A Statistical Evaluation Of Early-Or-Mid-Lactation Dairy Cow Responses toSodium Bicarbonate Addition
by Wenping Hu and Michael R. Murphy
TAKE HOME MESSAGES
- Mixed model statistical analysis suggested that NaHCO 3 addition affects the performance of early- or mid-lactation dairy cows fed corn silage based diets.
- Addition of NaHCO 3 at 0.70 to 1.0 % of feed dry matter, based on contrast analyses, appeared optimal for early- or mid-lactation cows fed corn silage based diets.
- Buffer treatment effects differed with forage type (corn silage or other); however, these were confounded with diet content of acid detergent fiber.
Addition of buffers to diets for lactating dairy cows has been extensively investigated. Initially, studies were conducted to evaluate buffers in fat-depressing diets where only 5 to 15 % forages were fed. Results suggested that buffering helped prevent or alleviated milk-fat depression, thus buffer addition has been recommended primarily for diets when depression of milk fat might otherwise occur. Effects of buffer addition to more typical diets on feed intake, milk production, and acid-base balance of lactating dairy cows have also been examined. However, responses varied because of the complexity of buffer sources, animal and diet factors involved, and the interaction among them, such as the degree to which cows are stressed by metabolic acid load, acidity and buffering capacity of the diets, etc.
The most common buffer used in the dairy industry is NaHCO 3; we also chose it to help elucidate the role of buffer addition to dairy diets. Our analyses were based on data collated from studies of early- or mid-lactation dairy cows fed diets with different added amounts of NaHCO 3. Our objectives were to examine effects of NaHCO 3 addition on the performance of lactating dairy cows, and to determine if an optimal amount of buffer addition exists for early- or mid-lactation dairy cows.
MATERIALS AND METHODS
Twenty-four studies of NaHCO 3 effects on early- or mid-lactation dairy cows from 1980 to 1998 were compiled into a database in which 27 trials, 66 feeding treatments, and 314 cows were involved. Over 90% of the data were from Holstein cow records.
Sodium bicarbonate was the only buffer considered; therefore, data from trials with other buffers (e.g., MgO) were omitted, except one trial in which 0.11% MgO was included as a Mg supplement. Data from late lactation were also excluded. Dietary NaHCO 3 addition was categorized according to the amount in the diet as control (0%, C group), moderate (0.7 to 1.0 %, M group), or high (1.2 to 1.5 %, H group); buffer additions outside these ranges were not included. Forage type was categorized as corn silage (CS) when CS was the sole or main forage in the diet, and as forage other than CS (NCS) when alfalfa or bermuda grass hay, or silages based on alfalfa, barley, sorghum, or wheat were included in the diet.
Effects of NaHCO 3 addition, forage type, and their interaction on lactating dairy cows, and potential relationships between milk fat percentage and independent variables such as dietary acid-detergent fiber (ADF) content, and acetate: propionate ratio were investigated across studies using mixed model analysis of variance with study as a random effect. Regressions are presented to quantify the relationship between milk fat percentage and measured variables across studies. Significance was declared at P≤ 0.05.
RESULTS AND DISCUSSION
Dry matter intake did not differ among buffer treatments for cows fed NCS; however, cows fed CS without NaHCO 3 addition consumed 1.2 kg/day less dry matter (P = 0.04) than cows fed buffer. Increased feed intake may be attributed to reduction in acidity of CS by NaHCO 3 compared to NCS, because CS generally has a lower pH than alfalfa silage or hay-crop silage.
Milk production was unaffected by buffer treatments regardless of forage type; however, milk fat percentage for CS diets was 0.3 percentage units higher when buffer was added (P = 0.02). Milk fat yield and FCM were also higher for buffered compared to unbuffered diets, by 113 g/day (P < 0.01) and 1.9 kg/day (P = 0.03), respectively. Dose of buffer (0.7 to 1.0 % compared to 1.2 to 1.5 %) did not affect measured variables.
Changes in milk fat have been linked to the dynamics of ruminal volatile fatty acids. As others have reported, our analyses also revealed that a positive relationship existed between milk fat percentage and both ruminal acetate molar percentage and acetate to propionate ratio (Figure 1); whereas, milk fat percentage was negatively related to molar percentage of ruminal propionate.
Figure 1. Milk fat percentage as affected by ruminal acetate: propionate ratio (C 0 %, M 0.7 to 1.0 %, and H 1.2 to 1.5 % of dietary dry matter as NaHCO 3 and for CS corn silage, or NCS other forages).
As suggested in previous studies, addition of NaHCO 3 did not affect responses when NCS based diets were fed. Our analyses substantiated this observation statistically. Forage type was confounded with ADF content of the diet. Figure 2 suggested a strong positive relationship between ADF content of the diet and milk fat percentage, regardless of the buffer treatments to which they belonged. Therefore, differences in response to buffer treatments related to forage type might be explained by variation in the ADF contents of the forages themselves. Effectiveness of buffer addition for NCS with lower ADF needs to be investigated further.
Figure 2. Milk fat percentage as affected by dietary acid detergent fiber content (C 0 %, M 0.7 to 1.0 %, and H 1.2 to 1.5 % of dietary dry matter as NaHCO 3 and for CS corn silage, or NCS other forages).