Effects on rumen pH and feed intake of a dietary concentrate challenge in cows fed rations containing pH modulators with different neutralizing capacity

Forty-five Holstein lactating cows (41 & PLUSMN; 8.8 kg/d of milk yield, 96 & PLUSMN; 35.6 days in milk, and 607 & PLUSMN; 80.4 kg of body weight) were enrolled in this study to assess the effects of diets supplemented with sodium bicarbonate or a magnesium-based product and their corresponding differences in dietary cation-anion difference (DCAD) on rumen pH, rumen microbial population, and milk performance of dairy cattle exposed to an induced decrease in rumen pH through a dietary challenge. Cows were randomly allocated to 3 total mixed rations (TMR) differing in the type of supplement to modulate rumen pH: (1) control, no supplementation; (2) SB, supplemented with 0.82% of sodium bicarbonate with a neutralizing capacity (NC) of 12 mEq/g; and (3) MG, supplemented with 0.25% of magnesium oxide (pHix-Up, Timab Magnesium) with a NC of 39 mEq/g. Thus, SB and MG rations had, in theory, the same NC. The 3 TMR differed for control, SB, and MG in their DCAD-S (calculated considering Na, K, Cl, and S), which was on average 13.2, 21.2, and 13.7 mEq/100 g, respectively, or DCAD-Mg (calculated accounting for Mg, Ca, and P), which was 31.4, 41.2, and 35.2 mEq/100 g, respectively. The study lasted 63 d, with the first 7 d serving as a baseline, followed by a fortnightly progressive decrease of dietary forage-to-concentrate ratio (FCR) starting at 48:52, then 44:56, then 40:60, and finishing at 36:64. Individual dry matter intake (DMI) was recorded daily. Seven cows per treatment were equipped with elec-tronic rumen boluses to monitor rumen pH. Control and SB cows consumed less dry matter (DM; 23.5 & PLUSMN; 0.31 kg/d) than MG cows (25.1 & PLUSMN; 0.31 kg/d) when fed dietary FCR of 44:56 and 40:60. Energy-corrected milk decreased from 40.8 & PLUSMN; 1.21 to 39.5 & PLUSMN; 1.21 kg/d as dietary FCR decreased, independently of dietary treatments. Rumen pH decreased and the proportion of the day with rumen pH <5.8 increased as dietary FCR decreased, and at low dietary FCR (i.e., 36:64) rumen pH was greater in MG cows than in control and SB cows. Reducing the DCAD-S from 28 to 18 mEq/100 g or the DCAD-Mg from 45 to 39 mEq/kg had no effects on DMI or milk yield. Cows supplemented with-62 g/d of magnesium oxide (pHix-Up) maintained a greater rumen pH and consumed more DM than cows supplemented with-200 g/d of sodium bicarbonate when fed a diet with low FCR.

Automated summary

This study aimed to assess the effects of diets supplemented with sodium bicarbonate or a magnesium-based product and their corresponding differences in dietary cation-anion difference (DCAD) on rumen pH, rumen microbial population, and milk performance of dairy cattle exposed to an induced decrease in rumen pH through a dietary challenge. The study found that supplementation with magnesium oxide can increase rumen pH and increase dry matter intake. This is particularly important for cows fed low FCR diets.