Beef cows are resilient animals. They are programmed to adjust in real time to the environment they experience. Some cows respond better than others, and this is often referred to as “adaptability.”
The early lactation period represents the most energetically expensive phase of the annual production cycle. For perspective, during the last trimester, a 1,300-pound cow in good body condition requires about 12.7 megacalories of net energy for maintenance (Mcal NEm) to support her pregnancy and maintain her own body weight.
Compare that to the same cow producing 24 lbs. of milk during peak lactation at 19.3 Mcal NEm per day. Interestingly, there is one set of spring-calving cows at Oklahoma State University (OSU) that typically give 30 lbs. of milk at peak lactation.
Consider that each kilogram of milk contains about 0.33 Mcal NEm. For perspective, grass hay at 56 percent total digestible nutrients (TDN) contains 0.54 Mcal NEm per pound (dry matter basis). Lush spring forage at 68 percent TDN contains 0.71 Mcal NEm per pound of dry matter.
Therefore, if the genetic capacity for milk production was increased from 24 lbs. to 25 lbs., the cow would need to consume about 0.6 lbs. more low-quality forage, or 0.46 lbs. more high-quality forage to keep from losing weight. While the relationship between milk yield and forage intake in beef cows is not well understood, recent work suggests that for each 1 lb. increase in milk production, low-quality forage intake increases by around 0.25 lbs., and high-quality forage intake increases by around .35 lbs.
This is likely due to the limited capacity of the rumen. Cows can only eat so much forage in a day. When they are already consuming forage at capacity, increased energy requirements will be difficult to meet unless the energy concentration in the diet is increased.
With a 0.2 lb. increase in hay intake, only an additional 0.14 Mcal NEm is available to offset the increased energy requirement of 0.33 Mcal for the additional pound of milk. The mismatch is not quite as dramatic with the lush forage because daily energy intake can be increased by about 0.25 Mcal (0.35*0.71).
Perhaps the take-home message is that genetic potential for milk production needs to be matched to the forage system. Research has clearly documented that excessive genetic potential for milk production can lead to cows that are thin at the time of weaning. Excessive genetic capacity for milk production and extended periods of time when forage quality is moderate to low during lactation leads to negative energy balance that has to be made up at some point.
Either that or increased supplementation of more energy-dense feeds will be required to close the gap between energy requirements and energy availability. Long term, if cows are consistently too thin (or open) year after year at weaning time, a selection program designed to lower the genetic potential for milk should be considered. — David Lalman and Mark Z. Johnson, OSU Department of Animal and Food Sciences Extension