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- For calves fed milk replacers at a fixed rate (10 percent of birth weight), a 22 percent crude protein milk replacer resulted in 12.6 percent more weight gain than an 18 percent crude protein milk replacer.
- Increasing protein content of calf starter from 18 percent to 22 percent did not significantly affect weight gains or starter intake, but improved efficiency of starter use for weight gain by 11.6 percent.
- Increasing protein content in either milk replacer increased plasma urea concentrations, indicating less efficient use of dietary nitrogen.
- Milk replacer with 18 percent crude protein and calf starter with 18 percent crude protein provided acceptable calf gains and health.
Breakdown of dietary amino acids in excess of requirements results in excretion of nitrogen into the environment. Minimizing the amount of protein fed to calves so that requirements for amino acids are met, but not greatly exceeded, increases profitability of dairy producers and helps to minimize odor and environmental impacts of dairy production. Many Illinois dairy producers use commercial milk replacers to feed young calves because they are lower-cost sources of nutrients than whole milk. What constitutes an adequate but not excessive amount of crude protein (CP) in milk replacers remains controversial. The feed industry has been polarized for years into two camps, with one major manufacturer promoting 22 percent CP and the other promoting 20 percent. This issue is important economically because the reduction of CP content by 2 percentage units decreases the cost of a 50-lb bag of milk replacer (the approximate minimum amount to feed one calf to the time of weaning) by $2 to $3, or about 5 to 10 percent.
In reality, however, milk replacer CP contents of either 20 or 22 percent may be higher than necessary for satisfactory health and growth of young calves. The protein requirement of calves is most affected by the requirement for growth, with maintenance needs being a relatively small component. Most heifer calves being raised as herd replacements in Illinois and the US are not fed for maximal growth. Rather, calves typically are fed restricted amounts of milk replacer (typically 8 to 12 percent of body weight daily), with free access to calf starter. Such management stimulates early consumption of dry feed, which promotes early development of the rumen and allows weaning to dry feed by an age of 4 to 8 weeks. Calves fed these amounts of liquid feed consume only enough energy for maintenance plus body weight gains of 200 to 300 g/d. Growth rate increases as calves begin to consume greater amounts of starter, which provides energy to fuel growth; consequently, demands for protein also increase.
Our calculations indicate that a typical two-week-old calf fed milk replacer (reconstituted to 12.5 percent solids) at a rate of 10 percent of birth weight should easily meet its needs for protein when the milk replacer contains 18 percent CP. These calculations assume that the milk replacer contains only highly digestible and efficiently utilized milk proteins. As calves consume more starter and grow more rapidly, the requirement for protein also increases. If calves are fed calf starter that contains 18 percent CP as specified by the current National Research Council guidelines, the milk replacer might need to contain as much as 22 percent CP by the end of the liquid feeding period. A more economical approach might be to use a lower-CP milk replacer (e.g., 18 percent) coupled with a starter with a higher CP content (e.g., 22 percent) to provide the necessary amount of CP as starter consumption increases. Protein could be supplied at less cost in starter than in milk replacer. Therefore, the objective of our experiment was to determine whether a relatively low CP content in milk replacer combined with a relatively high CP content in calf starter will produce more efficient growth and nitrogen utilization at lower cost.
MATERIALS AND METHODS
Sixty female Holstein calves born at the University of Illinois Dairy Research Unit were administered 3 L of colostrum within 30 min of birth. Calves were placed into individual outdoor hutches and fed colostrum at 8 percent of birth weight for 2 days. On day 3, calves were assigned alternately to one of four treatments, which were combinations of milk replacers and starters in a 2 x 2 factorial arrangement. Treatments were 1) 18 percent CP milk replacer, 18 percent CP calf starter, 2) 18 percent CP milk replacer, 22 percent CP calf starter, 3) 22 percent CP milk replacer, 18 percent CP calf starter, and 4) 22 percent CP milk replacer, 22 percent CP calf starter. Treatment 3 represents the most widely used industry standard currently and is consistent with current National Research Council recommendations. Our hypothesis was that treatment 2 should produce comparable growth to treatments 3 and 4, at lower cost and with less nitrogen excretion into the environment.
Protein in milk replacers was supplied entirely from milk sources (whey protein concentrate and dried whey). Protein in calf starter was increased by additional soybean meal. Milk replacers were reconstituted to 12.5 percent solids and fed a rate of 10 percent of starting body weight; this rate was held constant through 28 days of age. Calf starters and water were available continuously to each calf. During days 29-35, the amount of milk replacer was decreased to half of the amount fed during the previous week. Calves were weaned abruptly from milk replacer on day 36. Calves remained in the hutches through day 49. Starter intake was measured weekly. Calves were weighed and measured (height at withers, heart girth, and body length) weekly. Body temperature was measured and blood sampled on one day each week. Feces of each calf was scored daily on a 5-point visual scale (1=normal to 5=watery diarrhea with blood). Data were analyzed as a completely randomized design. Variation attributable to milk replacer, calf starter, and the interaction of milk replacer and starter were determined by ANOVA.
RESULTS AND DISCUSSION
Calves fed the higher CP milk replacer tended to gain weight more rapidly (Table 1). Calves fed 22 percent CP milk replacer also had greater gains of heart girth, but gains of height at withers and body length were not affected significantly. The protein content of calf starter had no effect on average daily gains of weight or stature.
Intakes of starter did not differ significantly among treatments. However, calves fed the 22 percent CP starter had significantly greater efficiencies of starter use for body weight gain (Table 1). Greater efficiency resulted from numerically greater gains for calves fed both 22 percent CP milk replacer and 22 percent CP starter, and the non-significant decreases of starter intake for calves fed 22 percent CP starter. The concentration of urea in plasma was increased by the greater protein content in either milk replacer or starter. A significant interaction of milk replacer and calf starter protein concentrations for plasma urea concentration indicated that urea was increased more by the 22 percent CP starter for calves fed the 18 percent CP milk replacer than for calves fed the 22 percent CP milk replacer.
This experiment raises several interesting points. First, calves fed the higher CP milk replacer tended to grow more rapidly regardless of which calf starter they received. This finding is similar to other recent experiments from our laboratory. Whether the relatively modest improvements in weight gain justify the greater cost of the higher CP milk replacer is doubtful unless the higher protein results in long-term improvements in health or subsequent milk production. Calves will be followed through the first lactation to see if treatments had any marked effect on milk production.
Second, increasing the CP content of calf starter did not improve calf growth with the lower-CP milk replacer, as we had hypothesized it would, but did have a numerical advantage when fed with the high-CP milk replacer. The higher CP starter was used with greater apparent efficiency. Finally, plasma urea concentrations were greater for calves fed the 22 percent CP starter. This indicates that more of the dietary protein was degraded and not re-incorporated into microbial protein in the developing rumen of the calves. Perhaps formulation of a higher-CP starter with more attention to inclusion of protein sources that are more resistant to degradation, such as heat-treated soybeans or soybean meal, fish meal, or corn gluten meal, would show even more benefit.
As forumulated, the 22 percent CP starter would cost about $22/ton more to producers, assuming soybean meal at $300/ton and corn at $2.50/bu. For calves fed the 22 percent CP milk replacer, the cost of gain is about $0.09/lb less for the higher-CP starter.
This project was supported by the Illinois Council on Food and Agricultural Research (C-FAR). The authors thank Gene McCoy, Nancy Bower, and Bob Riggs for assistance.
Table 1. Growth and feed intake by heifer calves fed milk replacers and starter containing either 18% or 22% crude protein (CP).
|18% CP Milk replacer||22% CP Milk replacer|
|Variable||18% CP Starter||22% CP Starter||18% CP Starter||22% CP Starter||SE||Significance (P<)1|
|ADG, lb||1.17||1.10||1.23||1.32||0.036||MR (0.09)|
|Heart girth gain, cm/42 d||13.5||13.5||15.0||15.8||0.83||MR (0.03)|
|Body length gain, cm/42 d||8.1||7.4||7.7||8.6||0.72||NS|
|Wither height gain, cm/42 d||18.3||18.7||18.9||21.7||1.28||NS|
|Starter intake, lb/d||1.52||1.39||1.61||1.45||0.045||NS|
|Plasma urea, mg/dl||5.6||8.4||7.9||8.8||0.45||MR (0.006), CS (0.0004), MR*CS (0.04)|
1 Significance of main effects or interactions. MR = milk replacer; CS = calf starter; NS = not significant (P > 0.10).
2 Calculated as BW gain divided by starter intake. Milk replacer intake was fixed as a percentage of starting BW.