As the dairy industry changes and science unlocks the mysteries of nutrition, the way cows are fed has evolved.
'It's a different world now. We need to think about feeding groups, not feeding a single cow,' Dr. Bill Weiss, professor of dairy cattle nutrition at The Ohio State University, told listeners during the April 'Hoard's Dairyman' webinar.
The presentation, co hosted by Steve Larson, Hoard's Dairyman, and Dr. Mike Hutjens, University of Illinois, was sponsored by Kuhn (www.KuhnNorthAmerica.com).
There are lots of reasons to separate cows into groups, Weiss said. It's a barn-specific issue that can hinge off production, parity or state of lactation.
'However you decide to group cows will have an effect on formulation because how you group affects the average milk yield of that group and, more importantly, the distribution of milk yields,' Weiss said.
Grouping also affects feed intake. There will be a pen average intake, but assuming a normal distribution of animals with a typical bell-shaped curve, every pen has cows with very low through high requirements.
'You might have cows eating 40 pounds; you might have cows eating 60 pounds. This must be considered in diet formulations,' Weiss pointed out.
From a nutritional point of view, the priority for grouping should be fresh cows separated from later lactation cows. Separating fresh cows boosts nutritional efficiency, Weiss emphasized.
The major factors affecting requirements and formulation are body weight, days in milk, milk yield and milk composition, Weiss pointed out. However, body weight does not have a huge effect on requirements, nor does milk composition or parity (growth).
'These three are not major drivers. They have a small effect so we can use pen averages or bulk tank,' he said. 'Milk yield and days in milk can have a huge effect on formulation.'
However, bigger cows and higher-producing cows have higher requirements, but they generally eat more, so it's a wash.
The big issue is days in milk and the key factor is consumption. 'Days in milk has, essentially, no independent effect on requirements, but this factor has a huge effect on intake,' Weiss underlined.
In early lactation, cows lose the relationship between milk yield requirements and intake. In his example, a cow producing 95 pounds of milk in early lactation was only eating 46 pounds of dry matter. At 110 days in milk, she was producing 95 pounds of milk and eating 55 pounds of DM.
'Think about this: there's a 20 percent difference in intake and the same milk yield,' Weiss pointed out. 'This disconnect between intake and milk yield caused by early lactation is why separating cows by days in milk is so important, nutritionally.'
The first question to ask when formulating diets is if the pen to be fed includes fresh cows, defined as cows three or four weeks in milk. If it does not, take the pen average and multiply by 1.15 or 1.2. 'This is about the maximum milk I'm going to expect from that pen,' he observed.
If the pen is all fresh cows, the formulation goal is 'feed intake, feed intake. I put much more emphasis on feed intake than on meeting requirements,' Weiss said. 'I cannot overemphasize the importance of formulating from intake in a fresh group.'
Formulating for pens that hold only later lactation cows is much easier than mixed pens. Pens with both fresh and later lactation cows is difficult. It is nutritionally inefficient and it can become expensive, Weiss said.
'Because early lactation nutrition has effects on milk peaks, you have to give these fresh cows a lot of weight because if they're not fed right, you are going to lose production the whole lactation,' he pointed out.
Group 'safety factors'
Weiss uses a 'safety factor' (SF) for nutrients, defined as the degree of overfeeding relative to the pen average requirements. For example, if the pen average production is 80 pounds, the diet is formulated for 80 × a SF of 1.1, which would come to 88 pounds. The SF is always greater than one, he noted.
Nutrients should be treated independently. Think about how quickly a cow will respond, production-wise, to lesser amounts of a nutrient, Weiss advised. For example, a cow can be deficient in magnesium for months and never show it, whereas a deficiency in energy will show up very quickly.
Also consider the accuracy of the model being used and the effects of formulating for excess. 'Think about what over-supplying does. For example, you might end up with a bunch of fat cows if you overfeed energy,' Weiss observed.
Another point is whether milk is the driver of the requirement. Minerals, for instance, are not important for milk.
Weiss's No. 1 rule is never formulate for the average cow. 'When you feed the average, you don't get a benefit from all the below average cows, but you do see a negative for all the above average cows,' he pointed out. 'You always feed more. The question is how much more.'
For minerals and vitamins, SF are independent of production or grouping strategy. Depending on the certainty of delivery and absorption, Weiss suggests feeding about 20 percent extra. 'If I have a lot of antagonisms, I might go 50 percent,' he noted.
Rumen degradable protein (RDP) is an extremely important nutrient because it helps drive and optimize rumen production. It is relatively inexpensive and can stimulate dry matter intake, but overfeeding tends to increase urinary nitrogen loss.
Since milk yield does not come into play, Weiss suggested overfeeding by 10 to 10.5 percent.
Metabolizable protein (MP) is important because inadequate amounts will limit milk yield. However, it is a very expensive nutrient, so excessive MP will inflate feed costs, as well as increase nitrogen output.
MP specs for a pen with no fresh cows is the mean plus one standard deviation. If the standard deviation is not known, assume that it's 16 percent of the mean.
For energy (NEL) for groups without fresh cows, Weiss recommends about 1 to 1.1 times average milk.
When fresh cows are involved
In farm situations where the fresh cows can't be separated from high production cows, the safety factor depends on how many fresh cows (less than 25 days in milk) are in the pen.
Assuming 10 percent of a diverse pen is fresh cows, he figured the safety factor for MP-allowable milk bumps up to the mean times 1.24. 'I have to overfeed protein more because of the fresh cows in this pen, but I argue that cost is worth it because I won't lose the peaks,' he explained.
Safety factors for group formulations are affected by the distribution of milk yields within the group, the relationship between feed and milk price, ingredient consistency and feeding management consistency.
'The more confident you are in what is being fed, the less you have to overfeed,' Weiss said. 'Variability increases risk; uncertainty increases risk.'