GRAZED pasture represents the cheapest source of nutrients for dairy cows, but managing a pasture system is challenging.
Grazing management is the foundation of a successful and profitable pasture based system.
For dairy producers who adopt a grazing system, proper management of low cost pasture is critical. In order to maintain or improve profitability, emphasis needs to be on reducing costs and/or improving efficiency at the farm level.
Increased reliance on grazed forages offers considerable opportunity to reduce costs. In the USA, the estimated cost of pasture is about one-half the cost of ensiled legume or grass forages on a dry matter basis.
The intake by the dairy cow and the efficiency with which low cost pasture is utilized is the single most important factor determining profitability. In grazing, it is about intake.
Potential Pasture Intake and Milk Output
Research from several counties has demonstrated that with well-managed grazing systems, pasture intakes of 35 to 40 lb. of dry matter (DM)/cow/day can be achieved by Holsteins with pasture as the only feedstuff. This is about 3% of the bodyweight for Holstein cows.
In a study at Penn State University, we obtained a daily pasture intake of 45 lb/DM/cow with high genetic Holstein cows fed grass pasture as the only feedstuff. This intake can provide adequate energy to theoretically support up to 50 to 60 lb. of milk/day with Holsteins.
However, most cows may still lose substantial body condition to achieve this production since energy outgo exceeds energy intake. Pasture intake and milk yields of this magnitude may only be achieved in the spring or early summer when pasture growth and quality are high.
With the generally favorable price of milk in relation to grain supplement in the USA, it is most economical for most graziers to feed supplemental concentrates and feedstuffs rather than feed only pasture.
Factors Influencing Pasture Intake and Milk Output
In confinement operations, DMI is determined by feeding management, the amount fed, frequency of feeding, and other factors. Pasture intake by the grazing dairy cow is largely determined by how effective the cow harvests the pasture in the field.
How full is the pasture feedbunk? This depends primarily on the grazing time and the rate of intake during that grazing period.
The amount of pasture consumed is characterized by the amount of time spent grazing (grazing time, GT); the rate at which pasture is taken into the mouth (biting rate, BR), and the amount of pasture DM eaten with each bite (intake per bite mass or bite size).
This can be written more simply as pasture intake = grazing time x biting rate x bite size. Grazing time and biting rate are primarily animal factors, which means that dairy producers have little control of these factors.
High yielding cows have a stronger hunger drive than low yielding cows, and consequently graze for longer times (500 to 700 minutes/day) and have high biting rates (up to 65 bites/minute).
However, the major factor influencing pasture intake is the amount of herbage intake per bite, or bite mass. Bite mass can be controlled by management. Small increases in the intake/bite can have a major effect on daily pasture intake and animal performance.
Bite size is primarily influenced by sward factors such as grass height and density of pasture, and the proportion of green leaf in the sward.
If intake per bite declines, as it inevitably does on short swards, the behavioral constraints on biting rate and grazing time mean a reduction in daily forage intake.
The amount of time spent grazing increases as the amount of pasture decreases, which is why high producing cows need to be provided a dense sward with at least 6 to 8 inches pasture height depending on the type of grass.
We conducted a study at Penn State where we compared concentrate supplementation when high yielding Holstein cows grazed at two pasture allowances (55 vs. 90 lb. of pasture DM/cow/day). Cows were equipped with electronic recorders to monitor eating and chewing behavior (see Table 1).
Cows fed only pasture grazed about 617 minutes/day (10 hr), and averaged 56 bites/minute. This resulted in about 35,000 bites/day. This suggests that grazing cows may need stronger jaws and mouths than cows fed in confinement!
To be continued…