Overview of the new NRC nutrient requirements of small ruminants (Proceedings)

Twenty-five pages of numbers

The new Small Ruminant NRC is a big book with lots of features. My background as a university professor suggests that I have a tendency to, ahem, pontificate — to go on and on and on and on. But I will gallantly refrain from such grandiloquent oratory, even though, frankly, parts of this edition almost beg for it. Instead, I'll be succinct. This month, I'll focus on the real heart of this book — the reference tables listing the nutrient requirements for sheep. I'll use a broad brush to describe the big things, so you'll know where to look when you try to balance a diet.

The reference tables of sheep nutrient requirements now take up 25 pages, plus another 23 pages of reference tables for feedstuff composition. Whew! Compare this to the 1985 Sheep NRC edition, which listed its nutrient requirements on just seven pages. And the difference is not a larger font. The tables in this new edition definitely contain more items and more categories.

An interesting aside: this section of the book is called "Nutrient Requirements of Sheep Tables." I kid you not. At first I thought this was a new scientific breakthrough. I didn't know that sheep tables had nutrient requirements. Were the authors referring to specialized ovine turning cradles or just routine shearing tables? The wooden tables in my shop don't seem to have nutrient requirements. But what do I know.

Anyway, back to the numbers. Whenever you balance an animal's diet, you must first look up its nutrient requirements — based on the type of animal you're trying to feed, its stage of production, its level of production, its body weight, and perhaps some other minor factors. For the broad animal categories, this NRC lists requirements for mature ewes and rams, replacement ewes, and growing/finishing lambs.

Let's first discuss mature ewes. Reference table 15-1 divides the annual production cycle of a ewe into logical periods, similar to previous editions, but some details are critically different. The new production periods are maintenance, breeding, early gestation, late gestation, early lactation, mid-lactation, and late lactation. The word breeding here means flushing, with the nutrient requirements during this period increased 10% over maintenance. The major change in the production cycle, however, is that lactation is now split into three periods rather than just early and late. The new NRC defines early lactation as the first 6 weeks after lambing, mid-lactation as weeks 7–12, and late lactation as beyond week 12.

Another important change: this NRC edition includes requirements for ewes with singles, twins, and triplets, for all of gestation and lactation. The previous edition contained no requirements for triplets. (Don't get me wrong. It's not that ewes raising triplets had no nutrient requirements; it's just that we didn't know enough to list any numbers). The new reference tables list gestation and lactation for ewes with "Three or more lambs." I'm comfortable with three, but the phrase "or more" may be a bit optimistic.

As a new feature for the dairy sheep industry, the lactation requirements now include sections for ewes in milking parlors — for all three stages of lactation.

The new NRC has also adapted to other recent changes in the sheep industry: the range of ewe weights is now much wider than in previous editions. The 1985 NRC listed mature ewes from 50–90 kg (110–198 lb). This new NRC lists ewes from 40–140 kg (88–308 lb). In recognition, no doubt, of the many flocks with smaller hair sheep breeds and also of those monsters I would never want to shear but undoubtedly someone does. Or possibly reflecting some Columbia-Bison crossbreeding trials that I have not yet heard about.

The NRC now includes a section on ram requirements, which is good. The ram production cycle is simpler than ewes: just maintenance and prebreeding. Either you are letting rams wander around the field doing nothing (maintenance), or you are pumping them up in preparation for their work period (prebreeding). Of course, most rams also require extra nutrition during their post-breeding recovery period, but this NRC lists no numbers for it. Which actually makes a lot of sense because rams exhibit so much individual variation in their level of emaciation by the end of the breeding season.

The requirements for young growing and finishing lambs are relatively straightforward. Table 15-2 contains different categories for early-maturing and late-maturing lambs, and for different ages and rates of gain. This table also lists separate requirements for ram lambs, because the NRC committee judged that ram lambs need slightly more energy for maintenance than ewe and wether lambs.

Now, however, we come to replacement ewes and ewe lambs. The new NRC tables are extensive but somewhat confusing. I've studied these tables carefully, and here's my interpretation:

The NRC essentially describes three types of replacement ewe lambs: (1) farm flock ewes first bred as ewe lambs, (2) farm flock ewes first bred as yearlings, and (3) range flock ewes first bred as yearlings.

For farm flock ewes first bred as ewe lambs (to lamb at 12 months of age), their few months of growth after weaning are covered in Table 15-2 in the section on "Growing Lambs and Yearlings." Once they are exposed to rams at 35 weeks of age, their requirements are listed in Table 15-2 in the breeding and gestation sections under "Yearling Farm Ewes." After they lamb, their lactation requirements are listed in Table 15-1 in the lactation sections under "Yearling Farm Ewes." Then after they wean their lambs, but during the 19 weeks before they are put with a ram again, their requirements are listed in Table 15-2 in the "Maintenance Plus Growth" section under "Yearling Farm Ewes." Finally, when the ram is put back into the flock, their requirements are listed in Table 15-2 in the breeding and gestation sections for "Yearling Farm Ewes".

For farm flock ewes not bred as ewe lambs (so therefore they will first lamb at two years of age), their entire first year of postweaning growth is listed in Table 15-2 in the section on "Growing Lambs and Yearlings." Let's skip to the end of the second year: breeding and gestation are covered in Table 15-2 under the breeding and gestation sections for "Yearling Farm Ewes." The middle period — as old croppers after their first birthday but before they are bred — contains some months of growth, and therefore those requirements are listed in Table 15-2 under "Maintenance Plus Growth" and some months of simple maintenance while waiting for the ram. Those months are covered in Table 15-1 in the "Maintenance Only"section under "Yearling Farm Ewes."

For replacement range flock ewes, the new NRC simply doesn't believe that they will be bred as ewe lambs. The requirements for their entire first year after weaning are listed in the section for "Growing Lambs and Yearlings" in Table 15-2. The word "Yearlings" in the title suggests that these requirements also apply to these range ewe lambs until they are exposed to a ram at 18 months of age. After that, their requirements are listed in Table 15-2 in the breeding and gestation sections under "Yearling Range Ewes." Once they lamb at 24 months, their lactation requirements are listed in Table 15-1 in the lactation sections for "Yearling Range Ewes" (even though they are really older than two years of age).

Then again, if these range ewe lambs are not bred until they are older than 24 months, the requirements for their second year are listed in Table 15-2 under "Maintenance Plus Growth," or if they have reached their mature size, in Table 15-1 under "Maintenance Only." Then in their third year, once they are put with a ram, all their requirements for breeding and gestation are listed in Table 15-1 under "Mature Ewes".

Who's on first?

Energy and protein, new

Let's continue our description of the sheep nutrient requirements in the new Small Ruminant NRC book. This month, I'll concentrate on the reference values for energy and protein.

Although this NRC book is so important to the industry, a funny thing is that very few people will actually read it. Or even own it. At 362 pages retailing for $116, not many folks will give it as a wedding present, and it probably won't be a best seller on Amazon.com. Another practical reason is that all its numbers are in metric. In the scientific community, the metric system rules, and all weights are in kg, g, mg, etc., and energy values are in calories, kcal, Mcal (kilo = thousand calories, M = mega = million calories) or even the cryptic megajoules per kilogram (MJ/kg). But on a working American sheep farm, how exactly would the shepherd feed 14 megajoules of alfalfa to a 70 kg ewe? So it will be up to nutritionists in feed companies, universities, and government offices to read this book carefully and translate its values into pounds per day or percentages of dry matter or square light-years per gallon.

Speaking of energy, the NRC reference tables list the energy values of feeds and the energy requirements of sheep in terms of ME — Metabolizable Energy. What is this ME, how does it relate to something we know, like TDN (Total Digestible Nutrients), and why does the NRC use it?

Some basic nutrition here. Feed energy is expressed in calories per kg of dry matter, and the different energy terms reflect the successive energy losses that occur as feed and nutrients move from the feedbunk through the digestive tract to the cellular level. The initial energy value of a feed before it goes into an animal's mouth is called the Gross Energy (GE). That's the feedbunk value. After a feed is consumed, the undigested portion is excreted in the manure. The GE minus the energy lost in the manure is called the Digestible Energy (DE) of the feed. This DE term is roughly equivalent to TDN, as we'll see below. To continue the digestion process, some energy is then lost as gas (primarily from the rumen) and in the urine. DE minus the gas losses and urine losses is called Metabolizable Energy (ME), which is the amount of energy that actually reaches the cells. But as cells use this ME, some energy is lost as heat. ME minus the heat loss is called Net Energy (NE). It's this NE that is finally used by cells to do metabolic work such as maintenance, muscular activity, and the manufacture of tissues like muscle, bone, milk, and wool. I'll summarize this sequence: a feedstuff contains GE which loses fecal energy to become DE which loses gas and urine energy to become ME which loses heat energy to become NE.

Historically, American livestock books described feedstuffs and nutrient requirements on the level of DE, as this most closely relates to TDN and is also easily measurable (just by collecting manure). But much of the world currently uses ME, particularly in the UK, Australia, and New Zealand, so this NRC committee shifted its terminology from DE to ME. The most precise energy term, NE, is very hard to measure, so nutritionists generally feel more comfortable on the ME level. Although the new NRC does indeed list NE values for growing lambs, things about NE are very complex and questionable, and I won't go into them here.

How does ME relate to TDN? Although ME and TDN don't measure the same thing, we can still interconvert them by using a couple of rule-of -thumb assumptions and a physical constant. One rule-of-thumb is that 1.0 lb of TDN equals approximately 2,000 kcal of DE. Another is that the sum of urine and gas losses equals 18% of DE, which means that ME is generally 82% of DE. The physical constant is that 1.0 kg equals 2.2 lbs. Here's an example from the first row of Table 15-1: a 40 kg ewe at maintenance needs 0.41 kg of TDN per day. Multiplying 0.41 x 2.2 x 2,000 x 0.82 and then dividing by 1,000 (to convert kcal to Mcal) gives an energy requirement of 1.48 Mcal ME per day, which is exactly the number listed in the table.

This new NRC also refers to feeds with 8, 10, and 12 MJ of ME/kg. Whoa! What's this? Well, many countries describe their feeds, especially forages, in terms of MJ/kg, so this NRC committee has decided to include those values in the book, perhaps with the hope that we'll all someday travel to New Zealand. The trick of converting joules, calories, and TDN is knowing the straight-ahead equivalency that 1.0 calorie equals 4.184 joules. Therefore, 1.0 Mcal ME equals 4.184 MJ ME. Here's an example: If a feed contains 66% TDN, I can start with this value (0.66 lb TDN per 1.0 lb of dry matter) and then multiply by all these factors to come up with a result of 10MJ/kg (I won't list all the arithmetic here — this is your homework assignment). Therefore, feeds containing 8, 10, and 12 MJ of ME/kg equate to 1.91, 2.39, and 2.87 Mcal ME/kg respectively, which equate to TDN values of 53.0%, 66.2%, and 79.5% respectively. Why do I even mention this? Because the fourth column of each requirement table lists dietary energy concentrations with these same weird numbers — 1.91, 2.39, or 2.87 Mcal ME/kg.

Let's switch to protein values, which may seem even more peculiar to most readers.

Most of us are comfortable with the concept of Crude Protein. We've used it forever as part of our daily vocabulary — buying and selling feeds, judging forages, and formulating rations. But this new NRC doesn't use Crude Protein, at least not directly. Instead, it bases its requirements on a number called Metabolizable Protein (MP). MP is actually a better way of describing the complex reality of protein nutrition, but most folks are probably not familiar with it.

Metabolizable protein refers to the amount of protein actually absorbed across the gut wall. If we think about it, the concepts of crude protein and metabolizable protein are roughly analogous to the concepts of gross energy and digestible energy. The first concept of each pair represents the nutrient amount in the feed when it first enters the mouth; the second concept in each pair refers to the nutrient amount actually moving from the digestive system into the blood. (Okay — for those of you who have a lot of knowledge in animal nutrition, you'll recognize that this is not a perfect match, but it is close).

I'll be brief here: the crude protein in feed is composed of two parts: the portion that the rumen microbes can degrade and the portion which passes through the rumen relatively unscathed because the rumen bugs can't degrade it. The first portion is called Degradable Intake Protein (DIP) and the second portion is called Undegradable Intake Protein (UIP). This second portion is also commonly known as Bypass Protein, but scientists tend to prefer the acronym UIP (possibly because it sounds more scientific). When degradable protein enters the rumen, things get rather complicated, but in the end, some DIP is lost to the animal because of microbial action, but most DIP is still available for absorption downstream in the lower tract. In contrast, the UIP is more efficient than the DIP because it "bypasses" the rumen and goes directly to the lower tract for absorption. MP is the sum of the amounts of DIP and UIP that are actually absorbed across the gut wall into the blood.

We need this background because the nutrient requirement tables in the new Small Ruminant NRC contain three columns for protein requirements, not just one as in previous editions. These three columns differ in the percentage of UIP in total amount of dietary protein — 20%, 40%, or 60%. The higher the percentage of UIP, the more efficiently the crude protein is digested and the less total amount is needed in the feed. For example, a 70-kg mature ewe in late gestation carrying triplets needs 222 g/day of crude protein if her diet contains 20% UIP, 212 g if her diet contains 40% UIP, and only 203 g if her diet contains 60% UIP, while her MP requirement is simply 149 g.

To use these columns for ration-balancing, we have to estimate the amount of UIP in the diet. Values for UIP in feedstuffs can be found in this book's main feed ingredient table (Table 15-12) and also other current textbooks and nutritional references. As a practical matter, however, as I look across all sheep categories, I notice that crude protein requirements seem to decrease by only approximately 10% as the percentage of dietary UIP rises from 20% to 60%.

These NRC reference tables also list requirements for MP and DIP. Although the DIP number is not very useful — it's just listed for reference — the MP number is the true basis for all the other protein requirements. The value for MP was calculated from underlying equations that add up the amounts of protein actually used for tissue growth and replacement. The NRC uses a simple formula to convert MP to CP, based on the percentage of UIP in the diet. It's clear that new ration-balancing software will use all these numbers to formulate rations, rather than just the traditional crude protein values of yesteryear.