Everyone likes numbers. We track football scores, market reports, medical charts and election results (well, sometimes, maybe). In the sheep world, we have our favorites, such as lambing rate, average daily gain (ADG) and, occasionally, the amount of profit. A few years ago, I wrote a column about some intriguing variations of the definition of lambing rate. But, in the end, numbers are just numbers – scratchings on paper and screen – unless they’re used. Their real value is what these numbers mean to your operation and how to use them to make management decisions. As I write this, we’re nearing the lambing season, so let’s explore the ins and outs of some important lambing numbers.
But, before we start calculating lambing percentages and other ratios, we need to collect the raw production numbers that go into those percentages. Here are some critical numbers to record: number of ewes exposed, number of ewes lambing, dates of first and last lambing, number of live lambs at birth, number of lambs born dead, number of lambs raised on milk replacer and number of lambs weaned. We won’t concern ourselves in this article about the lambs sold after weaning because each flock is so different with its own replacement rate, age structure, marketing plan, etc. Instead, we’ll focus entirely on the breeding/lambing/weaning numbers.
Let’s create a hypothetical 200-ewe flock with some hypothetical numbers. For simplicity, let’s say that all our ewes are adults (before this article, we cleverly shipped all our ewe lambs to a friend in southern California, so they’ve become his problem).
Total number of ewes exposed = 200
Number of ewes lambing = 190
Date of first lambing = March 1
Date of last lambing = April 25
Number of live lambs at birth = 360
Number of lambs born dead = 25
Number of lambs raised on milk replacer = 55
Number of milk replacer lambs that died before weaning = 6
Total number of lambs weaned = 327
These are just raw numbers. Now, let’s put them into useful percentages. (I’ll use the forward slash to indicate division because the classic divisor symbol doesn’t always appear properly in print.)
The first and most obvious percentage is lambing rate. Let’s use this definition: [total number of live and dead lambs at birth] / [number of ewes lambing]. The total number of lambs born = 385 (= 360 + 25). Therefore, for our hypothetical flock, the lambing rate = 202.6% (= 385/190).
Not bad. But, we should recognize that a 200% lambing rate means that some ewes dropped triplets and a few may have dropped quads. That’s good, but it also means that some ewes dropped singles. This last point is important. If we culled those ewes, our lambing rate would automatically go up.
Here’s an important ratio: % ewes not lambing = 5% (= 10/200). The question is: What happened to those ewes? If we can identify what happened, then we’ll have insights into managing the breeding and gestation periods and maybe the period prior to breeding. And here’s the logical follow-up question: Are these ewes still in the flock? If so, why?
Length of the lambing period = 56 days. This time period equates to three estrous cycles, plus some slack to allow for some biological variation (not all ewes conceive on the first day). Hmm. This is a long lambing period for a flock this size. Apparently, some ewes did not conceive until their third cycle. Was there enough ram power? Were the fields too hilly or large or variable during breeding so the rams couldn’t find the ewes at the right time? Were any rams not functioning properly, either because of poor semen quality or because they didn’t like ewes? (Yes, this can definitely be a problem.) Were the rams exposed to extremely hot weather three to six weeks prior to breeding?
Percent of live lambs at birth = 93.5% (= 360/385). This is the proportion of all lambs who came through the lambing process alive. Conversely, we can calculate % lambs born dead = 6.5% (= 25/385). These include the pulled dead fetuses and also lambs that died during the lambing process.
Can we determine what happened to the ewes and embryos before lambing, during gestation? That’s a bit of a black box; our numbers don’t show these details. Any fetal losses during this period are part of the 5% of ewes that did not lamb. We don’t know exactly how many ewes never cycled or how many ewes successfully conceived embryos but lost them during gestation. There are issues such as embryo loss during very early gestation, abortions, etc., but with our raw numbers, we have no way of knowing those details. If we wanted to investigate this further, we could begin by using marking harnesses during the next breeding season.
Now, let’s look at the numbers after lambing. The number of lambs that died between birth and weaning = 33 (= 360-327). But, this number can be misleading because it’s composed of two independent values – one for ewe-raised lambs and one for artificially raised lambs – so we need to parse it into meaningful numbers. Most lambs were raised on their ewes, but 55 lambs were raised artificially. Let’s say that six of those orphan lambs didn’t make it to weaning, which represents a death loss of 10.9% of the artificially raised lambs (= 6/55). This calculation also gives us the number of ewe-raised lambs that died, which equals 27, not 33.
I’ll add a few words about the orphan lamb management. Those 55 orphan lambs represent 28.9% of our 190 ewes who lambed with a lambing rate of 202.6%. The 9% loss of orphans prior to weaning is not bad, but better management could probably reduce this loss. Let’s round these numbers for ease of use. We are looking at an orphan rate of approximately 30% for a flock with a 200% lambing rate. If we have a large commercial flock and use these numbers to help plan for lambing, we can prepare good facilities in advance for a 30% orphan rate. This is good management and good business.
Percent of ewe-raised lambs lost between birth and weaning = 8.9% (= 27/305). These are the lambs that didn’t make it to weaning, even though they were raised on their ewes. (Note: 305 lambs raised on the ewes = 360-55). This percentage primarily reflects mothering ability and perhaps a dash of coyotes. If the flock was confined indoors or around a barn during this period, then we should also factor in the possibility of a contagious disease such as scours or pneumonia. (Although, coccidiosis can also occur on pasture.) Rather than focusing on losses, some folks may be more comfortable expressing this concept as % ewe-raised lambs weaned = 91.1% (= 278/305).
Now, we can calculate a number that is financially critical: total number of live lambs who made it from birth to weaning = 327. We can also look at the success of getting these animals to weaning: weaned lambs as a % of all lambs born alive = 90.8% (= 327/360). This takes into account every lamb weaned, whether raised on its ewe or raised as an orphan. We can also relate these 327 weaned lambs back to the ewes that lambed by calculating the weaning rate = 172.1% (= 327/190). Recall, however, that this number includes lambs of two flavors and doesn’t necessarily reflect mothering ability as well as the 91.1% value calculated in the previous paragraph.
Whew, that’s lots of numbers! If your eyes are slightly glazed over, well, you’re not alone. That’s why we have spreadsheets. You should design a spreadsheet to do these calculations automatically. Or somehow get one of your friends to design a spreadsheet. Trade a lamb or two for the service. Then the hypothetical flock stops being hypothetical; it’s your flock. You can then input the original raw numbers every year, step back and gaze admiringly at the results.
But the numbers and percentages by themselves are not the endpoints; we need to use them. If this hypothetical flock belonged to a hypothetical client, here are some of the things I would recommend:
- Focus on only a couple of things; don’t try to fix everything.
- Get rid of those open ewes. They’re not your best animals.
- Plan to raise orphan lambs as a high percentage of the number of ewes.
- That long lambing period can be shortened. Figure out what happened, then make some changes. If necessary, cull the ewes that waited too long to lamb. Again, they’re not your best animals.
If you shorten the lambing period by two weeks, you can treat yourself to a well-earned vacation, finally. Two weeks saved from lambing can translate into two weeks on a warm tropical island with palm trees, a blue lagoon and long, white-sand beaches. You can set up a hammock in the shade of one of those palm trees, pull out your laptop and look at the spreadsheet. Yes, numbers can really improve your management.