When you are creating a closed plan and herd size varies dramatically, you might consider using a more refined method for adjusting the grazing periods than that suggested in Step 15 of the Aide Memoire. This method involves a number of calculations. To start, you determine a herd size adjustment factor, which is based on the difference between average and actual herd sizes in both the nongrowth and drought reserve periods. Then you adjust the grazing periods plotted in Step 14 based on the herd size adjustment factor:
1. Calculate the average herd size for the nongrowing months and then for the drought reserve period:
• Multiply the number of SAUs in the herd (row 34) by the number of days the herd remains the same size (SAUs × days of same herd size). Record the resulting AD figure in brackets in row 34. When herd size changes, do the same thing again, and continue doing so until the end of the planned period (nongrowth or drought reserve).
• Add up all the AD figures and divide by the total days in the planned period. This gives you the average herd size. Record this figure in the “Remarks” section.
Example: Nongrowing Months. Assume the planned period is 200 days. Herd size during the first 60 days is projected to be 450 SAU (thus 450 × 60 = 27,000 ADs); then calves are weaned, dry cows are culled, and herd size is reduced to 300 SAU for 45 days (13,500 ADs); then yearling heifers are added to the herd, and size goes back up to 375 SAU for 95 days (35,625 ADs). Total ADs accumulated is therefore 76,125. That figure divided by 200 (the total days in the planned period) gives an average herd size of 381 SAU:
Example: Drought Reserve Period. Assume the drought reserve is 60 days. Herd size during the first 15 days is projected to be 375 SAU (5,625 ADs) but will increase to 450 SAU during the next 30 days after 75 pregnant cows are bought (13,500 ADs). It will increase again, to 500 SAU over the last 15 days as calves start to come (7,500 ADs), for a total of 26,625 ADs. That figure divided by 60 (the total days in the drought reserve period) gives an average herd size of 444 SAU.
2. Calculate an adjustment factor for each different herd size by dividing the average herd size by the actual herd size:
Example: Nongrowing Months. If the average herd size is 381 SAU, then in the first 60-day period, when actual herd size is 450 SAU, the adjustment factor would be 0.9 (381 ÷ 450). In the next 45-day period, when herd size decreases to 300 SAU, the factor becomes 1.3 (381 ÷ 300). In the final 95-day period, when herd size increases to 375 SAU, no adjustment is required (381 ÷ 375 = 1).
Example: Drought Reserve Period. If the average herd size is 444 SAU, then in the first 15-day period, when actual herd size is 375 SAU, the adjustment factor would be 1.2 (444 ÷ 375). In the next 30-day period, when herd size increases to 450 SAU, no adjustment is required (444 ÷ 450 = 1). In the last 15 days, when herd size increases to 500 SAU, the factor is 0.9 (444 ÷ 500). Obviously, the greater the fluctuation in herd numbers, the greater the range of adjustment factors.
3. Record the herd size adjustment factors (in brackets) to the right of the herd size figures in row 25. Then multiply each of those figures by the actual grazing period in column 4 (or the drought reserve column to its right). Round off the total to the nearest whole day (or half day) and pencil in the adjusted grazing period line. Occasionally, herd size will change during a planned grazing period in an individual paddock. That is especially common when you have few paddocks and therefore long grazing periods. The following procedure should be used when that happens, especially if herd size is going to change dramatically and if the calculated actual grazing period is very long.
4. Calculate how many ADs would be harvested from the paddock based on the average herd size. For example, assume the actual grazing period recorded in column 4 is 25 days and the average herd size for the planned period is 300. That would result in 7,500 ADs harvested with the average herd size. Make a note of that number so you don’t forget it.
5. Next, calculate how many ADs will be accumulated with the actual herd size up to the time the herd size changes. For example, if herd size is 200 SAU in the first 10 days (of a 25-day grazing period), total ADs harvested would be 2,000. That leaves 5,500 ADs in that paddock for the remainder of the grazing period (7,500 – 2,000). If herd size after day ten is planned to increase to 280 SAU, then there are 20 days of grazing left in the paddock (5,500 ADs divided by 280 SAU). That means the grazing period can be extended from 25 to 30 days (10 + 20), which you should pencil in on the chart.