**4. Implications**

146 Milk Production – An Up-to-Date Overview of Animal Nutrition, Management and Health

(Dunklee et al, 1994; Veerkamp et al, 1993, 1994, 1995).

(e.g. Van Arendonk et al, 1991; Li et al, 1998; Zamani, 2005).

Ashtiani et al (2005) to 0.56 by Buttazzoni and Mao (1989).

lactation.

**3.2. Genetic factors** 

1995; Zamani et al, 2008).

These findings reveal that many nutritional factors may affect the efficiency of lactation. Generally, it is necessary to optimize different dietary components to maximize efficiency of

Many evidences show genetic variation of the lactation efficiency. In several studies, the cows selected for high yield traits were more efficient in use of feed for milk production

There are many reports on estimates heritability (h2) for different measures of feed efficiency. Heritability estimates for gross energy efficiency had a wide range of 0.09 to 0.86

Buttazzoni and Mao (1989) estimated a heritability of 0.32 – 0.49 for net energy efficiency (NEE) of lactation. The estimate of heritability for NEE in another study was 0.34 (Miraei Ashtiani et al, 2005). In different studies NEE had moderate positive genetic correlations to milk yield. Genetic correlation of NEE to milk yield has reported from 0.51 by Miraei

Several studies have shown the genetic variation of residual energy intake (REI) in dairy cattle. Van Arendonk et al (1991) and Kennedy et al (1993) have reported heritability estimates of 0.19 and 0.14, respectively for REI. Veerkamp et al (1995) reported a heritability of 0.30 to 0.38 for REI, depending on the way of calculating the energy requirements from phenotypic regressions. In another study, the estimated heritability and repeatability for REI were 0.15 and 0.53 from univariate, and 0.21 and 0.60 from multivariate models, respectively (Zamani et al, 2008). On the other hand, Ngwerume and Mao (1992) and Svendsen et al (1993) found no evidence for any additive genetic variation in REI, where they have reported heritability estimates of 0.016 and 0.00 – 0.11 for REI, respectively. Unlike to other measures of efficiency, REI is, to some extent, independent of yield traits. In different studies REI had low and negative genetic correlations with milk yield (Madgwick et al, 1991; Van Arendonk et al, 1991; Kennedy et al, 1993; Veerkamp et al,

Genetic variation of protein efficiency has been rarely studied in dairy cattle. Li et al. (1998) reported a value of 0.13 as estimate of heritability for crude protein efficiency of lactation. In another study, heritability estimates for crude protein efficiency in 90 and 305 days of lactation were 0.10 and 0.31, respectively (Ageeb, 1999). In a recent study Zamani et al (2011a) reported heritability estimates of 0.07, 0.40 and 0.03 for gross efficiency of crude protein, crude protein balance and residual protein intake, respectively. In their study, noticeable genetic correlations were observed between different measures of protein efficiency and some of yield traits. In this study this has been proposed that the protein efficiency could be improved by direct selection against crude protein balance, while other measures of protein efficiency, including gross efficiency of crude protein and residual protein intake did not seem to be suitable for direct selection. They proposed the selection Efficiency of nutrient utilization is a major factor affecting profitability in modern dairy farms. Several criteria are used to measure efficiency of lactation. Different measures of feed efficiency could be categorized as general, energetic, protein and economic criteria. Feed efficiency is controlled by different environmental and genetic factors. Feed efficiency could be improved by optimizing environmental effects, including nutritional factors, direct selection for feed efficiency or indirect selection for feed efficiency by the selection on yield traits.
