Abstract
We aimed to evaluate the effects of dietary supplementation with magnesium oxide and calcium-magnesium dolomite on milk fat synthesis and milk fatty acid profile or persistency in milk fat synthesis after their cessation in dairy cows under milk fat depression conditions. Twenty-four multiparous dairy cows in early lactation (mean ± standard deviation; 112 ± 14 d in milk) were used in a randomized complete block design. Milk fat depression was induced in all cows for 10 d by feeding a diet containing 35.2% starch, 28.7% neutral detergent fiber, and 4.8% total fatty acid (dry matter). The experiment was conducted in 2 periods. During the Mg-supplementation period (d 1-20), cows were randomly assigned to (1) the milk fat depression diet used during the induction phase (control; n = 8), (2) the control diet plus 0.4% magnesium oxide (MG; n = 8), or (3) the control diet plus 0.8% calcium-magnesium dolomite (CMC; n = 8). Compared with the control group, feeding the magnesium-supplemented diets increased milk fat concentration and yield by 12% within 4 d. During the 20-d Mg-supplementation period, both the MG and CMC diets increased milk fat concentration and yield, as well as 3.5% fat-corrected milk and energy-corrected milk yield, without affecting dry matter intake, milk yield, and milk protein and lactose concentrations. In the Mg-cessation period (d 21-30), all cows received the control diet, which resulted in a greater milk fat concentration and yield in the cows that had already received the MG and CMC diets in the Mg-supplementation period. Whereas, milk fat concentration and yield remained high after discontinuation of the magnesium-containing alkalizer until d 27. The difference in milk fat synthesis was associated with lower trans-10 C18:1 (-22%) and higher trans-11 C18:1 (+12.5%) concentrations in milk during the Mg-supplementation period. Furthermore, it was evident that within 2 d of supplementation, the trans-10:trans-11 ratio was lower in MG and CMC cows compared with cows receiving the control. This suggested that the effect of magnesium-based alkalizers on milk fat synthesis was mediated via a shift in ruminal biohydrogenation of cis-9,cis-12 C18:2 in the rumen. In conclusion, abrupt addition of magnesium oxide and calcium-magnesium dolomite increased milk fat synthesis, which persisted for 7 d after cessation of magnesium-based alkalizers. A similar ability to recover milk fat synthesis and normal fatty acid biohydrogenation pathways was observed for magnesium oxide and calcium-magnesium dolomite.
Highlights
Increasing dietary NFC is a common approach to maximizing milk production in high-producing dairy cows
It was evident that within 2 d of supplementation, the trans-10:trans-11 ratio was lower in magnesium oxide (MG) and CMC cows compared with cows receiving the control. This suggested that the effect of magnesium-based alkalizers on milk fat synthesis was mediated via a shift in ruminal biohydrogenation of cis-9,cis-12 C18:2 in the rumen
We investigated the effect of magnesium sources on rumen fermentation and daily rumen pH patterns; that study was not designed to measure changes in milk fat synthesis due to experimental conditions (Razzaghi et al, 2021)
Summary
Increasing dietary NFC is a common approach to maximizing milk production in high-producing dairy cows. Feeding highly fermentable carbohydrates results in increased concentration of VFA in the rumen and decreased rumen pH, which can affect the microbial composition of the rumen (Calsamiglia et al, 1999; Dijkstra et al, 2012) These changes in rumen pH and microbiota composition are associated with shifts in rumen biohydrogenation (BH) pathways (Fuentes et al, 2009; Sandri et al, 2020), leading to increased rumen synthesis of BH intermediates capable of inducing milk fat depression (MFD), such as trans-10,cis-12 CLA and trans-9,cis-11 CLA (Baumgard et al, 2000; Perfield et al, 2007). Razzaghi et al.: MILK FAT DEPRESSION AND ALKALIZERS IN DAIRY COWS men pH and milk fat synthesis in dairy cows (Erdman et al, 1982; Bach et al, 2018) This additional magnesium oxide supplementation may increase feeding costs, whereas alternative magnesium sources may have similar effects while being less expensive
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