Intensive rotational grazing (IRG) and bovine somatotropin (bST) are two topics that have received much press during the last several years. Both are management tools that are available and being used by some dairy producers to improve dairy farm profitability. A real question is whether a grazing system and bST are management techniques that should be used separately or together. Are the two management technologies mutually exclusive or complementary? Can a dairy producer use IRG and bST together? Whereas extensive research has been conducted with bST under nongrazing conditions, more limited research has evaluated bST plus grazing.
Several studies have confirmed that pasture-fed cows will respond to bST. Research from New Zealand indicated that the response to bST with grazing is closely related to pasture growth and availability, being high in the spring and low in the summer. The quantity and quality of pasture are likely the major factors in the response obtained to bST. This situation should not exist in the United States where we can economically provide supplemental forages and grain during periods of low pasture availability. Additionally, grain is usually fed in the United States and is often uneconomical to feed in New Zealand. The responses to bST are contingent upon cows being adequately fed, whether grazing or not grazing. Both grazing and bST have the potential to improve profitability.
At Penn State we recently published a study evaluating the response to bST of high producing dairy cows with three different feeding systems combining pasture and TMR (Bargo et al., 2003). Overall, the milk response for the 21-week trial was greater for cows fed the TMR (777 lb) than for cows fed pasture plus a partial TMR (596 lb) or pasture plus concentrate (503 lb). This trial did not have a group of control cows with no bST. The expected response to bST when compared to no bST is typically 8 to 10 lb of milk/day. Cows will respond to bST when intensively grazed, and bST should be considered in well-managed dairy operations where pasture and total feed intake is maximized. The challenge is to combine pasture and bST into a system which allows for the most profitable production. Some dairy producers are successfully combining both. Proper feeding plays a big role in the successful use of the two technologies.
Administration of bST results in increased milk yield, usually within the first few days. Feeding management, particularly during the first few weeks, is very important to obtaining a profitable response to bST. The following management guidelines should be evaluated in order to maximize the milk response of cows administered bST in a grazing system.
- Provide top quality and quantity of pasture free choice (greater than 20 hours/d) to allow maximum dry matter intake. Monitor stocking rates and available pasture, and move cows to a new paddock before the quantity of pasture is limiting, which usually occurs when pasture height is less than 3 inches.
- Monitor pasture quality during growing season. As the growing season progresses, pasture quality tends to decrease; therefore, complement lower quality pasture, particularly during the summer, with higher quality forage (i.e., corn silage, haylage, hay) and/or adjust the grain and total ration for milk yield and body condition. The response to bST will diminish as pasture quantity and quality decreases unless adjustments are made in supplementary forage and concentrations to balance the diet for milk production and body condition.
- Work closely with a nutritionist to feed balanced diets formulated to meet or exceed NRC Dairy Guidelines based on performance and body condition, and consider supplementing with a TMR. Cows eat more feed and need the proper feeding management condition, which maximizes feed intake.
- Optimize cow comfort and allow continual access to fresh, clean water.
- Monitor body condition at dry off and calving.
If bST and a grazing system are being used together, good management practices are needed to obtain a profitable response. Dairy producers need to monitor milk yield on an individual cow basis, if possible, or at least record daily bulk tank weights. Often, grazing cows may have a lower body condition than cows under non-grazing. bST and grazing are independent management technologies that have the potential to improve profitability and should be considered together if they can improve dairy profitability. Good nutritional and grazing management will enhance the opportunity of obtaining a profitable response from the two technologies.
Published as pages 77-78 in proceedings from "Nutrition of Dairy Cows on Pasture-Based Systems" held March 31, 2003 in Grantville, PA.