Effect of cereal grain type and corn grain harvesting and processing methods on intake, digestion, and milk production by dairy cows through a meta-analysis

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Abstract

A meta-analysis was performed to determine the influence of cereal grain type and corn grain harvesting and processing methods, dietary starch, rumen-digestible starch, and forage NDF concentrations on intake, digestion, and lactation performance by dairy cows using a data set comprising 414 treatment means from 102 peer-reviewed journal reports from 2000 to 2011. Categories for corn processing were dry ground, cracked or rolled corn (DRY), high-moisture shelled or ear corn (ENS), and steam-flaked or -rolled corn (STM); categories for kernel mean particle size were 500 to 1,000, 1,000 to 1,500, 1,500 to 2,000, 3,000 to 3,500, and 3,500 to 4,000 µm for dry corn and <2,000 and ≥2,000 µm for ensiled corn. Dietary starch and forage NDF concentrations were used as continuous variables. Data were analyzed using PROC MIXED in SAS (SAS Institute Inc., Cary, NC), with treatment as fixed and trial as random effects. Total-tract starch digestibility was reduced and milk fat content was greater for DRY compared with ENS or STM. Total-tract digestibility of dietary starch was reduced for both DRY and ENS as particle size increased. Increased dietary starch concentrations increased milk yield and protein content, but decreased ruminal and total-tract NDF digestibilities and milk fat content. Dry matter intake, total-tract starch digestibility, and milk protein concentration decreased as forage NDF in the diet increased. Total-tract starch digestibility was positively related to ruminal (percentage of starch intake) and postruminal (percentage of duodenal flow) starch digestibilities.

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Physical and chemical characteristics of dietary ingredients and their interactions can have a large effect on dry matter intake (DMI) of lactating cows. Physical limitations caused by distension of the reticulo-rumen or other compartments of the gastrointestinal tract often limit DMI of high producing cows or cows fed high forage diets. Fermentation acids also limit DMI from a combination of increased osmolality in the reticulo-rumen and specific effects of propionate, although the mechanisms are not clear. The specific physical and chemical characteristics of diets that can affect DMI include fiber content, ease of hydrolysis of starch and fiber, particle size, particle fragility, silage fermentation products, concentration and characteristics of fat, and the amount and ruminal degradation of protein. Site of starch digestion affects the form of metabolic fuel absorbed, which can affect DMI because absorbed propionate appears to be more hypophagic than lactate or absorbed glucose. Dry matter intake is likely determined by integration of signals in brain satiety centers. Difficulty in measurement and extensive interactions among the variables make it challenging to account for dietary effects when predicting DMI. However, a greater understanding of the mechanisms along with evaluation of animal responses to diet changes allows diet adjustments to be made to optimize DMI as well as to optimize allocation of diet ingredients to animals. This paper discusses some of the characteristics of dietary ingredients that should be considered when formulating diets for lactating dairy cows and when allocating feeds to different groups of animals on the farm.

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Intake and digestibility of feeds by ruminants are influenced by characteristics of the feed, animal and feeding situation. Integration of these characteristics in mathematical models is critical to future progress in forage evaluation and optimal formulation of diets for ruminants. The physiological and physical theories of intake regulation can be described by simple mathematical equations. These equations indicate that intake is a linear function of animal characteristics, such as body weight and production level, and a reciprocal function of feed characteristics, such as fill effect and energy content. Theoretical equations were developed to predict intake when the neutral detergent fiber and energy content of the diet and the energy requirements of the animal are known. The theoretical model also can be used to predict the maximum intake that will maintain a given level of animal production by solving the physiological and physical intake equations at their intersection. Psychogenic intake regulation, which is related to the animal's behavioral response to factors not related to physiological or physical characteristics, can be described mathematically as a multiplier. Digestibility can be predicted by summing the contents of ideal nutritive entities in feeds, which have true digestibilities near 100%, subtracting their associated endogenous losses and adding the variable digestible fiber content. Steady-state models indicate fractional rates of digestion and passage can be used to define ideal nutritive entities and predict digestibility over a range of kinetic characteristics. The steady-state solutions are particularly useful in understanding and predicting the depression in digestibility associated with changes in rates of passage at high levels of feed intake.

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In the United States, cattle are commonly fed diets containing cereal grains. The presence of starch and sugars reduces fiber digestion, which may in turn depress intake. In this paper, chemical constraints that may be responsible for the decrease in fiber digestion are explored. A major factor appears to be rumen pH. Moderate depression in pH, to approximately 6.0, results in a small decrease in fiber digestion, but numbers of fibrolytic organisms are usually not affected. Further decreases to 5.5 or 5.0 result in depressed growth rates and decreased fibrolytic microbes, and fiber digestion may be completely inhibited. Proliferation of organisms on readily fermentable carbohydrates may increase the need for total nitrogen as both ammonia and amino acids. The value of amino acids to cellulolytic organisms appears to be primarily as sources of isobutyric, isovaleric, and 2-methylbutyric acids. This reinforces the need to establish dietary requirements for nonprotein nitrogen, degradable protein, and isoacids. Other factors affecting fiber digestion, such as inhibition of cellulytic enzymes and plant concentrations of lignins and phenyl propanoids, are also discussed.