Our analysis used means from 993 individual calves used in 20 trials conducted at our research unit in Ohio, which have all been published in peer-reviewed journals. Each trial used male Holstein calves that were two to three days old when received at our research unit and they remained on trials for eight weeks.
The trials evaluated protein content of dry starter feeds, fiber content of starter feeds, feeding rate of milk replacer powder, content of fat and protein in milk replacer powders and age at weaning. All calves in these trials were housed and cared for individually in an unheated nursery.
The variables that were measured and evaluated for influencing average daily gain (AGD) in these calves included initial serum protein concentration; concentration of fat and protein in the milk replacer powder; intake of milk replacer powder; intake of dry starter feed; body weight at the start of the trial; age at weaning; number of days with abnormal feces; and minimum, maximum and average air temperature in the nursery during the trials.
These variables were used in regression analysis to determine which had influence on predicting the ADG of calves over the first eight weeks of life. A description of the data collected is in Table 1.
After the regression analysis was complete, the variables that best predicted ADG in neonatal dairy calves were starter feed intake; milk replacer powder intake; milk replacer fat and protein content; number of days with abnormal feces; initial body weight; and interactions of starter feed intake, milk replacer powder intake and milk replacer fat and protein content.
Together these variables could explain over 88% of the variation in ADG observed in this dataset. Interestingly, many of the variables excluded by the regression analysis did appear related to ADG in these calves. It is possible that their influence was being masked by other variables.
Our initial objective was to determine if serum protein concentration (used as a proxy for immune status) had an impact on growth of these calves. It became apparent that the relationship between serum protein and ADG of the calves was almost entirely random.
It is still possible that actual measures of passive immunity such as serum Ig status would have had an impact on growth, but within our dataset, total serum protein was not related to growth.
The NRC calf sub-model has factors to adjust growth of neonatal calves when the air temperature is below 60°F. The NRC model increases the calves’ maintenance energy requirements as the air temperature drops.
We did not observe this loss of growth in the calves represented in this dataset. Although not statistically significant, it was apparent that for each temperature measure, as the air got warmer, the ADG of the calves over the eight-week period declined.
Further, this could be seen to occur at temperatures below that which NRC has defined as the lower critical point (60°F). It is worth noting that the neonatal calves tended to nest in their bedding.
This may have created microenvironments around the individual calves such that the air temperature in the nursery was not representative of the level of stress the calves were experiencing.
Two other factors that appeared to influence growth in neonatal calves were sickness (as estimated by number of days with abnormal fecal scores) and bodyweight at birth.
Bodyweight appeared to be related to starter feed intake, with larger calves consuming more starter (Figure 1) which resulted in faster rates of gain (Figure 2).
However, sick calves appeared to repartition nutrient intakes to support the immune system, resulting in small but significant reductions in growth.
In conclusion, nutrient intake from a combination of starter feed and milk replacer powder best predicted growth of neonatal calves, with intake of starter feed being the largest predictor.
Factors that resulted in reduced starter feed intake and increased incidence of sickness depressed growth of neonatal calves over the first eight weeks of life.
Contrary to published information from NRC, heat depressed the growth of neonatal calves more than cold depressed the growth of neonatal calves, within the temperatures represented in this dataset. PD