Illinois dairy managers have had BST (bovine somatotropin) available since February of 1994 as a tool to manage the milk lactation curve and improve the profitability of their herds. BST technology has been extensively researched and studied which has provided valuable knowledge to producers. Field reports and experiences have added to our knowledge. Seven Illinois herd volunteered their DHI data to be used to evaluate farm responses this new technology.

EVALUATING FARM RESPONSES

Most dairy managers depend on DHI or other cow milk records to evaluate response to BST. If the manager had access to daily milk weights with a 14 day summary, an individual cow response would be measured. The following methods can be used to measure farm responses.

Change in milk yield from the previous month adjusted for milk persistency (normal milk decline due to stage in the lactation curve) can be monitored. First lactation cows drop 5 to 7 percent a month after peak milk production while older cows drop 8 to 1D percent a month. If cows maintain their current production, then milk yield actually went up even though bulk tank readings remained constant. However, several other factors should be considered:

• How typical was the previous month's milk yield prior to BST injection?
• How constant and normal is the individual cow's lactation curve?
• Did an unrelated factor (change in feed quality or intake, milking order, heat stress, es us, off-feed, or health factor affect this month's milk yield) cause an unexpected response?

Follow the projected 305 day actual or mature equivalent record as calculated by the computer to evaluate response. If your treated cows are more persistent or increase in milk production, the 305 when the rolling DHI average and bulk tank remain constant. Midst tes DHI Records Center can provide individual MLM on each cow on a monthly basis for one dollar to evaluate individual cow response.

Listing BST-treated cows in one string will allow the computer to summarize this group (milk yield, milk components, days in milk, and somatic cell count) for comparison to other groups in the herd, especially if a similar untreated group exists.

Persistency values are calculated on some Illinois DHI reports. Milk production at 100 percent persistency would be normal for Illinois herds.

The use of bulk tank measurements and test day average milk on DHI has limited value because environmental changes, cow factors(fresh and dried off cows), and health factors can have large effects. Also, dairy managers should be consider when BST is injected when evaluating test day milk. If the cows are near the end of the fourteen day injection period, milk yield can be declining due to the cyclic nature of the product. Based on research results, milk yield at day 3 and 10 after injecting injection period.

ILLINOIS FARM RESPONSES AND RESULTS

Seven Illinois herds volunteered to provide their DHI records and additional cow data from January, 1994, through May, 1994, to evaluate BST response in their herds. Table 1 illustrates the range of responses and patterns.

Herd A BST-treated cows continued to maintain milk production in the second month after an initial increase in the first month (referred to as ramping up). In the third month, the BST-treated cows followed typical persistency values. Herd average, test day milk, and MLM values indicate a milk response occurred.

Herd B had a large increase in the first month after BST administration and then followed a typical lactation curve response. Herd average did not change while test day milk and MLM did respond.

Herd C BST-treated cows continued to increase the second and third months above the first month's large increase. This herd has the highest milk production with a high energy diet. All DHI measurements indicated positive responses.

Herd D followed the pattern seen in herd A, but the first month response was lower and ramp up in the second month was intermediate. Herd average actually declined while test day milk and MLM illustrated only modest increases.

Herd E illustrates the wide variation in individual cow milk yield on monthly DHI test day. On this farm, the time cows can enter the milking parlor can vary by two hours due to parlor management and one group. It is difficult to evaluate BST response looking at individual cow records because of wide milk yield swings. All DHI milk measurements (herd average, test day milk, and MLM) indicate improvement in milk production and the herd owner is very pleased with the response based on cash flow.

Herd F has an excellent response to BST with a response of 20 pounds per cow based on monthly MLM records on individual cows. Herd average and test day production showed only modest increases cows injected.

Herd G has a typical response and followed the pattern seen in herd persistency changes in months two and three). Herd average increased while MLM and test day milk did not increase.

All seven herd owners reported an economical response, but the pattern varied. Based on field experiences, the dairy manager should evaluate the BST-treated cows as a group. Individual cow records are available. Farm response should be conducted because a low milk response may indicate significant health, management, orf nutritional limitations exist on the farm. Well managed BST cowsb and herds responded.

Non-responding cow alternatives

In the Illinois herds (excluding Herd E), 7 of 183 cows (3.8 percent of all injected cows) did not response compared to earlier DHI test prior to BST use. Each cow should receive four to five injections to allow an opportunity for a response to measured. This time period would normally allow for two DHI test results. In Illinois field related to their metabolism, health status, or lack of BST receptors. At some point, it may make economic sense to stop treating the non-responding cow. If these cows were not injected study five of the non-responding cows in the first month responded the second month. Non-responding cows could be with BST, follow their milk response to see if a response occurred. Herd F in our study stopped injecting 11 cows (no reason listed) and observed a drop of 9.0 pounds the first DHI test after stopping and an additional 6.1 pounds in the second DHI test. Stopping BST injections is a method to lower milk levels making the drying off process easier on the mammary gland and cow. Herd F stopped injecting 8 cows one month prior to dry off and measured an average drop of 23 pounds in the following DHI test prior to drying off. Herd G stopped injecting five cows one month prior to drying off and observed an average drop of 18.8 pounds per cow (range of -9 to -26 pounds of milk).

NUTRITION AND MANAGEMENT FIELD REPORTS

Milk yield responses

Milk production increases under controlled conditions reported in 29 field herds ranged from 5.7 to 18.5 pounds per cow per day. Illinois field reports are higher ranging from 10 to 20 pounds per cow per day. "Ramping up" occurred on Illinois field study farms with 38 cows of 121 increasing milk production in the second DHI month test. The cyclic pattern (peaking 3 to 11 days after injecting followed by declines after days 13 and 14) was observed.

Dry matter intake

Feed intake lags milk production response by 3 to 6 weeks after injecting BST. Body reserves or a higher quality ration murt be fed to support higher milk yield. Digestibility and partial efficiencies of nutrient utilization are not altered by BST administration. Illinois field reports parallel research guidelines of 4 to 8 pounds more dry matter intake. One concern is some dairy managers do not know if cows have increased dry matter intake and can not make correct feeding adjustments (increase nutrient levels or change ration ingredients).

Body condition score

One concern was BST-treated cows could become thin due the higher milk yield and lower nutrient intake. Minnesota researchers reported body fat and condition score were lower with BST-treated cows (3.7 for control cows and 3.0 for cows injected). Field reports indicate BST cows are not thinner and maintain weight.

Heat stressed cows

Research studies concluded that untreated and BST-treated cow will respond similarly with lower feed intake and reduced milk yield due to heat stress. Field reports in June by Illinois dairy farmers indicated that BST-treated cow declines in milk were similar to untreated cows. Because BST-treated cows consume more dry matter to support higher milk yield, more heat of digestion must be dissipated. Body condition of BST-treated cows should be monitored under heat stress if milk production is maintained and dry matter declines.

Milk component changes

Composition of milk from cows supplement with BST is similar to control cows if the nutrient needs of the cows are met. If energy is limiting, fat composition can decline while milk protein content will decline if protein needs are not satisfied. In Illinois field herds, milk components did not change. Herd A had lower milk fat for the entire herd, but the lower fat test was in the fresh untreated cows (2.8% fat) compared to the BST-treated cows at (3.1% milk fat). Monitor milk fat to milk protein test ratio to avoid undesirable rumen fermentation and evaluate nutritional status of BST-treated cows.

Cow health and reproduction

No negative field responses have been reported by dairy producers. Somatic cell counts were lower or similar to untreated cows. BST treated cows were later in the lactation cycle which is associated with higher somatic cell counts. Reproduction, foot and leg, and metabolic disorders did not increase in Illinois BST field herds.

FUTURE CONSIDERATIONS

BST technology is a new tool available to Illinois dairy managers. As more field observations occur, recommendations and strategies can be refined. Unfortunately, in Illinois, little discussion occurred which limits knowledge transfer. Several guidelines are listed below.

1. Initial cow selection criteria to use BST was too restrictive: body condition score 3 or higher, cows should be confirmed pregnant, and older cows (2nd and over lactation). Because BST treated cows are not becoming thin, BST treatment can be initiated earlier without negatively affecting health, production, or reproductive performance. Heifers (first lactation cows) are also responding if they are well grown (Holstein heifers weighing 1250 pounds after calving). The following criteria may be useful guidelines.

• Healthy cows (free of mastitis, low somatic cell count, no feet or leg problems, free of metabolic disorders, and no disease).
• Cows should be in positive energy balance (typically 80 to 100 days postpartum) and not thin (<2.5 BCS).
• Heifers should be well grown.

2. Dairy managers should use available milk records to evaluate groups of BST-treated cows to determine if management, environment, or nutritional factors are affecting the herd. BST can be a tool to identify problem areas in herds. List all BST-treated cows in a DHI string which allows the computer to summary monthly production, somatic cell count, days in milk, components, age, or other management patterns. MLM on individual cows prior to BST treatment and after will quickly and easily monitor cow responses. Herd average and test day milk is limited in value, especially if less than half of the herd is injected.

3. If a farm does not use BST technology, the manager may not realize $0.50 to $1.25 per day net profit per cow. Typical daily investment was $0.36 (now $0.42) for BST per day, $0.03 per pound of milk for added feed costs ($0.06 per pound of ration dry matter), and $0.01 to 0.02 per day for labor and miscellaneous costs. For a 12 pound response, the cost would total $0.74 per day with a milk return of $1.44 (12 pounds times $0.12 per pound of milk).

4. Some milk processing plants have asked farms not use BST. Using the example listed above of $0.70 profit per cow per day, a farmer would make $70 per lactation if the cow was injected 100 days (conservative application) or $140 per lactation if cows were injected 200 days (aggressive application). If the herd averages 20,000 pounds of milk (200 hundred weight or cwt), the manager should receive $0.35 per cwt more not to use BST if cows are injected 100 days or $0.70 per cwt if cows are injected 200 days.

5. Dry cow programs must be optimal to insure healthy cows postpartum and achieve desired body condition scores. Weight gains of one pound per day during the dry period is possible if dry cows need to add weight. A one pound weight gain for 60 days represents an increase in body condition score of one half score (for example from 3.0 to 3.5).

Funds to conduct this field study were provided by the Illinois Cooperative Extension Service 1994 Competitive Block Grants.

Table 1. Evaluating milk response in seven Illinois DHI herds after injecting BST.