Feed sorting appears to be affected by acidosis risk with severely acidotic cows selecting feed particles that should increase ruminal pH.
Selective consumption of the most palatable TMR components (Leonardi and Armentano, 2003 ; DeVries et al., 2007) can imbalance dairy cow diets. Low forage diets allow higher degrees of sorting against longer forage particles and for smaller grain concentrate particles (DeVries et al., 2007). Early lactation cows are typically fed low forage diets. Sorting behavior coupled with rapidly increasing DMI after calving (Kertz et al., 1991) can greatly skew proportional intakes of short and long particle, drastically reducing intake of physically effective fiber. Consequently, VFA production increases and buffering capacity is reduced in the rumen (Cooke et al., 2004; Stone, 2004). Thus, feed sorting may significantly contribute to ruminal acidosis problems in early lactation cows (Penner et al., 2007).
Some research results suggest cows with low ruminal pH will select dietary components (ie – longer forage particles) that should improve rumen function (Beauchemin and Yang, 2005; Cooper et al., 1996; Keunen et al., 2002; Yang and Beauchemin, 2006). The extent to which cows will select feed to attenuate ruminal acidosis is uncertain. The following work evaluated this phenomenon.
Ruminally cannulated cows were assigned to 1 of 2 acidosis risk levels by diet and milk production; low risk (LR, miday lactation cows fed a 60% forage diet) or high risk (HR, early lactation cows fed a 45% forage diet). An acidosis challenge, consisting of a 1-h meal of 4 kg of ground barley/wheat before the TMR, was conducted in each of two 14-d periods. Each period consisted of 3 baseline days, a feed restriction day (restricting TMR to 50% of ad libitum itake), the acidosis challenge day, and a recovery phase. Ruminal pH was measured continuously for the first 9 d of each period. Intake was measured and particle size of both offered and consumed diets was determined.
Both acidosis risk groups sorted feed to selectively consume particle fractions that were higher in starch and lower in NDF than the original diet. This sorting reduced ruminal pH variables, although the specific pH variable best correlated with the sorting variable differed between diets.
Cows changed their sorting behavior after acidosis challenge. For the HR cows in period 1, there were some small adjustments to sorting behavior, particularly on day 2 post challenge. In period 2, the most acidotic HR cows (Dohme et al., 2008), dramatically changed their sorting behavior, particularly on day 1 postchallenge. These cows selectively consumed medium particles over short and fine particles, suggesting that the cows were attempting to increase their physically effective fiber intake (i.e., forage particles within the medium fraction), and reduce their starch intake (i.e., grain particles within the short and fine fractions). It is possible this was done to attenuate the effects of the very low ruminal pH that animals were experiencing during acidosis challenge. Regression analysis found that the cows with the lowest ruminal pH performed the most sorting for longer particles in the diet, while sorting against the shorter particles.
The LR cows also changed their sorting behavior in response to the acidosis challenge in both period 1 and 2. Most changes occurred on day 2 postchallenge and the recovery day when pH had returned to baseline values. On day1 postchallenge for period 1, those cows with the lowest maximum ruminal pH during day 1 postchallenge selected more long and fine particles, and less medium and short particles. In period 2, a greater daily swing in ruminal pH resulted in less sorting for short and fine particles. LR cows with lower mean ruminal pH values also sorted against fine particles.
Overall, there were clear associations between sorting and ruminal pH on day 1 postchallenge for both LR and HR cows. The HR cows did appear to alter their sorting behavior in response to acidosis, particularly those animals experiencing severe acidosis. The alteration of TMR sorting in response to the acidosis challenge supports the theory that ruminants will alter their feeding behavior to feel better (Provenza, 1995). Researchers have previously shown that ruminants will alter their diet selection in response to low ruminal pH, including the selection of sodium bicarbonate ( Cooper et al., 1996; Phy and Provenza, 1998), preferring long hay over-pelleted forage (Keunen et al., 2002), and sorting a TMR for long particles when fed diets that caused low ruminal pH (Beauchemin and Yang, 2005; Yang and Beauchemin, 2006). The present study provides the first evidence to suggest that acidosis severity will influence the extent of TMR sorting by lactating dairy cows to increase ruminal pH. Oddly, prior to the acidosis challenge, the sorting behavior of these cows likely lowered their ruminal pH. Further research is needed to determine the exact point at which acidosis becomes severe enough for the cows to alter their sorting behavior in an attempt to increase rumen pH. Also, given the relatively short duration of ruminal acidosis following the challenge (Dohme et al., 2008), it is unclear whether sorting behavior can effectively increase ruminal pH during a prolonged acidosis event.
Early lactation cows fed a low forage diet and mid lactation cows fed a high forage diet both sorted against the longest and finest TMR particles and sorted for medium length TMR particles. Sorting was performed to a greater extent by the cows at high risk for acidosis; sorting was related to low ruminal pH. The HR and LR cows altered sorting behavior in response to acidosis challenge. For the HR cows, severe acidosis was associated with increased sorting for the longer particles in the diet and against the shorter particles, likely to attenuate the effects of the very low ruminal pH. Feed sorting appears to be affected by acidosis risk with severely acidotic cows selecting feed particles that should increase ruminal pH.
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