Neville W E, Tucker S V, Utley P R
University of Georgia, Tifton 31793-0748.
J Anim Sci. 1988 Jul;66(7):1606-13. doi: 10.2527/jas1988.6671606x.
This 4-yr study was conducted with Angus (A)-sired and Polled Hereford (PH)-sired crossbred heifers that were exposed to either A or medium-frame Brahman (B) bulls from primiparous calvings either as 2- or 3-yr-olds. Each year, heifers were allotted at random within breed composition and age of dam to breeding groups on pasture for an 84-d breeding period that began in late March or early April. Results indicated that differences due to age of dam (AOD), calf sire breed (CSB) and dam sire breed (DSB) were similar for calving rate, earliness of calving in the calving period and percentage of calves born alive, percentage weaned of those born alive and percentage weaned of cows exposed. The 3-yr-old cows had less calving difficulty and their calves had higher birth weight, average daily gain (ADG) to weaning, 205-d weight and 205-d weight/cow exposed than the 2-yr-old cows. Compared with the B-sired calves the A-sired calves had lower birth weight, ADG to weaning and 205-d weight; however, 205-d weight/cow exposed was similar. Compared with the PH-sired cows, the A-sired cows had less calving difficulty, and their calves had higher ADG to weaning and higher 205-d weights/cow exposed. Under conditions similar to those of this study, it would be economically advantageous to calve first at 2 yr of age rather than at 3 yr of age.
这项为期4年的研究使用了安格斯（A）种公牛和无角海福特（PH）种公牛所生的杂交小母牛，这些小母牛在初产时作为2岁或3岁的母牛与A种公牛或中等体型的婆罗门（B）种公牛配种。每年，根据品种组成和母牛年龄，将小母牛随机分配到牧场的繁殖组，进行为期84天的繁殖期，繁殖期于3月下旬或4月初开始。结果表明，在产犊率、产犊期产犊的早期程度、活产犊牛百分比、活产犊牛的断奶百分比以及参与配种母牛的断奶百分比方面，母牛年龄（AOD）、犊牛父本品种（CSB）和母牛父本品种（DSB）的差异相似。与2岁的母牛相比，3岁的母牛产犊难度较小，其犊牛出生体重更高，断奶前平均日增重（ADG）、205日龄体重以及每头参与配种母牛的205日龄体重更高。与B种公牛所生的犊牛相比，A种公牛所生的犊牛出生体重、断奶前ADG和205日龄体重较低；然而，每头参与配种母牛的205日龄体重相似。与PH种公牛所生的母牛相比，A种公牛所生的母牛产犊难度较小，其犊牛断奶前ADG更高，每头参与配种母牛的205日龄体重更高。在与本研究类似的条件下，2岁首次产犊在经济上比3岁首次产犊更具优势。
