Weaning young ruminants: Why formulation needs to move away from traditional patterns

By Nicolas Anglade, Category Manager Ruminants, Hamlet Protein

What does an older car engine have in common with the gut of a young ruminant? More than you might think. Both depend on a well‑managed start‑up phase. In my many years of experience, both systems – the car engine and early rumen fermentation – need to be “ignited” to ensure stable and long-term performance.

In this article, I make the argument that milk and starter feed must work together from day one to build a healthier, more efficient rumen. I also challenge some typical misconceptions in the industry – such as the belief that dairy proteins are always safer or that starter feed plays only a secondary role – because we must rethink the formulation to secure the transition.

Weaning, a major physiological turning point that is too often underestimated

Weaning is undoubtedly one of the most important stages in the management of a young ruminant. It involves far more than simply stopping milk feeding; it marks the transition from a monogastric phase, with simple digestion, to the emergence of a functional ruminant capable of efficiently digesting a solid, plant-based diet.

Despite this, milk and solid feeds are still too often approached as two separate nutritional components, when in reality, they are deeply interconnected. In my view, this lack of coordination in feed formulation is a key factor behind the post-weaning growth slump, a clear sign that the transition has not been optimally managed.

From milk to microbial digestion: a delicate metabolic shift

During weaning, the calf transitions from a rapidly digestible liquid diet to solid feeds that require efficient microbial fermentation, a fundamental shift toward the ruminant mode of digestion. This transformation relies initially on proper rumen development, both in terms of volume and the structural growth of rumen papillae, alongside the establishment of a functional microbial population capable of producing volatile fatty acids (VFAs) and microbial protein.

Nutritionally, this transition can be visualized through a simple diagram showing the crossover between two feeding curves: the planned decline in protein intake from the liquid phase (Calf Milk Replacer, or CMR) and the progressive, yet unstructured, increase in intake from the Calf Starter Feed (CSF), which depends entirely on the animal's voluntary consumption.

Figure 1: The nutritional transition around weaning: The crossover between liquid and solid feed intake. Weekly protein intake from milk replacer and solid feed from Weeks 1–13. Liquid protein intake peaks before weaning and declines, while solid‑feed protein intake increases and surpasses milk during the weaning period (Weeks 9-11). Total protein intake remains stable through weaning and rises as solid feed becomes the main source.

The weaning period is the critical window where this transition occurs. If the synchronization between liquid and solid feeds is inadequate, the calf may face a nutritional gap, leading to the all-too-common post-weaning growth slump seen in many field situations. This mismatch between nutritional supply and physiological demand can severely impair the growth trajectory of the young ruminant.

Is your Milk Replacer helping or hindering weaning?

Let’s revisit the flagship feed of a young ruminant’s first weeks of life. Traditionally, milk replacers are formulated using proteins of dairy origin, with skimmed milk powder as the benchmark ingredient. This doctrine ensures good palatability and reassures amino acid balance as well as high digestibility of both protein and energy.

However, such a formulation principle can have unintended downside: the calf becomes too "comfortable" with its milk, showing little motivation to start consuming actively solid feed. This excessive comfort, while initially reassuring, can actually hinder the next stage of development. In reality, it is the intake of starter feed that initiates rumen fermentation and drives the maturation of the digestive system in the transitioning ruminant.

From the field, I often hear the same observation: calves – and lambs – raised on highly dairy-based diets frequently experience a drop in growth and body condition at weaning. The well-known "weaning dip"...

But alternatives do exist. The once-sacred all-dairy formula has already moved away from milk fat, thanks to the now common use of vegetable oils, without major impacts on digestion or performance. This shift benefits both the industry – which reserves cream and butter for human food – and the farmer, who reduces costs on milk replacer formulations.

One promising approach is the controlled introduction of digestible plant proteins, such as enzymatically treated soybean meal (e.g., HP 100), into the milk replacer. This strategy enables a form of “soft vegetalization” designed to support nutritional transition. As shown in the table below, the inclusion of HP 100 in CMR significantly stimulates solid feed intake, according to a recent study on newborn Holstein dairy heifers from the University of Warmia and Mazury.

Graph 1: Intake of calf milk replacer (CMR) and dry feed (Starter feed + forage) of calves fed a CMR based on skimmed milk powder (SMP), a CMR with partial replacement of SMP by HP 100 (HP 100) from day 7 to day 75 (weaning).

Heifers fed with CMR containing 12% of HP 100 replacing 22% of the initial 50% SMP formulation demonstrated a higher appetite to dry feed from the beginning to the end of the trial. This additional dry intake is a powerful lever for successful weaning. Moreover, it comes with a substantial cost advantage: depending on market trends, dairy proteins can be four to six times more expensive than processed plant-based alternatives.

A common ingredient in the CMR and the starter... What if it was the solution to ensure better synchronization of liquid and solid feed at weaning time

Starter feed as the rumen’s ignition system

The starter feed plays a far more strategic role than that of a simple complementary feed: it acts as a rumen activator. Its formulation should therefore not only focus on meeting nutritional requirements, but also on its ability to trigger early and safe ruminal fermentation.

To draw a parallel that I used to use: older engines required a manual choke to start smoothly in cold conditions, a temporary but essential mechanism to ensure stable combustion and prevent engine misfires. While modern engines rely on electronics, the principle remains the same: secure the start-up phase to guarantee long-term performance.

The same applies to young ruminants. Starter feed is the nutritional "ignition system" of the rumen. It must deliver the right substrates, easily fermentable, safe, and well-suited to microbial development, to initiate rumen activity under optimal conditions, without disrupting the animal’s delicate digestive balance.

This start-up phase is decisive: how well the rumen is activated early on will shape the calf’s digestive efficiency, feed conversion, and overall performance in the weeks and months that follow.

Why cheap proteins don’t pay off in young ruminants

In compound feed formulation, the cost of ingredients is still too often prioritized over their intrinsic nutritional quality. Optimization models typically rely on factors like price, availability, and target levels of energy or protein. But this economic logic has its limits, especially for very young animals preparing for weaning. The cheapest option isn’t always the most suitable, particularly for an immature digestive system.

Conventional soybean meal is the most common protein source I encounter in starter feed formulations. Its value lies in its high crude protein content, which also sets a market benchmark. However, when feeding very young ruminants – as with monogastric neonates – we must also consider the anti-nutritional factors that soy contains.

Soybean meal includes around 10% of soluble, rapidly fermentable sugars specific to legumes such as stachyose, raffinose, and sucrose. These sugars are quickly fermented in the young rumen by lactic acid bacteria, leading to an excessive production of lactic acid. This is a key driver of ruminal acidosis, a serious and harmful condition during this sensitive stage.

Expecting the rumen, still in its developmental phase, to handle this metabolic challenge is far from ideal. The calf is already managing several physiological transitions, and any early disruption of rumen function can hinder the proper development of this vital organ. In turn, this affects the animal's future ability to produce volatile fatty acids and microbial protein, both essential for its energy and protein needs as a functional ruminant.

HP 300 in starter feed: A smarter protein choice for early rumen development

The use of advanced plant-based proteins, such as HP 300, offers a valuable alternative in starter feed formulation. Highly digestible and free from anti-nutritional factors, HP 300 supports more efficient nitrogen utilization by the rumen from the earliest stages of development.

A recent study from the University of Illinois highlighted several benefits: enhanced growth of rumen microbial populations, increased microbial protein synthesis, and a marked reduction in non-protein nitrogen transfer to the intestine, as shown in the graph below.

Figure 2 A) Duodenal flow of NPN (g/kg DMI) and B) efficiency of microbial synthesis (microbial N, g/kg CP) as measured at the duodenum of calves fed calf starters with soybean meal (SBM) or HP 300 as main protein source. a,b indicate significant differences (p<0.05).

The data of this trial were published in a peer-reviewed journal: Ansia, I., Stein, H.H., Brøkner, C., Hayes, A., Drackley, J.K. 2021. Nutrient digestibility and endogenous protein losses in the foregut and small intestine of weaned dairy calves fed calf starters with conventional or enzyme-treated soybean meal. Journal of Dairy Science. 104.

This type of formulation is safer and more suitable for young ruminants, whose digestive systems are still maturing and whose immune resources must be preserved for challenges beyond intestinal disorders.

Developing a robust rumen fermentation strategy doesn’t require a complete overhaul of feed formulation. It starts with avoiding poorly digestible proteins or those containing anti-nutritional compounds, such as soybean, rapeseed, sunflower meal, and instead opting for high-quality sources like HP 300. Similarly, replacing fast-fermenting sugars with slower and various starch sources, such as those from cereals, helps maintain digestive balance.

Starting the rumen well means starting the calf well. A well-formulated starter feed goes beyond nutrient delivery; it is a key lever for health, growth, and stability at the critical weaning stage. From day one, prioritizing quality over cost is a smart, long-term investment.

Synchronizing to Secure: Thinking of Formulation as a Whole

This is where everything comes together: liquid and solid feeds must be designed in harmony, following a unified approach to the calf’s nutritional profile. To ensure a smooth transition, it can be helpful to use a milk replacer that is slightly less “comfortable,” encouraging earlier and greater starter intake.

In parallel, incorporating shared ingredients as HP 100 and HP 300 between the milk replacer (CMR) and the starter feed respectively help to ease the handover at weaning. And most importantly, the milk weaning schedule should be adjusted based on actual starter consumption, not just a rigid timeline.

On the ground, I still observe calves or lambs, showing poor appetite for solids, and then being weaned abruptly. The consequences are predictable: growth dips, elevated stress, and preventable health issues. All signs of a transition that wasn’t properly managed.

Breaking old habits in young ruminant nutrition

Why do outdated formulation models persist, despite scientific progress and field observations? The reasons are many: habits, a reluctance to adopt new ideas, insufficient knowledge transfer, etc. But also the fact that CMR and starter feeds are often developed by different companies, with little integration between their approaches.

In my opinion, several persistent preconceived ideas deserve to be challenged:

"Only dairy proteins are safe for calves and lambs."

→ Not true: carefully selected plant proteins can be both highly digestible and safe.

"Starter feed is secondary."

→ On the contrary: it is a central driver of rumen development, and its formulation requires just as much attention as liquid feeding.

"Formulating (starter feed) at the lowest cost is the priority."

→ This short-term view overlooks the fact that not all proteins are equal in bioavailability or ruminal impact. A well-managed weaning phase brings long-term economic benefits throughout the animal’s productive life.

Challenging these paradigms requires collective commitment, from formulators, technicians, and farmers alike, to rethink and coordinate nutritional strategies from the very first days.

Formulating shapes a ruminant’s future from the first weeks

Formulation is not just a matter of mathematical optimization. It is a strategic decision that influences the animal’s entire trajectory. When thoughtfully designed, it enables not only a successful weaning but also the development of a functional, resilient, and productive rumen and ruminant.

Formulating better means challenging outdated habits, embracing innovation, and thinking in terms of transition, not rupture. It involves selecting the right ingredients, adjusting strategies dynamically, and always keeping the long-term perspective in mind.

Because weaning is not the end… It’s the beginning!