The relationship between fertility and lactation characteristics in Holstein cows on United Kingdom commercial dairy farms.

Higher milk production is commonly associated with poorer fertility in dairy cows. This study used a biological model of lactation to define more closely which characteristics of lactation were linked to increased calving interval, an easily recorded measure of fertility in commercial dairy herds. Large data sets from a national milk recording scheme in the United Kingdom, collected over a 10-yr period, were used to calculate the genetic and phenotypic correlations between calving interval and a range of lactation traits within the Holstein breed. A lactation curve was fitted to each lactation, and several characteristics of each lactation were calculated. These were used in a series of mixed-model bivariate analyses with calving interval to derive the genetic parameters. When heifer lactation curve trait data were used, the highest genetic correlations were found with peak yield, maximum secretion potential, and total lactation milk yield (0.59±0.06 to 0.63±0.05), reflecting the observed phenomenon of poorer fertility and higher milk production. Genetic correlations for calving interval were also calculated with the rate of increase in milk yield in early lactation (0.46±0.08), persistency (0.36±0.09), day of peak yield (0.20±0.02), and relative cell death rate (-0.12±0.09). The daily production of milk components was highly genetically correlated with calving interval, with values for fat, protein, lactose, and water being 0.73±0.15, 0.48±0.13, 0.57±0.13, and 0.50±0.13. With these results and breeding values derived from these analyses, 2 possible strategies were suggested for improving the relationship between milk yield and fertility in dairy cows. First, animals that break the correlations described above could be selected (e.g., bulls with high peak yield and low calving intervals). Second, animals with lower peak yields but better persistency could be selected to maintain total milk yield.

Comparison of mathematical models applied to f1 dairy sheep lactations in organic farm and environmental factors affecting lactation curve parameter.

THE OBJECTIVE OF THIS STUDY WAS TO COMPARE THE GOODNESS OF FIT OF FOUR LACTATION CURVE MODELS: Wood's Gamma model (WD), Wilmink (WL), and Pollott's multiplicative two (POL2) and three parameters (POL3) and to determine the environmental factors affecting the complete lactation curve of F1 dairy sheep under organic management. A total of 5,382 weekly milk yields records from 150 ewes, under organic management were used. Residual mean square (RMS), determination coefficients (R(2)), and correlation (r) analysis were used as an indicator of goodness of fit for each model. WL model best fitted the lactation curves as indicated by the lower RMS values (0.019), followed by WD (0.023), POL2 (0.025) and POL3 (0.029). The four models provided total milk yield (TMY) estimations that were highly correlated (0.93 to 0.97) with observed TMY (89.9 kg). The four models under estimated peak yield (PY), whereas POL2 and POL3 gave nearer peak time lactation estimations. Ewes lambing in autumn had higher TMY and showed a typical curve shape. Higher TMY were recorded in second and third lambing. Season of lambing, number of lambing and type of lambing had a great influenced over TMY shaping the complete lactation curve of F1 dairy sheep. In general terms WL model showed the best fit to the F1 dairy sheep lactation curve under organic management.

Genetic parameters derived from using a biological model of lactation on records of commercial dairy cows.

The object of this study was to investigate the genetics of lactation curve parameters derived from a biological model of lactation and the relationships among them. This biological model fitted 2 logistic curves to mimic the initial increase in milk secretory cell numbers in early lactation and the progression of apoptosis in late lactation. Records from 82,255 Holstein-Friesian heifers from commercial dairy herds in the United Kingdom, recorded from 1994 to 2003, were analyzed. The heritabilities of 2 lactation curve parameters, maximum secretion potential and relative cell death rate, were 0.27 and 0.08 respectively. Maximum secretion potential was highly genetically correlated with peak yield (0.99), and relative cell death rate was highly correlated with persistency of lactation (0.84). Heritability values for the traits analyzed showed a characteristic pattern. Total milk yield traits, maximum secretion potential, and peak yield had similar and moderate heritabilities (approximately 0.3). Traits associated with late lactation had lower heritability values (approximately 0.1), whereas day of peak yield and early lactation traits had little genetic variation. The permanent environmental variance of the various traits ranged from 0.08 to 0.26 of the phenotypic variance. Parameters from the 2 logistic curves were not highly correlated, suggesting that selection programs could be devised to exploit genetic variation in both aspects of lactation independently.

Feeding strategies for rearing replacement heifers in small-scale dairy production systems in the highlands of Central Mexico.

The growth of Holstein heifers in the campesino dairy systems in the highlands of Mexico was evaluated in three feeding strategies. Thirty-three heifers from 11 farmers, grouped according to strategy, were weighed every 14 days for 28 weeks. The live weight change over each 14-day period was estimated by individually regressing live weight over period, taking the regression coefficient as an unbiased estimate of live weight change. Regression coefficients were analysed as a randomized design with feeding strategies as treatments. Strategies were as follows: S1: grazed or cut pasture all year, maize silage and maize straw in the dry season, and 1.0-1.5 kg concentrate/heifer per day. S2: maize straw in the dry season, cut pasture forage, grazing of native grass, weeds from maize fields, and 1.0-1.5 kg concentrate/heifer per day. S3: maize straw in the dry season, grazed native grasses and weeds in the rainy season. Live weight gains were: S1, 0.511 kg/heifer per day; S2, 0.271 kg/heifer per day; and S3, 0.252 kg/heifer per day. Despite the better gains in S1, they are 24% below recommendations, arriving to service at 20 months of age. Not rearing their replacements may be a better alternative for campesino farmers under current economic conditions.