236 R
.C. Dobos et al. Livestock Production Science 63 2000 235 –243
1. Introduction compared to heifers grown on an alfalfa based diet
22 CP. Milk production was not affected by Dairy heifers grown to achieve liveweight gains
either diet, LWG or LWG within diet. However, LWG greater than 800 g per d before puberty have
heifers on the high-CP diet produced 1.0 kg per d significantly reduced milk yields in some studies
less fat-corrected milk, independent of pre-pubertal Amir and Kali, 1974; Gardner et al., 1977; Little
LWG. and Harrison, 1981; Sejrsen et al., 1982; Valentine et
There is continuing pressure for dairy managers to al., 1987; Waldo et al., 1989, but not in others
reduce costs of production. Reducing age at first Capuco et al., 1988; van Amburgh and Galton,
breeding is an option but there is a need to determine 1994; Capuco et al., 1995; Pirlo et al., 1997. This
if diet composition can alleviate the negative effects lack of consistency has been attributed to the differ-
of high LWG during the pre-pubertal period. Sejrsen ent feeding standards used Sejrsen, 1994, and also
and Foldager 1992 suggested that type of diet may to diet composition Kertz et al., 1987.
not be important at lower LWG. They found no Kertz et al. 1987 suggested that the lower milk
effect on mammary parenchyma of diets based on yield because of high pre-pubertal LWG could be
straw in heifers at low feeding levels. prevented by increasing the dietary crude protein
The objective of this experiment was to determine CP concentration within this period. However,
the effect of dietary protein concentration and investigations conducted by van Amburgh and Gal-
amount of protein protected from rumen degradation ¨
ton 1994 and Mantysaari et al. 1995 indicated no REP within iso-energetic diets on mammary gland
effect of dietary CP concentration on milk pro- development and subsequent production of dairy
duction and mammary gland development, respec- heifers grown at LWG in excess of 900 g per d
tively. Pirlo et al. 1997 investigated the effects of before puberty.
various levels of dietary energy and CP fed to Italian Friesian heifers between 100 and 300 kg liveweight
on milk production during first lactation. Heifers
2. Materials and methods
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