Underperforming animals in a paddock full of feed

Nick Linden, Agriculture Victoria, Rutherglen

In many cases, the highlight of 2020 was the exceptional pasture growth from autumn all the way through to spring – coupled with high stock prices. It’s been a remarkable year for agricultural production. Currently, many farms have varying amounts of carry-over dry feed or even heavy stubbles. Yet the challenge remains of how to get the best animal production from these dry feed resources.

When assessing what’s driving animal performance (either good or bad) once animal health issues have been accounted for, we need to consider the suitability of the feed on offer. As with any feed budgeting question, there are initially two important considerations:

  • what you have
  • what your animals need.

Matching the two together becomes critical to getting the best out of your livestock.

The ‘what you have’ with dry feed is best described by looking at what you don’t have – and that is essentially protein. Protein in pastures (and hay and silage, for that matter) comes from nitrogen. When a feed test calculates the crude protein percentage (CP) of a sample, it tests how much nitrogen the sample contains. The nitrogen is held in the chlorophyll of plants. Chlorophyll is what captures sunlight and converts it into the carbohydrates that build plant growth. It’s also what makes pastures green – when we put nitrogen onto pastures, we are essentially boosting the chlorophyll levels, and so pastures typically become a darker shade of green following an application of nitrogen. When we see plants becoming less green, as in summer – we can safely assume that the nitrogen levels in the pasture are declining. Hence, our dry summer feeds are low in nitrogen, which equates to also being low in protein.

Now for some stock classes that’s not going to matter. Dry ewes post-joining only have a low protein requirement – six to eight per cent crude protein (as per the energy and protein requirements in livestock tables from Agriculture Victoria’s drought feeding books). If you have dry ewes running on dry pastures, these pastures are likely to meet the protein requirements for that class of stock. However, the energy requirements of the ewes may be a different story – especially by late summer when the digestibility of the pastures drops down to near 50%. Under these circumstances the energy requirements of the ewes may not be being met.

It’s worth remembering what protein is used for – two key demands are lactation and growth. So, it’s logical that the classes of stock that require higher levels of protein (nitrogen) are ewes in late pregnancy, ewes with lambs at foot and young stock (especially weaners). These stock classes require eight to 10 per cent, 12 to 14 per cent and 15 to 18 per cent crude protein respectively, in their diets to meet their protein requirements. A feed test of the dry summer feed is likely to give a result of around seven to eight per cent crude protein. At the end of spring and the start of summer, it’s likely that protein levels would have been higher – indeed, sheep are quite good at selecting out various parts of the pasture that will meet their requirements, so in early summer the crude protein levels of the diet would be higher as animals consume some clover burr and higher-quality parts of the pasture. By mid- to late summer, many of the higher protein components of the pasture will have been utilised, leaving the lower-quality materials. This decline in pasture quality can be a real issue for stock classes that have an increasing need for protein over this period. Summer rains may lead to a green pick in some paddocks, and this will be high in protein and often supply enough protein for most classes of sheep.

Although protein supplementation can help the animal utilise more of the energy in the dry feed (as cellulose), the need to supplement energy becomes more important as the amount of dry feed gets low and/or there is a green pick.

High-protein grains such as faba beans and lupins are an excellent means of supplementing protein to animals that need it. These provide a safe and cost-effective way of supplementing protein, although their availability has varied.

An additional means of supplementing dietary protein doesn’t involve feeding a form of protein at all. In this case, stock are fed a concentrated form of nitrogen – the most common being urea, often referred to by ruminant nutritionists as nonprotein nitrogen. Feeding urea has some potential risks to animal health and must be undertaken with a degree of caution. Using ‘dry feed blocks’ that contain a certain percentage of urea (often 10 per cent of the block weight) may be a lower-risk option, but if rainwater collects in the depression of the block, this water can be highly concentrated with urea and toxic. In some cases, urea will be added directly to a grain-based ration. There are also commercially available liquid supplements that contain set percentages of urea – all aimed at increasing daily nitrogen and dry feed intakes – and other additives such as molasses to improve palatability. Whatever the form of the urea (or nonprotein nitrogen) supplement, the mechanism remains the same – they don’t magically enable animals to squeeze more energy out of a given mouthful of feed, but they provide an energy source for the bacteria living in the animal’s rumen. This enables the population of rumen bacteria to increase, ultimately increasing the volume of feed that animals are able to consume. Hence, there is little point offering a urea-based supplement if there isn’t adequate feed on offer to support an increased feed intake. The increase in rumen bacteria leads to an increase in ‘microbial protein’ – which is the protein made available to the animal through the digestion of dead and decaying rumen bacteria, a highly important source of protein to ruminants. Research is currently looking at the value of canola meal to stock as a provider of bypass protein (protein that bypasses the rumen bacteria and can be used directly by the animal), but the implications, results and concepts of bypass protein are another story – for the next edition!

Although the use of urea blocks and liquid supplements may prove to be a convenient way of increasing nitrogen intakes of protein-deficient stock, it is always worth costing out the alternative ways this could be achieved (including canola meal, lupins and/or faba beans). Steve Cotton, from Dynamic Ag in South-West Victoria, has put together a three-page information note on the benefits of using lick blocks over summer (see the link under ‘References’) – for those about to load up their paddocks with blocks, it’s worth a read.

Protein also plays an important role in the animal’s immune function, which is tied to their ability to withstand infection from internal parasites such as black scour worms. As a late pregnant ewe prioritises her nutrient partitioning to the survival of her lambs, she potentially reduces her immune capacity, leaving herself exposed to parasitic infection. It is a similar scenario for weaners that allocate scarce resources to growth and development, at the expense of immune capacity. When deliberately infected with a known parasite load, weaner merino lambs supplemented with protein had significantly lower production losses and increased expulsion of the parasitic burden (see the second link under ‘References’).

The question remains for late pregnant ewes grazing on dry summer pastures: What allocation of faba beans (or other protein source) would they need to achieve the desired crude protein? A useful way to calculate this is to use a Pearson square, which calculates the ratio of two known feeds to achieve a desired level of feed. Figure 1 provides an example where we have dry feed that is seven per cent CP and faba beans that are 25 per cent CP. We put the target protein level in the ‘middle’ of the square and then calculate the difference between the two (working in a diagonal line).

Pearson square example showing that for every 18 units of a ration, two units is required of faba beans and 16 units of dry pasture to give a diet with a crude protein of 9 per cent as explained in the text below.

For every 18 units of the feed that are consumed, if two units are faba beans and 16 are dry pasture, the ewes will receive a diet that is nine per cent CP. It’s easiest to express this as a percentage of each component:

  • (2 units of faba beans/18 total units of feed) × 100 = 11% faba beans
  • (16 units of dry feed/18 total units of feed) × 100 = 88% dry feed

That’s close enough to call it as 10 per cent faba beans that will achieve the target protein percentage. Although the total amount of feed that the ewes will consume will depend on the size of the ewes, as well as the quality and quantity of feed, for ease of calculation, let’s assume that the ewes are consuming 2 kg of dry feed each day – 10 per cent of the ration as faba beans would equate to 200 g/head/day. As the ewes pass the point of lambing, their protein requirement will lift – up from nine per cent CP pre-lambing to 13 per cent CP. We could use the same method to calculate the adjusted mix of faba beans to dry feed to give the desired level of protein in the diet. Although some people like to be able to do their own ‘back of the envelope’ calculations, there are some good tools to do this, such as the NSW Drought and Supplementary Feed Calculator. This is a brilliant app or web-based tool that will do the same calculations, can include more than one supplement, and includes stock requirements to estimate cost-effective rations. See the article ‘Resources for assessing feed and animal requirements over summer’ in this newsletter for more information on these (including the Pearson square).

Key points

  • Dry feed with no green pick will be limiting in protein for any animals that are growing or reproducing.
  • Deciding on the best source of protein will often come down to costing out suitable sources of protein on a per unit basis.
  • Dry feed is often adequate for dry animals, and a green pick will often provide adequate protein for most sheep classes.
  • Making sure that the protein levels are matched to the requirements for key stock classes such as weaners and late pregnant ewes can be a critical component of managing stock on abundant amounts of dry standing feed.
  • When dry feed is limiting and/or there is access to a small amount of green pick, energy will be the prime requirement and adding excess protein would be wasted.

References

Page last updated: 26 Mar 2021