with monocarboxlate transport-1 primarily responsible for transfer across the basolateal membrane. Another major factor influencing the pH in the rumen is the entry


of buffers in the feed, or via saliva, although the contribution of the feed is comparatively small (Aschenbach et al., 2011). Much attention has focussed on the production of buffer via saliva and factors that influence this, although as discussed previously, the transfer of bicarbonate across the rumen epithelium may be more important in many dietary situations. Mixed saliva contains bicarbonate (125 mEq/L) and phosphate (26 mEq/L), although the former is the more important of the two. The main factors affecting saliva production include the length of time eating and ruminating, and the main drivers of rumination time being the chemical and physical properties of the diet (Beauchemin, 2018). The mean amount of time eating a TMR/PMR is 284 mins (just over 4.5 h), but can range from 141 to 507 min (1.7 to 8.5 h; White et al., 2017). In a review of the literature, White et al. (2017) also reported a mean ruminantion time of 436 min (7.3 h), ranging from 236 (3.9) to 610 min (10.2 h). Rumination time is however, negatively related to eating time, with overall chewing time being similar, with a maximum of approximately 16 h/d (Beauchemin, 2018). It is well accepted that increased chewing time increases salivary secretion and helps reduce the risk of SARA. However, the increase in salivary excretion per day with increased chewing activity is often small (Jiang et al., 2017; Beauchemin, 2018; Table 2). Increasing forage particle size also increases the length of time ruminating, with the response diminishing as particle size increases (Beauchemin, 2018). The precise dietary particle length at which no further response is obtained is difficult to define, but has been suggested to be approximately 10 mm (Allen et al., 1997). Increasing particle length does however appear to have a greater effect on eating time (Beauchemin, 2018).


Ruminating, L/d 80 87 93 94


Total L/d 232 237 246 249


Mean rumen pH 5.9 6.0 6.1 6.2


Feeding practice and potential impact on rumen pH In addition to dietary factors that can affect rumen pH, on farm feed management can have a major influence on what an individual cow consumes throughout the day, and can subsequently alter rumen metabolism and cow performance. In a recent survey of 50 UK dairy farms, Tayyab et al. (2018) reported considerable variation between farms in feeding practice. For example, the frequency of feed push-up per day had a mean of 4.6, but varied from once to 16 times, whilst length of feed mixing varied from 5 to over 60 minutes per mix (Tayyab et al., 2019). Using a modified Penn State Separator that contained additional pans with a 27 and 44 mm pore size, Tayyab et al. (2018) were also able to investigate the effect of mixing and diet selection. On many farms ration mixing was poor, with 58% of herds reported to have a moderately or poorly mixed ration, and 66% of herds having evidence of diet selection after 4 h of feeding (Tayyab et al., 2018). Perhaps of greatest concern was that 34% of the herds had no feed refusals on the morning of the visit, indicating underfeeding of a proportion or all of the cows. More limited access to feed results in fewer, larger meals (Crossley et al., 2017), with older cows consuming their feed faster than younger cows, with the consequence that first lactation animals will be most greatly affected by feed restriction (Beauchemin, 2018), although the impact on rumen pH is less clear.


Physically effective fibre and its effects on rumen pH and animal performance The particle size (PS) of the diet has been proposed as a key factor, along with neutral detergent fibre (NDF) and non-forage carbohydrate concentration to ensure a healthy rumen function (Zebeli et al., 2012), and maintain animal performance. Achieving the correct particle size (PS) and physically effective fibre (peNDF) in a ration can be reflected in the maintenance of a better environment for the growth of rumen microbes, a more efficient degradation of fibre, and as a consequence an increase in milk fat concentration (Mertens 1997, De Brabander et al., 2002). Additionally, increased microbial protein synthesis in the rumen is likely to be translated into greater metabolisable protein supply to the small intestine and therefore increase milk protein production (Sinclair et al., 2014). The term physically effective fibre (peNDF) was first defined by


Mertens (1997) as the fibre fraction that stimulates chewing and forms a floating mat of large fibre particles in the rumen. The peNDF is described as the product of a feeds neutral detergent fibre (NDF) content and its physical effectiveness factor (pef), where pef is the amount (% proportion) of a feeds particle size that is larger than that considered


FEED COMPOUNDER MAY/JUNE 2019 PAGE 41


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